History of botany (1530–1860)/Book 3/Chapter 1

by Julius von Sachs

CHAPTER I.

History of the Sexual Theory.

1. From Aristotle to R. J. Gamer Arius.

It will contribute to a correct appreciation of the discoveries made towards the end of the 17th century by Rudolph Jacob Camerarius and his successors in regard to the sexual relations of plants, if we first make ourselves acquainted with all that was known of the matter up to that time from Aristotle downwards; we shall learn at the same time how extremely unfruitful was the superficial observation of the older philosophy in a question in which inductive research only could lead to real results.

That Aristotle^[1] like many others after him reckoned sexual fertilisation among processes of nutrition, and thus failed to perceive the specific and peculiar character of the latter, is shown distinctly by his assertion, that the nutritive and propagative power of the soul is one and the same. This hasty generalisation was associated in Aristotle's mind with another error arising from very defective experience, which led him to bring sexuality in organisms into causal connection with their movement in space. He tells us in his botanical fragments, that in all animals which have the power of locomotion, the female is distinct from the male, one creature being female, another male, but both being of the same species, as in humankind. In plants on the contrary these powers are combined and the male is not distinct from the female; each plant therefore reproduces itself and emits no fertilising material; and he adds, that in animals which do not move, as those that have shells and those that live attached to some other substance, male and female are not distinguished, for their life resembles that of plants; at the same time they are called male and female by resemblance and analogy, and there is a certain slight distinction. In like manner some trees produce fruits while others do not, though they aid fruit-bearing trees in the production of fruit, as happens in the case of the fig-tree and the caprifig.

In comparison with these views of Aristotle those of his disciple Theophrastus^[2] appear to some extent enlightened, and to rest on a wider experience, but even his observation supplies nothing of interest on the subject; for he says that some blossoms of the 'mali medicae' produce fruit, and that some do not, and that it should be observed whether the same thing occurs in other plants, which he might easily have done for himself in his own garden. He is more concerned with putting his knowledge into logical order, than with answering the question whether there is any sexual relation in plants. It is certain, he says, that among plants of the same species some produce flowers and some do not; male palms, for instance, bear flowers, the female only fruit^[3]; and he concludes the sentence by the remark, that in this lies the difference between these plants, and those which produce no fruit, and that it is obvious that there must be a great difference in the flowers. In his third book 'De Causis' (c. 15, 3) he says, that terebinths are some male and some female, and that the former are barren and are therefore called male. That Theophrastus in all these matters trusted to the relations of others is shown by a passage in the same book (c. 18, i), where he says, 'What men say, that the fruit of the female date-palm does not perfect itself unless the blossom of the male with its dust is shaken over it, is indeed wonderful, but resembles the caprification of the fig, and it might almost be concluded that the female plant is not by itself sufficient for the perfecting of the foetus; but this cannot be the case in one genus or two, but either in all or in many.' We observe the grand style in which the Greek philosopher dismisses this important question, and how far he is from condescending to make an observation for himself.

It appears that in Pliny's time the hypothesis of a sexual difference in plants had grown up and become confirmed in the minds if not of writers, yet of those who occupied them- selves with nature; Pliny in his 'Historia Mundi,' describing the relation between the male and female date-palm, calls the pollen-dust the material of fertilisation, and says that naturalists tell us that all trees and even herbs have the two sexes^[4].

If this theme supplied little material for reflection to philosophers, it did not fail to excite the fancy of the poets. De Candolle cites the verses of Ovid and Claudian on the subject, and passing over the intervening centuries for a very sufficient reason notices the lively poetic description of two date-palms in Brindisi and Otranto by Jovianus Pontanus in 1505. But nothing was gained in this way for natural science.

Treviranus in his 'Physiologic der Gewachse 1 ' (1838), II. p. 371, has well described the state of knowledge on this subject among the botanists of Germany and the Netherlands in the 16th century. 'The idea of a male sex in such plants as Abrotanum, Asphodelus, Filix, Polygonum mas et femina, was founded only on difference of habit, and not on the parts which are essential to it. But it should be observed that it is the less learned among the older botanists, Fuchs, Mattioli, Tabernaemontan, who make most frequent use of this mode of designating plants; the more learned, as Conrad Gesner, de l'Ecluse, J. Bauhin employ it only in the case of a plant already known. De l'Ecluse it is true in describing the plants which he found often notes the form, colour, and even the number of the stamens; in Carica Papaya he calls the individual with stamens the male, and the one with carpels the female, since he holds them to belong to different sexes, though of the same species; but he is satisfied with saying, that it is affirmed that the two are so far connected, that the female produces no fruit if the male is separated from it by any great distance ('Curae posteriores,' 42).

The case of the botanists above-mentioned is simply one of ignorance; in the botanical philosopher Cesalpino on the contrary we see a consequence of the Aristotelian system, which leads him distinctly to reject the hypothesis of separate sexual organs in plants as opposed to their nature. It is difficult to understand how De Candolle, at page 48 of his 'Physiologic vegetale,' can say that Cesalpino recognised the presence of sexes in plants. His conception of vegetable seed-grains as analogous to the male seed in animals must have made it impossible for him to understand sexuality in plants. So too his notion that the seed is derived from the pith as the principle of life in plants, in connection with which he says at page 11 of the first of his sixteen books; 'Non fuit autem necesse in plantis genituram aliquam distinctam a materia secerni, ut in animalibus, quae mari et femina distinguuntur.' He regarded the parts of the flower which surround the ovary, or are separate from it, together with the stamens as simply envelopes of the foetus ; and though he knew, as has been already shown, that in some plants, the hazel, chestnut, Ricinus, Taxus, Mercurialis, Urtica, Cannabis, Mais, the flowers are separate from the fruit, and even mentions that the barren individuals are called male, and the fruit-bearing female, he understood this only as a popular expression, without really admitting a sexual relation. Respecting the words male and female he says at page 15: 'Quod ideo fieri videtur quia feminae materia temperatior sit, maris autem calidior; quod enim in fructum transire debuisset, ob superfluam caliditatem evanuit in flores, in eo tamen genere feminas melius provenire et fecundiores fieri aiunt, si juxta mares serantur, ut in palma est animadversum, quasi halitus quidam ex mari efflans debilem feminae calorem expleat ad fructificandum.'

There is no mention of the pollen here, still less any attempt to extend what had been observed in dioecious plants to the ordinary cases, in which flowers and pistil, as Cesalpino would say, are united in the same individual. His view of the relation between the seed and the shoot, cited above on page 47, shows that he conceived of the formation of seeds as only a nobler form of propagation than that by buds, but not essentially distinct from it. The idea of sexuality in plants was not in fact consonant with Cesalpino's interpretation of Aristotelian teaching.

Prosper Alpino's account (1592) of the pollination of the date-palm contains nothing new, except that he had seen it in Egypt himself^[5].

The Bohemian botanist Adam Zaluziansky^[6] made no observations of his own, but attempted in 1592 to reduce the traditional knowledge on the subject to some kind of theory. The foetus, he says, is a part of the nature of plants, which they produce out of themselves, and is thus distinguished from the shoot which grows from the plant, as a part from the whole, but the other as a whole from a whole. He quotes Pliny almost word for word where he says, that observers of nature maintain that all plants are of both sexes, but in some the sexes are conjoined, in others they are separate ; in many plants the male and female are united, and these have the power of propagation in themselves, like many androgynous animals; and he explains this, more explicitly than Aristotle, from defect of locomotion in plants. This is the case, he says, with the majority of plants. In some, as the palm, the male and female are separated, and the female without the male produces no fruit, and where the dust from the male does not reach the female plant by natural means, man can assist. Zaluziansky like other writers is anxious that plants of different sexes should not be taken for different species. He refers also to the popular distinction of many plants into male and female according to certain external peculiarities.

Jung again must certainly have known the facts and views that were current in his time; but there is nothing in his botanical writings to show that he entertained the idea of a real sexuality in plants, of the necessity of the co-operation of two sexes in the work of propagation. It might almost be believed that the most learned and serious men, such as Cesalpino and Jung, were just those, who regarded the hypothesis of sexuality in plants as an absurdity, and shrunk from its consideration. This impression is conveyed too by Malpighi's 'Anatomic des Plantes.' It was Malpighi who gave the first careful account of the development of the seed, and studied the earlier stages in the growth of the embryo in the embryosac; and yet even he says nothing of the co-operation of the dust contained in the anthers in the formation of the embryo, and does not once mention the views of former writers. Malpighi, like Cesalpino, regarded the formation of seeds as only another kind of ordinary bud-formation, and propagation as only another kind of nutrition. He mentions (p. 52) incidentally that plants with unfruitful flowers are designated as male, but treats this as a popular expression merely, and ultimately propounds the theory that the stamens and the floral envelopes remove a portion of the sap from the flower, in order to purify the sap for the production of the seeds (p. 56).

In all accounts of the theory of sexuality in plants, a botanist otherwise unknown in history, Sir Thomas Millington, is named as the person who first claimed for the stamens the character of male organs of generation. The only record of the fact, however, is contained in the following words of Grew in his 'Anatomy of Plants' (1682), ch. 5, sect. 3, p. 171 : 'In conversation on this matter (namely the connection of the stamens, called by Grew the attire^[7], with the formation of seeds) with our learned Savilian Professor Sir Thomas Millington, he told me he was of opinion that the attire served as the male organ in the production of the seed. I replied at once, that I was of the same opinion, and gave him some reasons for it, answering at the same time some objections that might be brought against it.' Grew gives on p. 172 the following summary of his ideas on the subject^[8]; it would appear, he says, that the attire serves to remove some superfluous parts of the sap, as a preparatory process to the production of seed. As the floral envelopes (foliature) serve to remove the volatile and saline sulphur-parts, so the attire serves to lessen and adjust the gaseous, in order that the seed may become more oily and its principles be better fixed. Here we find ourselves on the ground of the chemistry of the day, in which sulphur, salt, and oil play the chief parts. Consequently, continues Grew, the flower has usually a stronger smell than the attire, because the saline sulphur is stronger than the gaseous, which is too subtle to affect the sense. Closely adhering to Malpighi's view he goes on to compare these processes in the flower with processes in the ovary of animals, inasmuch as they qualify the sap in the ovary for the approaching formation of seed, and he says that as the young and early attire before it opens contains the superfluous part of the female organ, so after it is opened it probably performs the office of the male. But how confused his ideas still were on this point may be further seen by examination of the passage which follows in his book (page 172, section 7), where, speaking of the single flowers in the head of the Compositae, he regards the blade, that is the style and stigma, of the floral attire as a portion of a male organ, and the globulets (pollen-grains) and other small particles upon the blade and in the thecae (anthers) of the seed-like attire as a vegetable sperm, which subsequently when the parts are duly matured falls down upon the seed-case and so touches it with a prolific virtue.

He meets the objection, that the same plant must consequently be both male and female, with the fact, that snails and other animals are similarly constituted. That the pollen-grains communicate a prolific virtue to the ovary (uterus) or to its juices by simply falling upon it, he thinks is rendered probable by comparing this with the process of fertilisation in many animals, and here Grew has some curious remarks. The section closes with the observation that to expect complete similarity in this matter between plants and animals, is to require that the plant should not only resemble an animal, but should actually be one.

If now we ask ourselves, what it really was that was gained from Millington and Grew, we find that it was simply the conjecture, that the anthers produce the male element in fertilisation, and that this view was closely connected in their minds with the strangest chemical theories and analogies from animal life. It is remarkable by what indirect ways science sometimes advances. If Grew had only been prepared to assume some kind of sexuality in plants, he need only have taken up Theophrastus' statement, that the anther-dust of the male palm is shaken over the female to produce fertilisation; and since both Grew and Malpighi observed the pollen in the anthers, they might at once and in reliance on this experiment of a thousand years before have come to the conclusion that the stamens are the male organs. But Grew never mentions the ancient views and experiences. Like other writers before Camerarius, he made no attempt to answer the question by experiment. It was a step in advance, when Ray in his 'Historia Plantarum' (1693), I. cap. 10, p. 17; II. p. 1250, threw some light on the very obscure train of thought in Grew's mind, and did something to put it on the right track, by referring to the case of dioecious plants and to the old experience of the date-palm, but he too made no attempt to settle the question by experiment. The true discoverer of sexuality in plants, Camerarius, was however engaged in the experimental solution of the problem two years before the appearance of Ray's 'Historia Plantarum.' Ray's remarks on the subject in the preface to his 'Sylloge Stirpium' (1694) are only assertion founded on no experiments. But if any are prepared to attribute greater value to the utterances of Grew and Ray, the comparison of them with the way in which Camerarius addressed himself to the question will show at once, that it was he who so far advanced the theory of the subject as to make it accessible to experimental treatment, as he undoubtedly was the first who not only undertook experiments on the subject but carried them out with the skill which will appear in the following section. Linnaeus was right when he says in his 'Amoenitates ' (1749), I. p. 62, that it was Camerarius who first clearly demonstrated (perspicue demonstravit) the sexuality of plants and the mode of their propagation.

2. Establishment of the Doctrine of Sexuality in Plants by Rudolph Jacob Camerarius. 1691-1694.

We have seen that all that was known with regard to sexuality in plants up to 1691 was comprised in the facts related by Theophrastus concerning the date-palm, the terebinth, and the 'malus medica,' and in the conjectures of Millington, Grew, and Ray, while Malpighi's views in opposition to these later authors were considered to be equally well founded. The sexuality^ of plants could only be raised to the rank of a scientific fact in one way, that namely of experiment; it had to be shown that no seed capable of germination could be formed without the co-operation of the pollen. All historic records concur in proving, that Camerarius was the first who attempted to solve the question in this way, and that he followed up this attempt by many other experiments. It is quite another question how the fertilising matter reaches the germ which is capable of being fertilised, and this could not be entertained till experiment had established the fact, that the pollen is absolutely indispensable to fertilisation.

To Johann Christian Mikan, Professor of Botany in Prague, is due the merit of having collected the scattered and therefore almost forgotten writings of Rudolph Jacob Canerarius^[9] , and published them, together with some similar works of Koelreuter, at Prague in 1797 under the title, 'R. J. Camerarii Opuscula Botanici Argument!.' This book, apparently little known, will be my principal authority for the following remarks. The short preliminary communications are printed without alteration from the ninth and tenth year of the second, and from the fifth and sixth year of the third decury of the Ephemerides of the Leopoldina; the letter to Valentin, which will be noticed again further on, together with an abstract of the same and an answer of Valentin, are given according to Gmelin's edition of 1749.

Camerarius had observed, that a female mulberry-tree once bore fruit, though no male tree (amentaceis floribus) was in its neighbourhood, but that the berries contained only abortive and empty seeds, which he compared to the addled eggs of a bird. His attention was roused, and he made his first experiment on another dioecious plant, Mercurialis annua; he took in the end of May two female specimens of the wild plant (they were usually called male, but he knew them t'o be the female) and set them in pots apart from others. The plants throve, the fruit was abundant and filled out, but when half ripe they began to dry up, and not one produced perfect seeds; his communication on this subject is dated December 28, 1691. In the third decury of the Ephemerides, year 5, he relates that in a sowing of spinach he had found monoecious as well as dioecious plants, as Ray had observed in Urtica romana, and he himself again in three other species. The disregard of this fact was afterwards the cause of erroneous interpretation of the experiments and of doubt about sexuality. But Camerarius' chief composition on the subject of sexuality in plants is his letter 'De sexu Plantarum,' which is often mentioned but apparently little read, and which he addressed to Valentin, Professor in Giessen, on Aug. 25, 1694. It is the most elaborate treatise on the subject which had as yet been written, or indeed which appeared before the middle of the 18th century, and contains more profound observations than were made by any other botanist before Koelreuter. The style contrasts favourably with the style of the writers of the time, and is thoroughly that of modern natural science; it combines perfect knowledge with careful criticism of the literature of the subject; the construction of the flower is explained more clearly than it had ever been before, or was again for a long time after, and expressly for the purpose of making the meaning of his experiments on sexuality intelligible. The whole tone of the letter shows that Camerarius was deeply impressed with the extraordinary importance of the question, and that he was concerned to establish the existence of sexuality by every possible means.

After detailed examination of the parts of the flower, the anthers and pollen, the behaviour of the ovules before and after fertilisation, the phenomena of double flowers and similar matters, from all which he cautiously deduces the meaning of the anthers (apices), he proceeds to bring forward direct proofs. He says, 'In the second division of plants, in which the male flowers are separated from the female on the same plant, I have learnt by two examples the bad effect produced by removing the anthers. When I removed the male flowers (globules) of Ricinus before the anthers had expanded, and prevented the growth of the younger ones but preserved the ovaries that were already formed, I never obtained perfect seeds, but observed empty vessels, which fell finally to the ground exhausted and dried up. In like manner I carefully cut off the stigmas of Mais that were already dependent, in consequence of which the two ears remained entirely without seeds, though the number of abortive husks (vesicularum) was very great.' He then refers to his former communications to the Ephemerides on dioecious plants, and says that the case of the spinach confirmed these results. After alluding to similar relations in animals he continues, 'In the vegetable kingdom no production of seeds, the most perfect gift of nature, the general means for the maintenance of the species, takes place, unless the anthers have prepared beforehand the young plant contained in the seed (nisi praecedanei riorum apices prius ipsam plantam debite praeparaverint). It appears, therefore, justifiable to give these apices a nobler name and to ascribe to them the significance of male sexual organs, since they are the receptacles in which the seed itself, that is that powder which is the most subtle part of the plant, is secreted and collected, to be afterwards supplied from them. It is equally evident, that the ovary with its style (seminale vasculum cum sua plumula sive stilo) represents the female sexual organ in the plant.' Further on he assents to Aristotle's theory of the mixture of sexes in plants, and adduces Swammerdam's discovery of hermaphroditism in snails, which he says is the exception in animals but the rule in plants. One erroneous notion which was only seen to be erroneous a hundred years later by Konrad Sprengel, and not finally refuted till within the last few years, was his belief that hermaphrodite flowers fertilise themselves, and this by comparison with the snails he thinks is strange, though most botanists till down to our own times, in spite of Koelreuter and Sprengel, did not find it strange. That sexuality in plants was admitted by botanists, Ray excepted, at the close of the 17th century at most in a figurative sense, but that Camerarius conceived of it as in the animal kingdom, and sought to make this conception prevail, is apparent from the strong expressions, which he uses to show that in dioecious plants the distinction between male and female plants is not to be understood figuratively. He says that the new foetus, the young plant contained in the seed, is formed inside the coat of the seed after the plant has flowered, exactly as the new foetus is formed in animals. The authority of the ancients was still great at that time, for Camerarius thinks it necessary to insist that the views of Aristotle, Empedocles, and Theophrastus are not opposed to his sexual theory. Camerarius appears as the true investigator of nature, endowed with the true discerning spirit in disregarding the question which had already been raised with respect to animals, whether the ovum or the spermatozoid (vermis) produces the foetus, because the first thing to be done was to establish the fact of a sexual difference, not the mode of generation; he thinks it certainly desirable to examine and see what the pollen-grains contain, how far they penetrate into the female parts, whether they advance uninjured as far as the seed which receives them, or what they discharge if they burst before reaching it. He does full justice to Grew's services in connection with the knowledge of the pollen and its function.

It does all honour to the scientific spirit in Camerarius, that he raises a number of objections to his own theory; one was, that Lycopods and Equisetaceae produce, as he thinks, no young plants from their pollen; he suspected therefore that they have no seed. It should be remembered that the germination of Equisetaceae and Lycopods was not observed till the 19th century. An objection, more important at the time, was that a third ear of a castrated maize plant contained eleven fertile seeds, of whose origin he could give no account. He was even more disturbed by finding that three plants of hemp taken from the field and cultivated in the garden produced fertile seeds, and he tries to explain it by supposing various ways in which pollination might have taken place unobserved. This led him to make a fresh experiment ; next year he placed a pot containing seedlings of hemp in a closed room ; three male and three female plants grew up ; the three male were cut off (not by himself) before their flowers opened ; the female produced a great number of abortive seeds, but also a good many fruitful ones. His opponents and those who sought to appropriate his honours fastened, as is usual, on these failures, without being able to account for the experiments which had been successful. The statement of his failures is our best proof of the exactness of his observations, for we now know the cause of failure, which Camerarius himself observed, but did not apply in explanation. We may assume that he would have cleared up this point in his splendid investigations in a quieter time, for at the end of his letter he laments the unjust war then raging; it was the time of the predatory campaign of Louis XIV. To his letter is appended a Latin ode of twenty-six stanzas by an unknown poet, probably a pupil of his own; it is an epitome of the 'Epistola de sexu Plantarum,' as Goethe's well-known poem contains the chief points of his doctrine of metamorphosis, but it resembles Goethe's composition in no other respect; it begins

Novi canamus regna cupidinis,
Novos amores, gaudia non prius
Audita plantarum, latentes
Igniculos, veneremque miram.

3. Dissemination of the New Doctrine; its Adherents and Opponents. 1700-1760.

No part of botany has so often engaged the pen of the historian, as the doctrine of sexuality in plants ; but the majority of writers have not gone to the original sources for their information, and the consequence has been that the merits of the real founders and promoters of the doctrine have often been thrown into the shade for the benefit of others; even German botanists have ascribed the services of Camerarius to Frenchmen and Englishmen, because they were unacquainted with his writings, or were unable to judge of the question and its solution. We shall here endeavour to show from the records of the 18th century how far anyone before Koelreuter really contributed anything of value to the estab lishment of the sexual theory. As is usually the case in great revolutions in science, some simply denied the new theory, many adopted it without understanding the question, others formed a perverse and distorted conception of it under the influence of reigning prejudices, while others again sought to appropriate to themselves the merit of the real discoverer; there were but few who with a right understanding of the question advanced it by new investigations.

The botanists who endeavoured to aid in determining the matter by their own observations may be distinguished into those, to whom the important point was the enquiry whether the pollen is absolutely necessary to the formation of seed, such as Bradley, Logan, Miller, and Gleditsch, and those who like Geoffrey and Morland assumed that sexuality was no longer an open question, and who were bent on observing in what way the pollen effects fertilisation in the ovule. But there was another class of writers altogether, who, believing that they could deal with the subject without making observations and experiments of their own, either like Leibnitz, Burckhard, and Vaillant, simply accepted the results of the observations of others on general grounds, or like Linnaeus and his disciples, endeavoured to draw fresh proofs from philosophical principles, or like Tournefort and Pontedera, simply rejected the idea of sexuality in plants. Lastly, we might mention Patrick Blair who did nothing himself, but merely appropriated the general results of Camerarius' observations, and has had his reward in being quoted even by German writers as one of the founders of the sexual theory^[10].

We have now to see what was really brought to light by further experiment and observation. Bradley appears to have been the first who experimented on hermaphrodite flowers with a view to establish the sexuality of plants ('New improve ments in Gardening' (1717), I. p. 20). He planted twelve tulips by themselves in a secluded part of his garden, and as soon as they began to flower removed the anthers ; the result was, that not one of them produced seeds, while four hundred tulips in another part of the same garden produced seeds in abundance.

Twenty years pass by before another experiment is made James Logan^[11] Governor of Pennsylvania, an Irishman by birth, set some plants of maize in each corner of a plot of ground, which was forty feet broad, and about eighty long, and experimented on them in various ways. In October he noted the following results : the cobs of the plants, from which he had removed the male panicles when the stigmas were already dependent, presented a good appearance; but closer examination showed that they were unfertilised, with the exception of one which was turned in the direction from which the wind might have conveyed pollen from other plants. On the cobs, from which he had removed some of the stigmas, he found exactly as many grains as he had left stigmas. One cob, which had been wrapped in muslin before the appearance of the stigmas, produced only empty husks.

Miller's experiments in 1751, which Koelreuter has extracted from the 'Gardener's Dictionary,' part II^[12], are specially interesting, because the aid of insects in pollination was then observed for the first time. Miller planted twelve tulips, six or seven ells apart, and carefully removed the stamens as soon as the flowers began to open; he imagined that he should thus entirely prevent fertilisation; some days after he saw some bees load themselves with pollen in an ordinary tulip-bed and fly over to his imperfect flowers. After they were gone, he observed that they had left on the stigmas a quantity of pollen sufficient for fertilisation, and these tulips did in fact produce seed. Miller also kept some female plants of spinach apart from the male, and found that they bore large seeds without embryos.

Professor Gleditsch, Director of the Botanic Garden in Berlin, described in the same year ('Histoire de l'Academic royale des sciences et des lettres' for the year 1749, published in 1751 at Berlin), an experiment on the artificial fertilisation of Palma dactylifera folio flabelliformi, which was no doubt our Chamaerops humilis, since he says himself in page 105 that it was Linnaeus' Chamaerops, and Koelreuter speaks of the plant in his report by that name. This treatise, in point of scientific tone and learned handling of the question, is the best that appeared between the time of Camerarius and that of Koelreuter. We learn from the introduction, that in the year 1749 there were few who doubted the existence of sexuality in plants. The author says that he has endeavoured to attain to perfect conviction on the point by many years' experiments with plants of the most various kinds. Of late years he had chiefly selected dioecious plants for investigation, Ceratonia, Terebinthus, Lentiscus, and the species of date-palm which is commonly called Chamaerops. After relating the formation of fertile seeds in Terebinth and the mastic-tree produced by artificial pollination, he turns to Chamaerops, of which species Prince Eugene had repeatedly caused specimens of considerable size to be brought over from Africa; a specimen cost as much as a hundred pistoles; but they died without flowering. 'Our palm in Berlin,' he continues, 'is a female, and may be eighty years old; the gardener asserts that it has never borne fruit, and I have myself never seen fertile seeds on it during fifteen years.' As there was no male tree of the kind in Berlin, Gleditsch procured some pollen from the garden of Caspar Bose in Leipsic. In the course of the nine-days' journey the greater part of the pollen escaped from the anthers, and Gleditsch feared that it was spoilt; but he was reassured by the Leipsic botanist Ludwig, who had had experience in Algiers and Tunis, and who informed him that the Africans usually employ dry pollen that has been kept for some time for the purpose of fertilisation. Though the flowering of the female tree was nearly over, he strewed the loose pollen on its flowers, and tied the withered inflorescence of the male plant to a late-blowing shoot of the female. The result was that fruit ripened in the following winter, and germinated in the spring of 1750. A second attempt conducted in a similar manner produced an equally favourable result^[13].

Koelreuter, who repeats this account in his 'Historic der Versuche,' a record of the experiments made between the years 1691 and 1752 on the sexes of plants, ends his narrative with these words: 'These are, as far as I know, all the attempts which have been made and described since the year 1691 to prove the existence of sexes in plants.' Koelreuter's book was written to show that experiment only can determine the question of sexuality in the vegetable kingdom, and that no one beside Camerarius, Bradley, Logan, Miller, and Gleditsch had pursued this method up to 1752.

While these botanists occupied themselves with the question whether there was a distinction of sexes in the vegetable kingdom, we meet with two writers at the beginning of the 18th century who regard sexuality as proved, and who take up the question of the mode in which the pollen brings about the formation of the embryo. Both were adherents of the theory of evolution, bad observers, and not familiar with the literature of the subject. The first is Samuel Morland. In the 'Philosophical Transactions' of 1702 and 1703, p. 1474, he names Grew as the man who had observed that the pollen answers to the male semen, but he makes no allusion to Camerarius' experiments, the only ones which had as yet been made. He himself suggests that the young seeds may be compared to unfertilised ova, while the pollen-dust (farina) contains embryo plants, one of which must find its way into every ovule (ovum) in order to fertilise it. If so, the style must be a tube through which the embryos pass into the ova. He supposes the pollen in Fritillaria imperialis to be washed by wind and rain from the stigma through the style into the ovary, without reflecting that the movement must be an upward one in the hanging flower. If I could prove, he says, that embryos are never found in unfertilised seeds, this would be a demonstration; but I have never been so fortunate as to settle this point. He does not mention that Camerarius had shown this ten years before; he can only give as the main argument for his conjecture, that in beans the embryo lies near the orifice of the seed-coat (the micropyle), which shows that he was not aware that the two large bodies in the seed of the bean (the cotyledons) belong to the embryo, a fact which his countrymen Grew and Ray had already pointed out. It appears therefore, that Morland supplied no answer to the question how fertilisation takes place; his treatise contains nothing more than the assertion that the embryo is already contained in the pollen-grain, and that it reaches the seed through a hollow style and is there developed, an entirely erroneous and not even an original idea, for it was the off- spring of the theory of evolution which was at that time in vogue.

Geoffroy’s communications ('Histoire de l'Academic royale des sciences,' Paris, 1714, p. 210) contain a few more facts. He mentions neither Grew, Camerarius, nor even Morland, but connects his own observations of 1711 on the structure and purpose of the more important parts of the flower with those of Tournefort, who was a decided opponent of the doctrine of sexuality in plants. The parts of the flower are hastily described, figures are given of some forms of pollen-grains, and the notion that the style is a tube receives some apparent confirmation from the experiment of drawing water through the style of a lily. The view that the pollen is not an excrement, as Tournefort and Malpighi had maintained, is defended partly by arguments which prove nothing, for instance, by the erroneous assertion that the anthers are always so disposed that the extremity of the pistil must necessarily receive their dust. The only proof offered for the fact that seeds are infertile if deprived of the cooperation of the pollen, is a very hasty account of some experiments with maize and Mercurialis. The result of these experiments, as well as some other remarks of Geoffrey, remind us of the text of Camerarius' letter to an extent which mere accident will hardly account for. If Geoffroy really made these experiments, which is open to some doubt, yet they were made fifteen years later than those of Camerarius, who did make the same experiments among others and has described them better. Geoffroy next endeavours to show how the pollen effects the fertilisation, and offers two views on the subject; first, that the dust contains much sulphur and is decomposed on the pistil, the more subtle parts forcing their way into the ovary, where they set up a fermentation and cause the formation of the embryo ; the second view is, that the pollen-grains already contain the embryos, which find their way into the seeds and are there hatched. This is Morland's notion, who however is not mentioned. Geoffroy considers the latter to be the more probable hypothesis, chiefly because no embryo is found in the ovule before fertilisation, and also because the seed of the bean has an orifice (the micropyle); it does not occur to him that these facts speak as much for the first as for the second view.

Enough has been produced to show that Morland and Geoffrey contributed nothing either to the establishment of the fact of sexuality in plants, or to the decision of the question how the pollen effects fertilisation in the ovule. Nevertheless I have mentioned these two men immediately after those who really developed the sexual theory, because they at least took their stand on experience, and endeavoured, though unsuccessfully, to demonstrate conditions of organisation which should explain the process of fertilisation. We come now to the names of men Leibnitz, Burckhard, Vaillant, Linnaeus who are usually supposed to have aided in establishing the sexual theory, but who may be proved to have contributed nothing whatever to the scientific demonstration of that doctrine. First as regards the philosopher Leibnitz; he says in a letter of 1701, from which Jessen has quoted the most important parts in his 'Botanik der Gegenwart und Vorzeit,' 1864, p. 287: 'Flowers are closely connected with the propagation of plants, and to discover distinctions in the mode of propagation (principiis generationis) is very useful,' etc.; again, 'A new and extremely important point of comparison will be hereafter supplied by the new investigations into the double sex in plants,' alluding, according to Jessen, to those of Camerarius and Burckhard. We shall not expect to find that Leibnitz made experiments himself, and the words quoted merely indicate that he wished to see the parts of the flower employed for purposes of classification, because according to the observations of others they are the instruments of propagation. The remark applies in a still higher degree to Burckhard, who in his letter to Leibnitz of 1702, quoted above on p. 83, further developed the idea intimated by Leibnitz, for he too accepted the sexuality of plants as an established and self-evident truth. The address with which Sebastian Vaillant opened his lectures at the Royal Gardens in Paris in 1717 has often been noticed by the historians of botany. De Candolle, who assigns to him an important share in developing the sexual theory, says^[14], that in this address he propounded the sexuality of plants most expressly and as an acknowledged fact, and that he described very graphically the way in which the anthers fertilise the pistil, into which description little that was correct probably found its way, since it required Koelreuter, Sprengel, and the botanists of quite modern times to clear up this point. Vaillant therefore can only have the credit of an eloquent description of what was then accepted. However, De Candolle goes on to say what Vaillant's discoveries were, and on the following page we read that Linnaeus confirmed these discoveries in the year 1736 in his 'Fundamenta Botanica,' and made skilful use of them in the year 1735 in laying the foundations of his sexual system. We have already in the second chapter of the first book explained the confusion of ideas which lies at the bottom of these and many similar statements, and in the same chapter have sufficiently indicated our opinion respecting Linnaeus' share in the establishment of the doctrine of sexuality. It was the character of Linnaeus' mind to attach slight value to the experimental proof of a fact, even when, like that of sexuality, it could only be proved by experiment; from the point of view of his scholastic philosophy it was more important with him to deduce the existence of this fact, in what seemed to him the philosophic way, from the idea of the plant or from reason, and in doing so to drag in a variety of analogies from the animal kingdom; hence he acknowledged the services rendered by Camerarius, but troubled himself little about his experiments which alone could decide the question, while he undertakes himself to prove the existence of sexes in plants on grounds of reason and the like in his peculiar fashion. How he did this in the 'Fundamenta' and in the 'Philosophia Botanica' has been already shown. Here we will briefly notice the often-quoted

dissertation, 'Sponsalia Plantarum,' in the first volume of the 'Amoenitates Academicae' (1749). He first gives the views of Millington, Grew, Camerarius and others; then on p. 63 he accepts the statement of Gustav Wahlboom, that he, Linnaeus, had devoted infinite labour to this question in 1735 in the 'Fundamenta Botanica,' and had there (132-150) proved the sexes of plants with so great certainty that no one would hesitate to found on it a detailed classification of plants. Here then we have once more the construction of Linnaeus' so-called sexual system introduced into the question of sexuality, as if it had anything whatever to do with the establishing the existence of sexes in plants, and as to the infinite labour (infinito labore) which Linnaeus is supposed to have given to the question, the paragraphs cited from the 'Fundamenta' contain the scholastic subtleties quoted in Book I. chap. 2, but not one single really new proof. The arguments in the dissertation we are considering are of exactly the same kind, and it is itself only a lengthy paraphrase of Linnaeus' propositions in the 'Fundamenta Botanica,' illustrated by experiments made by others, and with the addition of a few unimportant observations, some of which are misinterpreted. We read, for instance, p. 101, 'Nectar is found in almost all flowers, and Pontedera thinks that it is absorbed by the seeds that they may be the longer preserved; it might seem that bees must be hurtful to flowers, since they carry away the nectar and the pollen;' but Linnaeus, differing from Pontedera, remarks that ' bees do more good than harm, because they scatter the pollen on the pistil, though it is not yet ascertained what is the importance of the nectar in the physiology of the flower.' This fact of the assistance rendered by insects, which was soon afterwards better described by Miller, is not further examined in this place, for Linnaeus goes on to speak of gourds, that they do not perfect their fruit under glass, because the wind is prevented from effecting the pollination.

One experiment only is mentioned, but not the person by whom it was made. We read at p. 99 that in the year 1723 in the garden of Stenbrohuld, the male flowers of a gourd in bloom were daily removed, and that no fruit was formed. Soon after allusion is made to the artifices used by gardeners to obtain hybrid varieties of tulips and cabbage, but the matter is treated rather as agreeable trifling. In the third volume of the Amoenitates of the year 1764, in which Koelreuter's first enquiries into hybridisation had been already published, we find a dissertation on hybrids by Haartman, which was certainly written as early as 1751. In this treatise the necessary existence of hybrid forms is concluded from philosophic principles, as Linnaeus had deduced sexuality from similar principles; no experiments are made, but certain forms are arbitrarily assumed to be hybrids; a Veronica spuria gathered in the garden at Upsala in 1750 is asserted to be the product of Veronica maritima as the mother and of Veronica officinalis as the father, but the only reason for assigning the paternity to the latter plant is that it grew close by. We find also a Delphinium hybridum stated on similar grounds to be the offspring of Delphinium elatum fertilised by Aconitum napellus, and a Saponaria hybrida to have arisen from the pollination of Saponaria officinalis by a Gentiana; and we are told among other things that Actaea spicata alba is the offspring of Actaea spicata nigra fertilised by Rhus toxicodendron. It is obvious that in all this there was no observation of decisive facts, but simple conclusions from arbitrary premises.

We conclude therefore that neither Linnaeus nor his disciples in the interval that elapsed between the labours of Camerarius and Koelreuter contributed a single new or valid proof to the establishment of the fact, that there is a sexual difference in plants and that hybrids are formed between different species; and if many later botanists talked of the great services rendered by Linnaeus to the sexual theory, and even regarded him as its most eminent founder, this arose partly from the fact that they were unable to distinguish between his scholastic deductions and scientific proof, and partly from that confusion of the idea of sexuality with a classification of plants founded on the sexual organs, to which we have before called attention. Such a confusion of ideas gave rise to the claims which Renzi asserted on behalf of Patrizi, but which Ernst Meyer, in his 'Geschichte der Botanik,' iv. p. 420, has refuted on this very ground. Even in our own century De Candolle has been blamed by Johann Jacob Roemer for not giving Linnaeus the credit of being the actual founder of the sexual theory.

A few words in conclusion on those writers, who after Camerarius' investigations still denied sexuality in plants, because they knew nothing of what had been written on the subject or were incapable of appreciating scientific proof. Tournefort must first be mentioned on account of the great authority which he enjoyed with botanists during the first half of the 18th century. In his 'Institutiones rei herbariae' of the year 1700 (Book I. p. 69), with which we have already made acquaintance, he treats of the physiological significance of the parts of the flower, apparently in entire ignorance of Camerarius' researches, and at any rate with a leaning to Malpighi's views. He makes the petals take up nourishment from the flower-stalks, which they further digest and supply to the growing fruit, while the unappropriated parts of the sap pass through the filaments into the anthers and collect in the loculaments, to be afterwards discharged as excreta. Tournefort even doubted the necessity of the pollination of the female date-palm. The truth is that he was not well acquainted with the facts, and was led astray by his preconceptions. The same was the case with the Italian botanist Pontcdera; in his 'Anthologia' of 1720 he reproduces Malpighi's unlucky notion, and at the same time makes the ovary absorb the nectar for the perfecting of the seed; he regarded the male flower in dioecious plants as a useless appendage.

Valentin, to whom Camerarius addressed his famous letter 'De sexu plantarum' in 1694, did his correspondent a disservice in publishing a short abstract of it, which contained some gross misapprehensions of the facts^[15]. Alston in 1756 relying on these incorrect statements disputed the conclusions of Camerarius, and doubted the sexual importance of the stamens on very insufficient grounds. More reasonable doubts were suggested by a German botanist, Moller, who observed that female plants of spinach and hemp produced seeds even after the removal of the male plants, and appealed to the apparently asexual propagation of Cryptogams; these objections were answered by Kästner of Göttingen, who pointed to the fact that dioecious plants, the willow for instance, sometimes bear hermaphrodite flowers. The botanists in question would never have entertained these doubts, if they had read and understood the writings of Camerarius, or had been acquainted with the literature of the subject.

4. The Theory of Evolution and Epigensis.

We have already observed the influence of the theory of evolution on the doctrine of the fertilisation of plants in the case of Morland and Geoffroy. We learn more about it in the work, already quoted, of the philosopher Christian Wolff, 'Vernünftige Gedanken von den Wirkungen der Natur,' Magdeburg, 1723; it will be well to give his own words, for they will serve to show at the same time the amount of knowledge possessed by a cultivated and well-read man in the country of Camerarius and thirty years after the appearance of his treatise on the sexuality of plants. In the second chapter of the fourth part, which treats of the life, death, and genera tion of plants, Wolff says: 'Ordinarily plants are produced from seeds, for the seed not only contains the plant in embryo but also its first food.' He says that propagation by means of buds is equally natural, for each bud contains a branch in little. 'We find inside in the flower a number of stalks disposed in a circle, and something at the top of each, which is full of dust and lets the dust fall on the tipper part of that which holds the seed; this organ is compared by some to the genitals of the animal, and the dust to the male seed; they think also that the seed is made fruitful by the dust, and that therefore the embryo must be conveyed by the dust into the seed-case and there be formed into seeds. I have proposed to examine into the matter, but I have always let it escape me.' . . . 'Since all that has been hitherto adduced is found also in flowers which spring from bulbs, and it is also certain that the leaves of bulbs have consequently embryos in them ... it is easy to see that the embryos must come from the leaves of the bulbs. And since they could as easily be conveyed from there into the seed-grains with the sap, as into the dust which is produced in the upper part of the flower, I am inclined to think that this is the true account of the matter and that it will be confirmed by experience. But now comes the main question, whence come the embryos into the sap; since they have not an external figure only but an internal structure also, it is not plain how they can be formed either by the mere inner movement of the sap, or by separation of certain parts. . . . And this is certainly more credible, that the embryos already exist in little in the sap and the plant, before they are brought by some change into the condition in which they are met with in the seed and in buds. l!ut there is the further question where they were previously. They must either lie one in another in a minute form, as Malebranche especially maintains, or they are brought from the air and the earth with the nourishing sap into the plant, an idea which Honoratus Fabri Advanced and Perrault and Sturm developed after him. According to the first opinion the first seed-grain must have contained everything in itself, which has grown from it to this hour.' But this demand goes beyond even Wolff's powers of belief; for, says he, it is too great a tax on the imagination to conceive of this inclosing of germs one in another like box within box. It is well known that such notions as these were very prevalent in the 18th century, and that the spermatozoids of animals were thought to lend considerable support to them; even Albert Haller after 1760 was an adherent of the theory of evolution. However confused Wolff's general train of thought may be, we should notice his perception of the fact, that the theory of evolution does away with the sexual significance of the anthers. We shall see by-and-bye, that Koelreuter was able to form a very different idea of sexual propagation. His great importance in the history of the sexual theory will be best learnt from a consideration of the speculative views of his predecessors and contemporaries. It will not be amiss therefore to disregard chronology for a while, and to notice here the views of the Baron von Gleichen-Russworm, and the feeble arguments of Kaspar Friedrich Wolff against the theory of evolution. The first-named writer in his work 'Das Neueste aus dem Reich der Pflanzen,' 1764, relying principally on microscopic observation of the contents of pollen-grains, supported the view that the granules in them answer to spermatozoids in animals, and that they find their way into the ovule and are there developed into embryos. Yet Gleichen was at the same time a zealous supporter of the sexual theory, and endeavoured to meet well-known objections to it by pointing to the occurrence of female flowers on male plants of spinach; he also made some experiments on maize and hemp in the interests of the theory. He did not perceive that hybrids supply convincing proof against the theory of evolution, but he rightly appealed to them as affording strong arguments in favour of sexuality. His real knowledge of hybrids is partly drawn from the statements of Linnaeus, with which we have already made acquaintance; he even describes a hybrid between a goat and a cow, and other similar ones, and he is angry with Koelreuter for fixing such narrow limits to the occurrence of hybrids; thus the first person who produced hybrids systematically in the vegetable kingdom must submit to be scolded for refusing to accept the imaginary hybrids of his contemporaries. Gleichen's book and the selection from his microscopic discoveries, which appeared in 1777, abound in good detached observations; he was the first who saw and figured the pollen-tubes of Asclepias, without of course suspecting their real nature and importance.

Kaspar Friedrich Wolff is usually said to be the writer who refuted the theory of evolution. It is certainly true that in his dissertation for his doctor's degree in 1759, the well-known 'Theoria generationis,' he appeared as the decided opponent of evolution; but the weight of his arguments was not great, and the hybridisation in plants which was discovered at about the same time by Koelreuter supplied much more convincing proof against every form of evolution. Wolff conceived of the act of fertilisation as simply another form of nutrition. Relying on the observation, which is only partly true, that starved plants are the first to bloom, he regarded the formation of flowers generally as the expression of feeble nutrition (vegetatio languescens). On the other hand the formation of fruit in the flower was due to the fact, that the pistil found more perfect nourishment in the pollen. In this Wolff was going back to an idea which had received some support from Aristotle, and is the most barren that can be imagined, for it appears to be utterly incapable of giving any explanation of the phenomena connected with sexuality, and especially of accounting for the results of hybridisation. Wolff may have rejected the theory of evolution on such grounds as these, but he failed to perceive what it is which is essential and peculiar in the sexual act.

5. Further Development of the Sexual Theory by Joseph Gottlieb Koelreuter, and Konrad Sprengel. 1761-1793.

Camerarius had shown by experiment that the co-operation of the pollen is indispensable to the production in plants of seeds containing an embryo, and later observers had confirmed the fact of sexuality by further and varied experiments. The next step in the strict scientific investigation of the matter was to determine by the same method of experiment the share of each principle, the male and the female, in the formation of the new plant which resulted from the sexual act. When pollen and ovule belong to the same individual plant, the offspring assumes the same form and the question remains undecided. It was necessary to bring together the pollen and ovule of different plants; this must show whether some characters are derived to the offspring from the pollen, and others from the ovule, and what the characters are which are thus distinguished, supposing of course that such a union of different forms is possible. The answer to these questions could only be obtained by experiment, that is by artificial hybridisation; for until hybrid forms had actually been produced in this manner, it must be quite unsafe to assume that certain wild plants owed their origin to cross-fertilisation.

Camerarius had already raised the question in his letter, whether cross-fertilisation in plants is possible, and had added another, whether the progeny varies from its parents (an et quam mutatus inde prodeat foetus). Bradley is our authority for the statement that a gardener in London had obtained a hybrid between Dianthus caryophyllus and Dianthus barbatus by artificial means as early as 1719; but Koelreuter^[16] was the first who investigated the question scientifically and thoroughly. He was the first moreover who recognised all its importance, and he applied himself to it with such admirable and unexampled perseverance and judgment, that the results which he obtained are still the best and most instructive, though a thousand similar experiments have been made since his time. He also made the first careful study of the different arrangements inside the flower in their connection with the sexual relation, discovered the purpose of the nectar and the co-operation of insects in pollination, and proposed that view of the sexual act, which with some considerable modification we must still in the main consider to be the true one, namely, that it is a mingling together of two different substances.

If we compare Koelreuter's writings, which are full of matter in a small compass, with all that was produced after Camerarius, we are astonished not only at the abundance of new thoughts, but still more at their wonderful clearness and perspicuity, and the sureness of the foundation laid for them in observation and experiment. In reading the observations of Linnaeus, Gleichen, and Wolff on the sexual theory we step into a world of thought which has long been strange and is scarcely intelligible to us, and which in the present day possesses only a historical interest. Koelreuter's works on the contrary seem to belong to our own time; they contain the best knowledge which we possess on the question of sexuality, and have not become antiquated after the lapse of more than a hundred years. We see by his example that one really gifted thinker with the requisite perseverance will effect more in a few years, than many less gifted observers in the course of many years. But the same thing happened now, which happens often in similar cases and which happened to Camerarius; a much longer time elapsed before others learnt to understand the meaning and importance of Koelreuter's labours, than he had found necessary for making his discoveries.

Koelreuter's most important and best-known work appeared in four portions in 1761, 1763, 1764 and 1766 under the title, 'Vorlaufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen und Beobachtungen'; we shall endeavour to give a brief summary of the more important results.

At different places in this work occur remarks and experi- ments on arrangements for pollination, which up to that time had been seldom and only hastily observed. As the pollen-tube had not yet been discovered, and Koelreuter himself set out with the view, that a fluid finds its way from the pollen-grains as they lie on the stigma to the ovules, it was important first of all to determine the quantity of pollen which is required for the complete fertilisation of an ovary; with this object in view Koelreuter counted the pollen-grains formed in a particular flower and compared them with the number required to be applied to the stigma in order to effect complete fertilisation, and he found that the latter number was much the smaller. For instance, he counted four thousand eight hundred and sixty-three pollen-grains in a flower of Hibiscus venetianus, while from fifty to sixty were sufficient to produce more than thirty fertile seeds in the ovary; in Mirabilis jalapa and Mirabilis longiflora he counted about three hundred grains of pollen in the anthers, while from two to three or even one sufficed for fertilisation in the one-ovuled ovary. He also tried, whether in flowers with divided and even deeply-cleft styles fertilisation could be effected in all compartments of the ovary through one of them only, and he found that it could.

Koelreuter directed special attention to the arrangements,

by which in the natural course of things the pollen finds its way from the anthers to the stigmas. He ascribed perhaps too much to the agency of the wind and the oscillations of the flower from any cause; at the same time he was the first who recognised the great importance of the insect-world to pollination in flowers. 'In flowers,' he says, 'in which pollination is not produced by immediate contact in the ordinary way, insects are as a rule the agents employed to effect it,' (later observation has shown that they are generally so employed even in cases where actual contact is possible), 'and consequently to bring about fertilisation also; and it is probable that they render this important service if not to the majority of plants at least to a very large part of them, for all the flowers of which we are speaking have something in them which is agreeable to insects, and it is not easy to find one such flower, which has not a number of these creatures busy about it.' He noticed the dichogamous construction in Epilobium, but did not further pursue his observation. He next examined the substance in flowers which is agreeable to insects; he collected the nectar of many flowers in considerable quantities, and found that it gave after evaporation of the water a kind of sweet-tasted honey; this honey was unpalatable only in Fritillaria imperialis, which is avoided by the humble-bees. He had no doubt therefore, that bees procure their honey from the nectar of flowers. How greatly he was interested in the relations between the existence of certain plants and that of certain animals, relations which were neglected till Darwin once more brought them into notice in quite recent times, is shown by his investigation into the propagation of the mistletoe (1763); he calls special attention to the fact, that not only must the pollination of this plant be effected by insects, but that the dissemination of its seeds is also exclusively the work of birds, and that the existence of the plant therefore is dependent on two different classes of living creatures.

Again we find observations on the movements of anthers and stigmas, especially those caused by sensitiveness. Count Giambattista dal Covolo had made the first observations in 1764 on the sensitiveness of the anthers of thistle-like plants, and had endeavoured to explain their mechanism. Koelreuter did not trouble himself about this point, so much as about the connection between the irritability of the anthers and the pollination of the stigmas. He took into consideration the sensitive stamens of Opuntia, Berberis and Cistus, which Du Hamel had already noticed, and discovered for himself the irritability of the lobes of the stigma in Martynia proboscidea and Bignonia radicans. He noticed that the lobes when touched close, but soon open again; but if pollen is placed upon them, they remain closed till fertilisation is secured.

How perfectly insects effect the pollination of flowers he showed by a comparative trial, in which he applied pollen himself to three hundred and ten flowers with a brush, while he left the same number to the operation of insects; the number of seeds formed in the latter case was very little less than in the former, though the insects had to contend with unfavourable weather.

He endeavoured also to ascertain the time required for the quantity of 'seminal matter' sufficient for fertilisation to reach the ovary after pollination; he also showed that pollination is followed by fertilisation without the aid of light; later botanists incorrectly maintained the contrary.

Koelreuter was less successful in his observations on the structure of pollen-grains ; here the microscope was indispensable and microscopes were still very imperfect. Nevertheless he discovered that the outer covering of the pollen-grain consists of two distinct coats, and noticed the spines and sculpturings on the outer coat and its elasticity; he observed the lids of the orifices in the exine of Passiflora coerulea, and went so far as to see the inner coat in moistened pollen-grains protrude in the form of conical projections, which then however burst and allowed the contents to escape. But he explained the pollen-tube, which he had thus seen, incorrectly by supposing that these projections were intended to prevent the bursting of moistened grains. It was not till sixty or seventy years later that the matter was fully understood. Koelreuter supposed the contents of the pollen-grain to be a 'cellular tissue,' and the true fertilising substance to be the oil which adheres to the outside of the grains, but is formed inside them and finds its way out through fine passages in the coat. The bursting of the pollen-grains, which his opponent Gleichen thought must take place to allow of the escape of his supposed spermatozoids, seemed to him an unnatural proceeding.

Starting from the hypothesis, that the oil which clings to the pollen-grains is the fertilising substance, Koelreuter propounds his view of the process of fertilisation in accordance with the chemical notions of the day; he first rejects the idea that the pollen-grains themselves can reach the ovary, and then says: 'Both the male seed and the female moisture on the stigmas are of an oily nature, and therefore when they come together enter into a most intimate union with one another, and form a substance which, if fertilisation is to ensue, must be absorbed by the stigma and conveyed through the style to the so-called ovules or unfertilised germs.' Koelreuter therefore made the fertilisation really take place on the stigma, the mingled male and female substance making its way into the ovary and there producing the embryos in the seed. He had expressed this view before in 1761; he repeated it in 1763 with the idea that the male and female moistures unite together, as an acid and an alkali unite to form a neutral salt; a new living organism is the result at once or later of this union. In an investigation which he made in 1775 into the conditions of pollination in Asclepiadeae he reverted to this idea, and insisted that the act of fertilisation in the whole vegetable and animal kingdom is a mingling of two fluids. But at a later period he seems to have no longer considered the moisture of the stigma to be the female principle, for experiment had taught him, that if a stigma exchanges the moisture from another stigma for its own, and is then dusted with its own pollen, hybrid form is produced^[17]. In any case Koelreuter had a more correct idea of the nature of sexual fertilisation than any of his predecessors, and it was one specially adapted to enable his contemporaries to understand the results of experiments in hybridisation, while the hybrids themselves supplied most convincing arguments against the prevailing theory of evolution.

We have arrived at Koelreuter's most important performance, the production of hybrids. Here was a case for skilful experimentation, not for microscopic observation, and here he obtained results in which nothing afterwards required to be changed, but which when combined with later observations have been used for the discovery of general laws in hybridisation. The first hybrid which he obtained by placing the pollen of Nicotiana paniculata on the stigmas of N. rustica, produced pollen that was impotent; but he soon after obtained hybrids from the two species which produced seeds capable of germination, and in 1763 he described a considerable number of hybrids in the genera Nicotiana, Kedmia, Dianthus, Matthiola, Hyoscyamus, and others. In the last portion of his great work (1766) he speaks of eighteen attempts to obtain hybrids with five native species of Verbascum, and submits Linnaeus' views on hybrid plants, which we have already described, to a withering criticism. He shows at the same time from experiment, that if the stigma of a plant receives its own pollen and pollen from another plant at the same time, the former only is effectual, and that this is one reason why hybrids which can be raised artificially are not found in nature. We must not attempt to give a detailed account of his famous hybrids of the third, fourth, and fifth degrees, nor of his experiments on other points, such as the reverting of hybrids to the original form by the repeated employment of its pollen; the value of these experiments for theoretical purposes was afterwards fully brought out by Nageli.

It is impossible to rate too highly the general speculative value of Koelreuter's artificial hybridisation. The mingling of the characters of the two parents was the best refutation of the theory of evolution, and supplied at the same time profound views of the true nature of the sexual union. It was shown by his numerous experiments that only nearly allied plants and not always these are capable of sexual union, which at once disposed of Linnaeus' vague ideas in the judgment of every capable person, though it was long before science candidly accepted Koelreuter's results. The plant-collectors of the Linnaean school as well as the true systematists at the end of the 18th century had little understanding for such labours as Koelreuter's, and incorrect ideas on hybrids and their power of maintaining themselves prevailed in spite of them in botanical literature. Hybrids were necessarily in- convenient to the believers in the constancy of species; they. disturbed the compactness of their system and would not fit in with the notion that every species represented an ' idea.'

Koelreuter's doctrines however did not always fall on un- fruitful soil ; two botanists at least were found in Germany who adopted them, Joseph Gärtner the author of the famous Carpology and father of Carl Friedrich Gärtner who at a later time spent twenty-five years in experimenting on fertilisation and hybridisation, and Konrad Sprengel who took up Koelreuter's discovery of the services rendered by insects and arrived at some new and very remarkable results.

Joseph Gärtner made no fresh observations on sexuality himself, but in the Introduction to his 'De fructibus et seminibus plantarum' (1788) he made use of Koelreuter's results for the purpose of distinguishing more clearly between different kinds of propagation, and strengthening his own attack on the theory of evolution. The germ-grains or spores of cryptogamic plants were at that time often regarded on insufficient grounds as true seeds; Gärtner distinguished them from seeds, because they are formed without fertilisation and yet are capable of germination, whereas ovules become seeds capable of germination only under the influence of the pollen. He distinctly denied the sexuality of the Cryptogams; it was not till fifty years later that strict scientific proof was substituted in this department of botany for vague conjecture, and it was more in the interest of true science in Gärtner's day to deny sexuality in the Cryptogams altogether, than to take the stomata in Ferns with Gleichen, or the indusium with Koelreuter, or the volva in Mushrooms for the male organs of fertilisation. Gärtner rightly appealed to Koelreuter's hybrids against the defenders of the theory of evolution; and to those who saw in the seed only another form of vegetative bud, he said, that the bud can produce a new plant without fertilisation but that the seed cannot. We have already given an account in the chapters on Systematic Botany of the services rendered by Gärtner to the knowledge of the seed in its immature and in its mature condition; as regards the process of fertilisation he adopted in the main Koelreuter's view, that it is the result of the union of a male and female fluid, from which the germ-corpuscle in the ovule is developed by a kind of crystallisation. Konrad Sprengel also fully committed himself to this view, and was thereby prevented from understanding the process of fertilisation in Asclepiadeae.

In Konrad Sprengel^[18] we encounter once more an observer of genius, like Camerarius and Koelreuter, who however surpassed them both in boldness of conception and was therefore even less understood by his contemporaries and successors., than they had been by theirs. The conclusions, to which his investigations led him, were so surprising, they suited so little with the dry systematism of the Linnaean school and with later views on the nature of plants, that they had become quite forgotten when Darwin brought them again before the world and showed their important bearing on the theory of descent. As Camerarius first proved that plants possess sexuality, and Koelreuter showed that plants of different species can unite sexually and produce fruitful hybrids, so now Sprengel showed that a certain form of hybridisation is common in the vegetable kingdom, namely the crossing of different flowers or different individuals of the same species. In his work, 'Das neu entdeckte Geheimniss der Natur in Bau und Befruchtung der Blumen,' Berlin, 1793, he says at page 43: 'Since very many flowers are dioecious, and probably at least as many hermaphrodite flowers are dichogamous, nature appears not to have intended that any flower should be fertilised by its own pollen.' This was however only one of his surprising conclusions; still more important perhaps was the view, that the construction and all the peculiar characters of a flower can only be understood from their relation to the insects that visit them and effect their pollination ; here was the first attempt to explain the origin of organic forms from definite relations to their environment. Since Darwin breathed new life into these ideas by the theory of selection, Sprengel has been recognised as one of its chief supports.

It is highly interesting to read, how this speculative mind by the study of structural relations in flowers, which were apparently trivial and open to the eyes of all men, first arrived at ideas which in the course of a few years were to lead to such far-reaching results. He says: 'In the summer of 1787 I was attentively examining the flowers of Geranium sylvaticum, and observed that the lower part of the petals was provided with slender rough hairs on the inside and on both edges. Convinced that the wise framer of nature has not produced a single hair without a definite purpose, I considered what end these hairs might be intended to serve. And it soon occurred to me, that on the supposition that the five drops of juice which are secreted by the same number of glands are intended for the food of certain insects, it is not unlikely that there is some provision for protecting this juice from being spoiled by rain, and that the hairs might have been placed where they are for this purpose. Since the flower is upright, and tolerably large, drops of rain must fall into it when it rains. But no drop of rain can reach one of the drops of juice and mix with it, because it is stopped by the hairs, which are over the juice-drops, just as a drop of sweat falling down a man's brow is stopped by the eye-brow and eye-lash, and hindered from running into the eye. An insect is not hindered by these hairs from getting at the drops of juice. I examined other flowers and found that several of them had something in their structure, which seemed exactly to serve this end. The longer I continued this investigation, the more I saw that flowers which contain this kind of juice are so contrived, that insects can easily reach it, but that the rain cannot spoil it; but I gathered from this that it is for the sake of the insects that these flowers secrete the juice, and that it is secured against rain that they may be able to enjoy it pure and unspoilt.' Next year, following out an idea suggested by the flowers of Myosotis palustris, he found that the position of spots of different colours on the corolla have some connection with the place where the juice is secreted, and with the same ready reasoning as before he came to the further conclusion: 'If the corolla has a particular colour in particular spots on account of the insects, it is for the sake of the insects that it is so coloured; and if the particular colour of a part of the corolla serves to show an insect which has lighted on the flower the direct path to the juice, the general colour of the corolla has been given to it, in order that insects flying about in search of their food may see the flowers that are provided with such a corolla from a long distance, and know them for receptacles of juice.'

He afterwards discovered that the stigmas of a species of Iris were absolutely unable to be fertilised in any other way than by insects, and further observation convinced him more and more, 'that many, perhaps all flowers, which have this juice, are fertilised by the insects which feed on it, and that consequently this feeding of insects is in respect of themselves an end, but in respect of the flowers only a means, but at the same time the sole means to a definite end, namely, their fertilisation; and that the whole structure of such flowers can be explained, if in examining them we keep in sight the following points, first, that flowers were intended to be fertilised by the agency of one or another kind of insects, or by several; secondly, that insects in seeking the juice of flowers, and for this purpose either alighting upon them in an indefinite manner, or in a definite manner either creeping into them or moving round upon them, were intended to sweep off the dust from the anthers with their usually hairy bodies or with some part of them, and convey it to the stigma, which is provided either with short and delicate hairs, or with a viscid moisture, that it may retain the pollen.'

In the summer of 1790 he detected dichogamy, which he first observed in Epilobium angustifolium. He found, 'that these hermaphrodite flowers are fertilised by the humble-bee and by other bees, and that the individual flower is not fertilised by its own pollen, but the older flowers by the pollen which the insects convey to them from the younger.' Having observed the same thing in Nigella arvensis, he afterwards found exactly the opposite arrangement in a species of Euphorbia, in which the stigmas can receive the pollen by the aid of insects only from older flowers.

He goes on to say that he grounds his theory of flowers on these his six chief discoveries made in the course of five years; he then gives his theory at length, first of all explaining the nature of juice-secreting glands (nectaries), and organs for receiving or covering the nectar, and the contrivances for enabling insects to find their way readily to the juice. He calls attention to Koelreuter's excellent observations on the fertilisation of nectar-bearing flowers by insects, and notices that no one has hitherto shown that the whole structure of such flowers has this object in view, and can be fully explained by it. He finds the chief proof of this important proposition in dichogamy.

'After the flower,' he says, 'has opened in dichogamous plants, the filaments have or assume either all at once or one after another a definite position, in which the anthers open and offer their pollen for fertilisation. But at this time the stigma is at some distance from the anthers and is still small and closed. Hence the pollen cannot be conveyed to the stigma either by mechanical means or by insects, for there is as yet properly no stigma. This condition of things continues a certain time. When that time is elapsed, the anthers have no longer any pollen, and changes take place in the filaments the result of which is that the anthers no longer occupy their former position. Meanwhile the pistil has so changed that the stigma is now exactly in the place where the anthers were before, and as it now opens, or expands the parts of which it is composed, it often fills about the same space as the anthers filled before. Now the spot, which was at first occupied by the ripe anthers and is now occupied by the ripe stigma, is so chosen in each flower, that the insect for which the flower is intended cannot reach the juice without touching with a por tion of its body the anthers in a young flower, and the stigma in an older; it thus brushes the pollen from the anthers and conveys it to the stigma, and so the pollen of the younger flower fertilises the older.' It has been already said, that Sprengel was also acquainted with the opposite form of dichogamy; and the result of his explanation of both kinds is the conclusion, that some flowers can only be fertilised by the aid of insects, and he adds that some cases are to be found, in which the arrangements in the flower are of such a nature as to involve the injury and even the death of the insect that gives its services. Further on he tells us, that all flowers, 'which are without a proper corolla and have no calyx of any importance in its place, are destitute of nectar, and are not fertilised by insects but by some mechanical means, as by the wind, which either blows the pollen from the anthers on to the stigmas, or shakes the plant or the flower and makes the pollen fall from the anthers on to the stigmas.' He observes, that such flowers always produce a light pollen and in large quantities, whereas the pollen of nectar-bearing plants is heavy. Then he shows how his principles explain all the physiological characters of flowers, position, size, colour, smell, form, time of flowering and the like.

Sprengel set out with the idea, that the nectar and certain arrangements in flowers are expressly intended for the service of insects; but his investigations led him ultimately to the conclusion, that insects themselves serve not only to effect the fertilisation of plants generally, but also in all ordinary cases to bring about the crossing of different flowers of the same plant or of different plants of the same species. There remained a question, which from Sprengel's strictly teleological point of view especially required an answer, what was the object of this crossing of flowers or individual plants? Sprangel was content, as we have seen, with simply stating the fact, and with saying, that nature apparently did not choose that any flower should be fertilised by its own pollen. Who would make it a reproach to the discoverer of such remarkable and widely-prevalent phenomena in nature, that he did not answer this question and give the final touches to the body of doctrine which he created, and which could only be developed by many experiments and the labour of long years? Neither his worldly circumstances nor the reception accorded to his work with all its genius were such as to encourage him to undertake the solution of this last and most difficult problem, even if he had been inclined to do so. Botanists were just at that time and for some time after preoccupied with views, which allowed such biological and physiological facts in vegetable life to lie neglected, nor were Sprengel's results at all favourable to the doctrine of the constancy of species ; from that point of view the wonderful relations between the organisation of flowers and that of insects must have seemed absurd and repulsive. In such cases it is the character of less-gifted natures, rather to deny the facts or to disregard them, than to sacrifice their own favourite views to them; this is one explanation of the neglect which Sprengel's book met with everywhere. Then notwithstanding the labours of a Camerarius and a Koelreuter there were many even at the beginning of our own century who still doubted the sexuality of plants. Even after Knight and William Herbert, with a right understanding of the question left open by Sprengel, had obtained experimental results which helped to answer it, the new doctrine did not make its way. The earlier simple-minded but consistent teleology had been succeeded by a rejection of all teleological explanations in the treatment of physiological questions, and this spirit conduced to make Sprengel's results seem inconvenient in proportion as they appeared to admit only of such explanation. With regard to phenomena of this kind botanists before 1860 were in a position, in which they were without the means of forming a judgment; they shrank from the teleological point of view and from believing with Konrad Sprengel, that every, even the least-obvious, arrangement in an organism was the direct work of a Creator; but they had nothing better to put in the place of this idea, and hence Sprengel's discoveries not being understood were neglected till Darwin recognised all their importance, and by opposing the theory of descent and selection to the principle of design was in a position not only to show that they had a scientific meaning, but also to employ them as powerful supports of the theory of selection. Then, too, it became possible rightly to appreciate the contributions of Knight, Herbert, and K. F. Gärtner to the further completion of Sprengel's doctrine, for their discoveries also were for a while neglected. A few years after the appearance of Sprengel's book, Andrew Knight^[19] relying on the results of experiments made for the purpose of comparing self-fertilisation and crossing in the genus Pisum, laid down the principle, that no plant fertilises itself through an unlimited number of generations; in 1837 Herbert summed up the results of his numerous experiments in fertilisation in the statement, that he was inclined to believe that he attained a better result, when he fertilised the flowers from which he wished to obtain seeds with pollen from another individual of the same variety or at least from another flower, than when he fertilised it with its own pollen; K. F. Gärtner came to the same conclusion after experiments in fertilising Passiflora, Lobelia, and species of Fuchsia in 1844. In these observations lay the first germ of the answer to the question left undecided by Sprengel, why most flowers are so constructed that fertilisation can only be fully effected by the crossing of different flowers or of different plants of the same species; the artificial crossings of this kind, which Knight, Herbert, and Gärtner compared with the self-fertilisation of single flowers, showed that crossing procures a more complete and vigorous impregnation than self-fertilisation. It was but a short step from this fact to the idea, that the arrangements in the flower discovered by Sprengel together with the aid of insects serve to secure the strongest and most numerous progeny possible. Darwin was the first who fixed his eye distinctly on this idea also, in order to employ it in his theory of selection, and sought support for it in a number of experiments made after 1857.

6. New opponents of Sexuality and their refutation by experiments. 1785-1849.

Those who have read the writings of Camerarius and Koelreuter carefully find it difficult to believe, that after their time doubts were still entertained not about the manner in which the processes of fertilisation are accomplished but about the actual existence of difference of sex in plants. And yet such doubts were expressed repeatedly during the succeeding sixty years in various quarters and with the greatest confidence, and this not in consequence of increased accuracy in experimental research or of contradictions that could be proved in the views of the founders of the sexual theory, but because a number of observers made unskilful experiments and obtained contradictory results, or made inaccurate observations on the plants on which they experimented, and generally had not the requisite experience and circumspection. Such were Spallanzani and later Bernhardi, Giron de Bouzareingue and Ramisch. Schelver, his pupil Henschel, and their adherents erred still more grossly and from a different cause ; they thought themselves justified by preconceived opinions and conclusions from the nature-philosophy in denying facts established by experiment. The destructive effects of the nature-philosophy on the powers of the understanding at the beginning of the 19th century was shown in the case of many botanists, who were no longer able to estimate the result of simple experiments, and to trace back the phenomena of nature to the scheme of causes and effects. As Linnaeus once imagined that he could prove sexuality in plants on philosophical grounds and paid comparatively slight attention to their behaviour as shown by experiment, so we have in Schelver a nature-philosopher who conversely endeavoured to prove the impossibility of sexuality in plants on philosophical grounds. As Linnaeus deduced sexuality from the nature or idea of the plant, Schelver denied it from the same nature or idea; as a matter of logic one was as much in the right as the other, but the question could not be decided in this way but only by experiment. However our nature-philosophers thought it advisable to get some empirical support for their theories, and they found it in Spallanzani^[20]. He published his enquiries into fertilisation in animals and plants under the title 'Experiences pour servir a l’histoire de la generation des animaux et des plantes,' Geneva, 1786; his account of those relating to plants, with which only we are concerned, betrays a very defective acquaintance with botanical literature, for he reckons Cesalpino among those who had admitted sexuality in plants. His experiments themselves testify to very slight knowledge of the biological considerations by which the cultivation of plants for experiment must be guided, and generally little botanical acumen, as is often the case with amateurs who without sufficient preparation suddenly turn their attention to questions of vegetable physiology; his treatment of his topics is superficial, his criticism of others is dogmatic and bitter without exciting confidence in the author's own skill and judgment. His experiments were often undertaken in haste and with little consideration, and some of them were made on plants the least suitable for such investigations, as for instance on Genista, beans, peas, radishes, Basilicum, Delphinium. It is no matter of surprise therefore that in the case of some plants, as Mercurialis and Basilicum, he arrived at the conclusion that the pollen is necessary to the production of fertile seeds, while he makes others, as the gourd, the water-melon, hemp, and spinach produce such seeds without fertilisation. His countryman Volta, a greater man, repeated his experiments and impugned the results which he had obtained from them.

Such was the character of the experiments to which Franz Joseph Schelver, Professor of Medicine in Heidelberg appealed in his 'Kritik der Lehre von dem Geschlecht der Pflanzen,' 1812. It is unnecessary to give a detailed account of this strange production of a mind misled, even though a consider- able number of German botanists as late as 1820 took its nonsense for profound wisdom. Schelver dismissed the experiments of Camerarius in four lines, and while he treated Koelreuter with contempt, he praised Spallanzani as the weightiest author on the subject. The statements of Camerarius and Koelreuter are true, he said, but they do not prove the fertilisation. He is more concerned to decide the question from the nature of vegetative life, and from this nature constructed by himself he concludes that the organs of plants are of no use at all, that they cannot even tend to be of use to one another and to propagate life together, because this one end of their action can be a living one only where all the parts are present at the same time, which of course disposes of the fertilising effect of the pollen ; accordingly he does not refer the effect of a male plant on a neighbouring female plant, which results in the formation of seeds, to pollination by the former, but it is the proximity itself which has the fertilising effect. But these are very insufficient specimens of his reasoning.

The writings of his pupil Henschel^[21] are even worse than those of his master, and the worst of these is his large work 'Von der Sexualitat dor Pflanzen' of 1820. He thought himself obliged to prove the doctrines of the nature-philosophy by countless experiments; but the way in which these are devised, managed and described displays the extreme of dulness and incapacity to form a sound judgment. The doubt which must occasionally rise in the mind of the reader as to the accuracy of his reports, and the remarks which have been made on this point by Treviranus and Gartner, are not needed to disgust him with the scientific efforts of this writer.

It would be superfluous to give an account of the contents of Henschel's book, which is interesting from the pathological rather than from the historical point of view. To what an extent better men than Henschel even later than 1820 lost under the influence of the nature-philosophy their capacity for judging such questions as we are discussing, how even investigators of merit thought it worth while to treat the productions of Schelver and Henschel with a certain respect, is shown among other works, by a collection of letters, which were published by Nees von Esenbeck as a second supplement to the 'Regensberg Flora' of 1821, and by the later remarks of Goethe on the metamorphosis of plants, to be found in Cotta's edition of his works in forty volumes (vol. xxxvi. p. 134) under the title 'Verstaubung, Verdunstung, Vertropfung.' But there were some who set themselves distinctly against these pernicious ideas, such as Paula Schrank ('Flora,' 1822, p. 49) and C. L. Treviranus, who published in 1822 a full refutation of Henschel in his 'Lehre von dem Geschlecht der Pflanzen in Bezug auf die neuesten Angriffe erwogen.' A few stray supporters of the dying nature-philosophy were still to be found at a later time; among them Wilbrand, Professor in Giessen, who ('Flora,' 1830, p. 585) adopted the very subtle distinction that there is in plants something analogous to sexuality in animals, but no real sexuality. We see in the whole literature of the nature philosophy an incapability of judging of experiments simply with the sound human understanding; an imaginary something was constantly introduced into the results of experiments which had not the remotest connection with their conditions and results.

The doubts expressed by Bernhardi in 1811, by Girou in 1828-30, and by Ramisch in 1837 were of a different kind: these men made experiments and judged of them in a scientific manner; but they were insufficiently acquainted with what had been done before them, and their experiments were not devised with the requisite knowledge of the conditions of the problem, or carried out with sufficient precautions. Camerarius and Ray had noticed in the previous century the occasional occurrence of male flowers on female plants of spinach, hemp and mercury; and yet the observers above mentioned chose these plants for their experiments without being on their guard against the possible appearance of these exceptional circumstances, or of other means of pollination.

We see then that doubts were entertained till as late as after 1830 with regard either to sexuality in plants altogether, or to its general prevalence in Phanerogams; the Cryptogams were not mentioned, for they were assumed to be devoid of sex in spite of many valuable observations of earlier times. The great majority of botanists however admitted the sexual significance of the organs of the flower; most of them rested in entire faith on Linnaeus' authority, while some were able to appreciate the experimental proofs of Camerarius, Bradley, Logan, Gleditsch and Koelreuter. But all who took up the subject in earnest between 1820 and 1840 were naturally led to desire that the question should once more be thoroughly examined. The Berlin Academy of science had offered in 1819 at Link's suggestion a prize for an essay on the question, whether there is such a thing as hybrid fertilisation in the vegetable kingdom, in the hope of stimulating botanists to new investigations into the decisive points in the sexual theory. The only reply to this offer, an essay by Wiegmann which was not sent in till 1828, did not come up to the requirements of the Academy, and was rewarded with only half the prize. The Dutch Academy at Haarlem was more successful when induced by Reinwardt in 1830 to propose the question in a somewhat altered form and in connection with practical horticulture. This prize was contended for by Karl Friedrich Gärtner^[22], whose essay delayed by circumstances till 1837 received the prize of honour and an extraordinary reward. But the whole body of his results, derived from the experimental researches of five-and-twenty years, were not published till 1849 and then in a large volume, 'Versuche und Beobachtungen über die Bastardzeugung,' Stuttgart, 1849, having been preceded by an introductory work of equal extent, 'Versuche und Beobachtungen über die Befruchtungsorgane der vollkommeneren Gewachse und über die natürliche und ktinstliche Befruchtung durch den eigenen Pollen.' The two works together are the most thorough and complete account of experimental investigation into sexual relations in plants which had yet been written. They are a brilliant termination of the period of doubt with respect to sexuality in plants which succeeded to the age of Koelreuter a termination which coincides in time with the lively discussion which was being maintained on the strength of microscopical investigations by Schleiden and Schacht on the one side and by Hofmeister on the other respecting the processes in the formation of the embryo.

Gartner's writings derive their importance not so much from new and surprising discoveries or brilliant ideas and unexpected combinations, as from their very searching examination into all the circumstances and relations which can come under consideration in the sexual propagation of Phanerogams. His experiments in hybridisation, of which he kept most exact accounts, exceeded the number of nine thousand ; in these and in normal cases of pollination he studied all the sources of error which could in any way affect his experiments, and took into careful consideration all the conditions of fertilisation connected with the development of the plant itself and with its external circumstances; at the same time he examined critically all that had been written on the subject, and submitted every experiment reported by former observers to the test of his own wide experience. The volume on self-fertilisation is a complete account of the biology and physiology of flowers. The phenomena connected with the unfolding and fertilisation of the flower are described from the writer's own observations, some of which are quite new; it specially investigates the relations between the calyx, the corolla, the secretion of nectar and the opening of the anthers, also the temperature of flowers and the physiological processes in the ovary, the style and the stigma; all that was then known of irritability and the phenomena of movement in the flower and in the organs of fructification was collected together and elucidated by fresh observations, and thus a picture was drawn complete to the smallest detail of the life of a flower, such as we do not yet possess of any other organ. It would be idle to think of giving in a small compass a clear idea of the wealth of these observations. But all this was only preliminary to the main point, the proof that Camerarius was right, that notwithstanding the objections of a hundred years the co-operation of the pollen is indispensable to the formation of the embryo in the growing seed, and that plants therefore have sexuality exactly as animals have it. Gärtner did not content himself with simply making new experiments in fertilisation; he refuted the objections of Spallanzani, Schelver, Henschel, Girou and others in detail from fresh experiments and from other sources of information, paying particular regard to all the circumstances which could come under consideration in each case ; he exposed the inaccuracy of the observations of the opponents of sexuality point by point, and finally called attention to a number of remarkable phenomena observable in the ovary even before fertilisation, and to the circumstances under which the pollen may find its way to it in cases where ordinary pollination has been apparently prevented. These observations once more confirmed the existence of sexuality in plants, and in such a manner that it could never be again disputed. When facts were observed in 1860, which led to the presumption that under certain circumstances in certain individuals of some species of plants the female organs might produce embryos capable of development without the help of the male, there was no thought of using these cases of parthenogenesis to disprove the existence of sexuality as the general rule ; men were concerned only to verify first of all the occurrence of the phenomena, and then to see how they were to be reasonably understood side by side with the existing sexuality, as had to be done also in the corresponding cases in the animal kingdom.

Gartner's work on hybridisation had been preceded by other enquiries into the same subject, those namely of Knight mentioned above at the beginning of the century, and Herbert's more ample investigations published in his work on Amaryllideae in 1837. Gärtner did not neglect to compare his observations at all points with the results of his predecessors, especially those of Koelreuter, and he deduced from the astonishing mass of material a number of general propositions respecting the conditions under which the production of hybrids is possible, the results of crossing, and the causes of failure. A special interest attaches to his mixed and compound hybrids, to his experiments on the various degrees of influence which foreign pollen exercises on the behaviour of the female organ, and the connection of this point with the formation of varieties. It is impossible to give a more distinct account of Gärtner's results without entering into discussions which would exceed the limits of a historical survey. It is the less necessary to do so, since Nägeli undertook in 1865 to give a summary view of all the important results to be found in the wealth of material supplied by Koelreuter, Herbert and Gärtner^[23]. Gärtner's experiments in hybridisation were conducted at Calw in Wiirtemberg, the place where Koelreuter had made his in 1762 and 1763. And thus it was in two small cities of Wiirtemberg that the foundations of the sexual theory were laid and the theory itself perfected, as far as it could be by experiment only, by three of the most eminent among observers. Camerarius in Tubingen, Koelreuter and K. F. Gärtner in Calw contributed so largely to the empirical establishment of the theory, that all that was done by others would seem of small importance, if artificial pollination only were in question. But Koelreuter was imperfectly acquainted with the methods by which pollination is usually effected in nature; Sprengel was the first who saw into all their more important relations, and the fact must not be concealed, that Gärtner in regarding Konrad Sprengel's observations as unworthy of serious consideration, neglected the most fruitful source of new and magnificent results. His careful study of the secreting of nectar and of the sensitiveness of the organs of fertilisation, and his many observations on other biological relations in flowers, would have found their natural termination, if he had connected them at all points with Sprengel's general conclusions respecting the relation of the structure of the flower to the insect world. This Gärtner entirely failed to do, and hence in this case also it was reserved for Darwin's wonderful talent for combination to sum up the product of the investigations of a hundred years, and to blend Koelreuter's, Knight's, Herbert's, and Gärtner's results with Sprengel's theory of flowers into a living whole in such a manner, that now all the physiological arrangements in the flower have become intelligible both in their relations to fertilisation, and in their dependence on the natural conditions under which pollination takes place without the aid of man. Here, as in morphology and systematic botany, Darwin found the premisses given and drew the conclusion from them; here too the certainty of his theory rests on the results of the best observers, on investigations which find in that theory their necessary logical and historical consummation.

7. Microscopic investigation into the processes of fertilisation in the phanerogams; pollen-tube and egg-cells^[24]. 1830-1850.

Those who were convinced of the sexuality of plants had endeavoured as early as the previous century to form some idea with the help of the microscope of the way in which the pollen effects the formation of the embryo in the ovule. We may pass over Morland's and Geoffrey's very rude attempts in this direction; Needham (1750), Jussieu, Linnaeus, Gleichcn, and Hedwig imagined that the pollen-grain bursts upon the stigma, and that the granules it contains make their way down wards through the style to the ovules, and are there either hatched into embryos or assist in their production. This way of conceiving the matter was closely connected with the theory of evolution which then prevailed, and seemed to find some countenance in the seed-corpuscles of animals; it was also supported by the observation that pollen-grains placed under the microscope in water often burst and discharge their con- tents in the form of a granular mucilage. It has been already mentioned that Koelreuter rejected this view; he declared the bursting of the pollen-grains to be contrary to nature, and con- sidered the oil which exudes from the grains to be the fertilising substance. This view was adopted by Joseph Gärtner and Sprengel, but it fell into disesteem, while that of Needham and Gleichen commanded some assent some years longer. The next question was, how the granular contents of the pollen- grain reach the ovules. Accident supplied a starting-point for further consideration. Amici, who was examining the hairs on the stigma of Portulaca for another purpose, saw on that occasion (1823) the pollen-tube emerge from the pollen-grain, and the granular contents of the latter, commonly known as the fovilla, execute streaming movements like the well-known movement in Chara. The desire to verify this remarkable fact, and to discover how the fertilising substance is absorbed by the stigma, led Brongniart in 1826 to examine a great number of pollinated stigmas. He succeeded in establishing the fact that the formation of pollen-tubes is a very frequent occurrence. The want of perseverance in following out his observation and a prepossession in favour of Needham's old theory prevented him from discovering the course of the pollen-tubes all the way to the ovules ; he supposed, indeed, that after penetrating into the stigma they open and discharge their granular contents, and he maintained distinctly that these are analogous to the spermatozoids in animals, and are the active part of the pollen. But now Amici addressed himself more earnestly to the question, and in 1830 he not only followed the pollen-tubes into the ovary, but also observed that one finds its way into the micropyle of each ovule.

Thus the question was suddenly brought near to its solution, when observers began to wander from the right path in different directions. Robert Brown showed in 1831 and 1833 that the grains in the pollen-masses of Orchids and Asclepiads put forth pollen-tubes as in other plants, and that fine tubes are found in the ovary of Orchids in which pollination has taken place; but he was in doubt about the connection of these tubes with the pollen-grains, and rather inclined to think that they were formed in the ovary, though possibly in consequence of the pollination of the stigma. Schleiden went wrong in a very different way, and by so doing made the question as prominent in botanical research, as was that of the origin of cells at this time. He published in 1837 some excellent investigations into the origin and development of the ovule before fertilisation, certainly the best and most thorough of the day. He at the same time showed that Brongniart's and Brown's doubts were unfounded, and confirmed the statement of Amici, that the pollen-tubes make their way from the stigma to the ovule, which they enter through the micropyle. But he made them push forward a little too far, for he asserted positively that 'the pollen-tube pushes the membrane of the embryo-sac before it, making an indentation, and its extremity then appears to lie in the embryo-sac. The extremity of the tube now swells out into a round or oval shape, and cell-tissue forms from its contents; the lateral organs, one or two cotyledons, are then produced, the original apical point remaining more or less free and forming the plumule. The portion of the tube underneath the embryo and the fold of the embryo-sac which envelopes it are divided off sooner or later and dis- appear, so that the embryo now really lies in the embryo-sac.' This view, which appears to rest on direct observation and is illustrated by figures which answer to the description, corresponds with the old theory of evolution and has a striking approximation to the ideas of Morland and Geoffrey; and if it were correct, it would like these imply the necessity of pollination to the formation of seeds that should contain embryos, but at the same time it would do away with that which is the essential point in the sexuality of plants, for the ovule would merely be the spot adapted to the hatching of the embryo formed from the pollen. Schleiden's idea was at once adopted by Wydler, Gelesnow and various other botanists, and especially by Schacht, but the most eminent microscopists withheld their assent. Amici was the first who openly opposed the new doctrine; before the Italian congress of savants at Padua in 1842 he endeavoured to prove that the embryo is not formed at the end of the pollen-tube, but from a portion of the ovule which was already in existence before fertilisation, and that this part is fertilised by, the fluid contained in the pollen-tube. But the choice of a gourd, a plant eminently unsuitable for his purpose, prevented his discovering the exact details of the process, and Schleiden did not hesitate to denounce his assertions in 1845 in the plainest terms. But in the next year (1846) Amici produced decisive proof for the views which he had maintained; he showed from the Orchidaceae, which were peculiarly well adapted for such investigations, not only that Robert Brown's doubts above mentioned were without foundation, but, which is the main point, that a body, the egg-cell, is present in the embryo-sac of the ovule before the arrival of the pollen-tube, and that this body is excited by the presence of the pollen-tube to further development, the formation of the embryo. He gave a connected account on this occasion for the first time of the whole course of these processes from the pollination of the stigma to the perfecting of the embryo.

The correctness of the account given by Amici was confirmed in the following year by von Mohl and Hofmeister, the latter of whom described in detail the points which were decisive of the question from a variety of plants, and illustrated them by very beautiful figures in a more copious work, 'Die Enstehung des Embryo der Phanerogamen,' Leipzig, 1849. Tulasne also came forward in opposition to Schleiden's theory, being thoroughly convinced that there was no actual contact of the pollen-tube with the egg-cell, denying indeed the existence of the egg-cell before fertilisation. Thus a vehement controversy arose on the subject; a prize offered by the Institute of the Netherlands at Amsterdam was awarded to an essay of Schacht's in 1850, which defended Schleiden's theory, and illustrated it by a great number of drawings giving incorrect and indeed inconceivable representations of the decisive points. Von Mohl says very admirably on this occasion (' Botanische Zeitung,' 1863, Beilage, p. 7): 'Now that we know that Schleiden's doctrine was an illusion, it is instructive, but at the same time sad, to see how ready men were to accept the false for the true; some renouncing all observation of their own dressed up the phantom in theoretical principles; others with the microscope in hand, but led astray by their preconceptions, believed that they saw what they could not have seen, and endeavoured to exhibit the correctness of Schleiden's notions as raised above all doubt by the aid of hundreds of figures, which had every thing but truth to recommend them; and how an academy by rewarding such a work gave fresh confirmation to an experience which has been repeatedly made good especially in our own subject during many years past, namely that prize-essays are little adapted to contribute to the solution of a doubtful question in science.' In this case the prize-essay had been refuted before it appeared by von Mohl, Hofmeister and Tulasne. Schacht adhered all the more firmly to Schleiden's theory; after further controversy, in which other writers of less authority took part, Radlkofer published in 1856 a complete review of the question, which fully confirmed Hofmeister's observations, and gave incidentally an account of Schleiden's views in the altered form which they had by that time assumed; this account showed in fact that Schleiden had completely retracted his former opinions, and in this retracta tion Schacht was soon after compelled to follow him, having become acquainted with facts observed in the ovule of Gladiolus, which were obviously irreconcilable with Schleiden's theory.

Hofmeister had from the first directed special attention to the questions, whether any bodies are found in the pollen-tube which answer in any way to spermatozoids, and whether any opening can be perceived at the end of the tube. He found indeed forms in Coniferae in 1851, which reminded him of the male organs of fertilisation in the higher Cryptogams; but the pollen-tube was closed both in them and in the rest of the Phanerogams, in which moreover its outer coat attains to a considerable thickness. There remained therefore only the hypothesis, that a fluid substance passes through the walls of the pollen-tube and of the embryo-sac and effects the fertilisation of the egg-cell; thus it was not the theory of preformation of the last century, to which Brongniart still adhered, but the view represented by Koelreuter, which ultimately proved to be nearer the truth, though it may be said that all that remained of that view was, that the fertilising substance in the Phanerogams is a fluid. The granular contents of the pollen-grains, which were supposed to be spermatozoids, have since been partly found to be only innocent starch-grains and drops of oil.

8. Discovery of Sexuality in the Cryptogams. 1837-1860.

By the year 1845 no one capable of forming a judgment on the question any longer doubted the existence of different sexes in Phanerogams. But it was not so with the Cryptogams, though a number of facts were acknowledged at this time which seemed to point to the conclusion, that a moment arrives sooner or later in the course of their development also, when a sexual act is accomplished. But the question had not as yet been systematically studied; no experimental investigations had been made, or observations of such a kind as to demonstrate the necessity of sexual union.

The great majority of botanists in the second half of the 18th century had no longer any doubt that the stamens were organs of reproduction, and they were anxious to prove the existence of similar organs in the Cryptogams; they rested in this matter on external resemblances and analogies, which they interpreted in a more or less arbitrary manner. The obvious external resemblance between the antheridia and archegonia in Mosses and the sexual organs in the Phanerogams led Schmidel and Hedwig to consider them to be stamens and ovaries, and the conjecture was correct, though the true nature of the moss-fruit had to be learnt in another way. Micheli, Linnaeus and Dillen, trusting still more to external appearance and with slight knowledge of these plants, had before this taken the fruit for a male flower, and in the case of the rest of the Cryptogams the best botanists were only feeling their way in the dark with no certain experience to guide them. It is not necessary to give a particular account of the views which originated in this way; one or two may be mentioned by way of example. Koelreuter regarded the volva of Mushrooms, Gleditsch and Hedwig certain tube-like cells in their lamellae, as the male organs of fertilisation. Gleichen took the stomata, Koelreuter the indtisium, Hedwig even the glandular hairs of Ferns for anthers. It was not yet suspected that the course of development and the whole morphology of the Cryptogams could not be so compared with that of the Phanerogams; correct and incorrect assumptions with regard to the sexual organs of the Cryptogams were alike devoid of scientific value, being mere guesses and vague conjectures. Nor was the state of things much better even in the first years of the 19th century; and if by that time a number of occasional observations had been made which could afterwards be turned to scientific account, these were as yet only isolated facts without scientific connection, and every one was at liberty to concede or to refuse sexual organs to the Cryptogams generally at his own discretion. Meanwhile observations gradually accumu lated, and towards 1845 it began to be possible by critical examination of them to arrive at something like a clearer understanding of this part of botany. The majority of botanists readily accepted Schmidel's and Hedwig's opinion with respect to the Mosses; Vaucher had as early as 1803 maintained that the long-known conjugation of Spirogyra was a sexual act; Ehrenberg observed in 1820 the conjugation of a Mould, Syzygites; Bischoff and Mirbel explained the organisation of the antheridia of the Liverworts in 1845, while Nees von Esenbeck saw the spermatozoids of Sphagnum in 1822 and Bischoff those of Chara in 1828, though they were at first taken for Infusoria, an opinion maintained by Unger as late as 1834. But it was Unger^[25], who in 1837, after careful study of the spermatozoids of the Mosses in 1837, declared them to be the male organs of fertilisation; in 1844 Nägeli discovered corresponding forms on the prothallium of Ferns, which had till then been called a cotyledon, and in 1846 the spermatozoids of Pilularia, the products of the small spores which Schleiden had explained to be the pollen-grains of that plant.

These facts were of the highest importance, but little was to be made of them as long as the female organ in the plants in question, the Mosses excepted, was unknown, and meanwhile it was only the resemblance between vegetable and animal spermatozoids which led to the conjecture, that the one had the same sexual significance as the other.

Light was suddenly thrown upon the subject, when Count Lesczyc-Suminsky discovered in 1848 on the supposed cotyledon (prothallium) of Ferns both the antheridia and the peculiar organs, inside which the embryo or young fern is formed. Though the statements respecting the structure and development of these female organs and of the embryo were inaccurate in some important points, yet the place was now indicated where it might be presumed that the fertilisation by the spermatozoids takes place ; and as the history of the germination of the rest of the vascular Cryptogams was to some extent known through the earlier labours of Vaucher and Bischoff, the organs of fructification of these plants might now be sought, where they are really to be found. But an erroneous idea respecting the meaning of the small spores of the Rhizocarps propounded by Schleiden had first to be put out of the way, and this was done by an appeal to the discovery of Nägeli mentioned above and by the investigations of Mettenius. Then in 1849 Hofmeister supplied a connected description of the germination of Pilularia and Salvinia, in which the decisive points as regards the sexual act were clearly set forth, and the connection of the spermatozoids with the fertilisation of the egg-cells in the archegonium was established. He did the same for Selaginella, which is very unlike the Rhizocarps and Ferns, and in which the spermatozoids are developed from smaller spores, and fertilise the egg-cells in archegonia formed in the prothallium of the large spores. By comparing the processes of germination in these plants with those of Ferns and Mosses, he succeeded in throwing entirely new light on the whole of the morphology of these classes of plants, and thus made it possible for the first time to compare them with one another and with the Phanerogams, and to form a right estimate of the sexual act in the Muscineae and Vascular Cryptogams in its relation to the history of the development of these plants. Hofmeister arrived at the following conclusion from his observations in 1849: 'The prothallium in the vascular Cryptogams is the morphological equivalent of the leaf-bearing Moss-plant, while the leafy plant of a Fern, of a Lycopodium and a Rhizocarp answers to the capsule of the Moss. In Mosses as in Ferns there is an interruption of the vegetative development by sexual procreation, an alternation of generations ; this takes place in the Vascular Cryptogams very soon after germination, in the Mosses much later.' The vast importance of this dis covery to systematic botany has been already noticed. The conception of these relations developed by Hofmeister was not less important to the doctrine of the sexuality of plants; it swept away at one stroke all the old false analogies between Phanerogams and Cryptogams and brought to light the real agreement; Hofmeister had detected in the archegonium of the Cryptogams the body which is developed there, as in the ovule of the Phanerogams, into an embryo after fertilisation, namely the germinal vesicle or egg-cell. Here was the point of departure for all further systematic comparison in the sexual propagation of Cryptogams and Phanerogams. All beside was of secondary importance, even the fact, that the fertilisation of the egg-cell in the Cryptogams is not effected by a pollen-tube, but by spermatozoids. It was now easy to show the corresponding relations of generation in the other cases which Hofmeister had not yet observed.

Hofmeister's statements and conclusions respecting Selaginella and Isoetes were confirmed and some additions made to them by Mettenius in 1850, and in 1851 appeared Hofmeister's exhaustive work 'Vergleichende Untersuchungen,’ in which the mode of production of the embryo in Coniferae was represented as an intermediate form between those of Phanerogams and Cryptogams. Further contributions were made to the knowledge of the subject; Henfrey confirmed Hofmeister's results in the case of Ferns; Hofmeister himself and Milde observed in 1852 the history of fertilisation in Equisetaceae, and the former supplied at the same time a more complete account of the development of Isoetes; in 1855 he described the decisive points in Botrychium and Mettenius in 1856 those in Ophioglossum.

The processes of development before and after fertilisation were now cleared up by all these discoveries, but the direct observation of the act of fertilisation was still wanting. Hofmeister ('Flora,' 1857, p. 122) describes the state of affairs in the following terms: 'While numerous investigations had thrown a clear light on the character of the male and female organs, and on the way in which the embryo is formed by repeated division of the egg-cell present before fertilisation, we continued quite in the dark respecting the particular nature of the fertilisation. Observation and experiment had established the fact, that the influence of the spermatozoids on the archegonia was required to produce an embryo in the latter. Female moss-plants^[26] separated from the male, macrospores in the Vascular Cryptogams separated from the microspores, had in all cases proved unproductive; but it was not even certainly known to what point in the female organ the spermatozoids force their way. It is true that Lesczyc and after him Mercklin had seen the entry of moving spermatozoids into the mouth of archegonia in Ferns; but Lesczyc's account of the part which he supposed them to play there afterwards, was proved to be an illusion. I had myself observed motionless spermatozoids halfway down the neck of archegonia of an Equisetum; but nothing was to be learnt of the manner in which the spermatozoid affects the egg-cell. Then it happened that in the spring of 1851, being engaged in observing the development of the organs of vegetation of Ferns, I repeatedly saw spermatozoids moving about in the basilar cells which enclose the egg-cell in the archegonia of Ferns, and the majority of them even playing about the egg-cell. Their movements were put an end to during the observation by the commencement of changes, which the contents of young vegetable cells which have been cut open usually experience under the prolonged influence of water.' Later observations leave no doubt now that in the Muscineae and Ferns single spermatozoids force their way into the naked egg-cell of the archegonium.

The question was first set at rest in the Algae, where the process of fertilisation could be seen directly and without exposing the objects to destructive influences. That sexual propagation occurs in the Algae also had seemed probable, since Decaisne and Thuret in 1845 discovered organs in species of Fucus, and Nägeli in 1846 in Florideae, which scarcely admitted of any other explanation. Alexander Braun also had called attention to the formation of two kinds of spores in a large number of freshwater Algae. But as yet there was only conjecture. Then Thuret proved by experiment in 1854, that in the genus Fucus the large egg-cells must be fertilised by very small swarming spermatozoids, in order to set up germination; both organs can be collected separately and in numbers in this genus, and be brought together at pleasure; Thuret even succeeded in obtaining hybrids. Pringsheim first observed in 1855 the formation of spermatozoids in the little horns of Vaucheria and established the fact that spores capable of germination are not formed unless the spermatozoids approach the egg-cell. To Thuret's statements he added the very important one, that the remains of spermatozoids may be recognised on the surface of the contents of the fertilised egg-cell of Fucus, which is already surrounded by a membrane. About the same time Cohn published his observations on Sphaeroplea annulina, which confirmed the fact of the approach of the spermatozoids to the egg-cells, which consequently, as in Fucus and Vaucheria, form a cell-wall and are rendered capable of further development.

Still the decisive observation had not yet been made; no one had yet seen how the two fertilising elements behaved at the moment of fertilisation. Pringsheim had the good fortune to make this observation in one of the commonest of fresh water Algae, Oedogonium. There he saw the moving spermatozoid first come into contact with the protoplasmatic substance of the egg-cell, and then force its way into it, blend with it and dissolve. And thus the first observation was made, which proved decisively that a real intermixture takes place of the male and female elements of fertilisation; this important fact was confirmed by De Bary in the same year.

Now that it was once established, that fertilisation in Cryptogams consists in the blending together of two naked bodies of protoplasm, the spermatozoid and the egg-cell, it was reasonable to conclude that conjugation in Spirogyra and generally in Conjugatae, was an act of fertilisation, only in this case the two fertilisation-elements are not of different size and shape, but similar in appearance. To this conclusion De Bary arrived in 1858 in his monograph of the Conjugatae. This extension of the idea of fertilisation to cases in which the uniting cells are to outward appearance alike, was of special value to the theory of sexuality, as was seen in the sequel, when other forms of fertilisation were observed which made it necessary still further to extend the idea of sexuality. In 1858 Pringsheim discovered arrangements for fertilisation in another group of Algae, the Saprolegnieae, which to outward appearance at least departed widely from those hitherto known in the lower plants.

Thus between the years 1850 and 1860 a number of fundamental facts were discovered, and were afterwards confirmed and extended by fresh observations in the course of the following years. It does not fall within the limits of this work to notice the many discoveries that were made in this part of botanical science after 1860; we will only remark, that between 1860 and 1870 the processes of fructification were observed by Thuret and Bornet in Florideae, and especially by De Bary and his pupils in Fungi, in some of which very peculiar forms were brought to light. No doubt any longer exists that difference of sex prevails generally in the Thallophytes also, though it is still an open question, whether it may not be wanting in some of the very simplest and smallest kinds.

One of the most important results of these investigations is obviously the striking resemblance between many of the processes of fertilisation in Cryptogams and in the lower animals; here is another confirmation of the fact, often brought out in other ways by modern zoological and botanical research, that the points of resemblance in the vegetable and animal kingdoms appear most plainly, if we compare together the simplest forms to be found in both; we have in this fact a plain proof also, that both kingdoms have been developed from like common elements, as the theory of descent implies. With respect to the true nature of fertilisation itself, which is evidently a similar process in the main in animals and plants, we can only say at present, that it amounts in all cases to a material blending together of the contents of two cells, neither of which is capable of further development by itself, while the product of the combination is not only capable of such development, but unites in itself the characteristics of the two parent forms and transmits them to its descendants. That fertilisation is not the intimate union of two bodies possessing a definite form, but that the male fertilising substance at least may be a simple fluid, appears to be distinctly shown by the process in Phanerogams; and we may assume, that in Cryptogams also, the sexual act is not affected by the form of the fertilisation-elements, though a certain shape and power of movement is necessary for the conveyance of the fertilising substance to that which is to be fertilised.

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1. See Ernst Meyer, 'Geschichte der Botanik,' I. p. 98, &c.
2. The edition here used is that of Gottlob Schneider, 'Theophrasti Ercsii quec supersunt opera,' Leipzig, 1818. See in addition to the passages noticed in the text the 'De Causis,' 1. I. c. 13. 4, and 1. IV. c. 4, and the 'Historia Plantarum,' 1. II. c. 8.
3. It should be understood that neither Theophrastus nor the botanists of the 16th and 17th centuries considered the rudiments of the fruit to be part of the flower; this, which was pointed out in the history of systematic botany, seems to have been overlooked by Meyer, 'Geschichte,' I. p. 164.
4. The passage is quoted in full in De Candolle's 'Physiologic végétale,’ 1 1835, ii. p. 44. It is said there of the pollen, 'Ipso et pulvere etiam feminas maritare.’
5. See De Candolle, 'Physiologic vegetale,' p. 47.
6. His 'Methodus Herbaria' is said to have been published in 1592. The remarks in the text are made in reliance on a long quotation from it in Keeper's translation of De Candolle's 'Physiologic,' ii. p. 49, who had before him an edition of 1604.
7. In the 'Compositae,' however, Grew called the single flowers the florid attire, see p. 37.
8. We may compare with this, pp. 38 and 39 of the first part of the work which appeared in 1671, where Grew ascribed no sexual significance to the stamens.
9. Rudolph Jacob Camerarius was born at Tubingen in 1665 and died there in 1721. I laving completed the course of study in philosophy and medicine, he travelled from 1685 to 1687 in Germany, Holland, England, France, and Italy. In 1688 he became Professor Extraordinary and Director of the Botanic Garden in Tubingen; in 1689 Professor of Natural Philosophy; and finally, in 1695, First Professor of the University, in succession to his father, Elias Rudolph Camerarius. He was afterwards succeeded by his son Alexander, one of ten children. There is an article on Camerarius in the 'Biographic Universelle,' from the pen of Du Petit-Thouars. His works on other subjects, as well as those on the question of sexuality in plants, are distinguished by ingenious conception and lucid exposition.
10. See Patrick Blair's 'Botanic Essays,' in two parts (1720), pp. 242-276. Even the Latin ode is borrowed without acknowledgment.
11. The account in the text is taken from Koelreuter's report in his 'Historic der Versuche über das Geschlechte der Pflanzen,' as given at p. 188 of Mikan's 'Opuscula Botanici Argument!.' Logan's work, 'Experimenta et Meletamata de Plantarum Generatione,' unknown to me, is said by Pritzel to have been published at the Hague in 1739. Koelreuter cites from a London edition of 1747.
12. Koelreuter's report in Mikan's collection is again the authority which is here relied on.
13. Koelreuter says that he sent pollen of Chamaerops in 1766 to St. Petersburg and to Berlin, where it w; s successfully employed by Eckleben and Gleditsch. He wished to try how long the pollen retains its efficacy.
14. See Vol. II. p. 502, of the 'Physiologie vegetale.’
15. See Mikan, 'Opuscula Botanici Argumenti,' p. 180.
16. Joseph Gottlieb Koelreuter was born at Sulz on the Neckar in 1733, and died at Carlsruhe in 1806, where he was Professor of Natural History, and from 1768 to 1786 Director also of the Botanic and Grand-ducal Gardens. On giving up the latter position he continued his experiments in his own small garden till the year 1790. Karl Friedrich Gärtner in his work 'Ueber Bastardzeugung' of 1849, at p. 5 says that after the latter date Koelreuter occupied himself with experiments in alchemy; but this must be a mistake. Gartner, loco cit., and the 'Flora' of 1839, p. 245, supply all that seems to be known of'the life of this distinguished man. The 'Biographie Universelle' contains no account of him. It would appear that he was in St. Petersburg before 1766.
17. See Gärtner, 'Ueber Bastardzeugung ' (1849), p. 62. I have unfortunately been unable to meet with the second continuation of Koelreuter's work.
18. Christian Konrad Sprengel, born in 1750, was for some time Rector at Spandau. There he began to occupy himself with botany, and devoted so much time to it that he neglected the duties of his office, and even the Sunday's sermon, and was removed from his post. He afterward lived a solitary life in straitened circumstances in Berlin, being shunned by men of science as a strange, eccentric person. He supported himself by giving instruction in languages and in botany, using his Sundays for excursions, which any one who chose could join on payment of two or three groschen. He met with so little support and encouragement that he never brought out the second part of his famous work; his publisher did not even give him a copy of the first part. Natural disgust at the neglect with which his work was treated made him forsake botany and devote himself to languages. He died in 1816. One of his pupils wrote a very hearty eulogium on him in the 'Hora' of 1819, p. 541, which has supplied the above facts.
19. See Hermann Mullcr, 'Befruchtung der Blumen durch Insecten,' Leipzig (1873), p. 5.
20. Lazaro Spallanzani was born at Scandiano in Modena, and died at Pavia in 1799, where he was for a long time Professor of Natural History. He made researches in very various questions of natural science, and especially in animal physiology; but they seem to have been conducted with the same want of care and deliberation which appears in his experiments on sexuality in plants. A long article in the 'Biographic Universelle' gives a detailed account of his scientific labours.
21. August Henschel was a practising physician and a University teacher in Breslau.
22. Karl Friedrich Gartner, son of Joseph Gartner, was born at Calw in 1772, and died there in 1850. He attended lectures on natural science at the Carlsacademie at Stuttgart, and then went first to Jena for medical instruction, and in 1793 to Göttingen, where he was a pupil of Lichtenberg. He took a degree in 1796 and settled as a physician in his native town. Here he occupied himself at first with questions of human physiology, and afterwards worked at the supplement to his father's 'Carpologia.' He collected notices and extracts for a complete work on vegetable physiology. This design was never fulfilled, but it led to his taking up the question of sexuality in plants, to which he devoted twenty-five years ('Jahresheft des Vereins fiir vaterl. Naturkunde in Würtemberg,' 1852, vol. viii, p. 16).
23. See also Sachs, 'Lehrbuch der Botanik,' Leipzig, 1874.
24. The more important works referred to in this section are Robert Brown's 'Miscellaneous Writings,' edited by Bennett, 1866-67; von Mohl on G. Amici, in the 'Dotanische Zeitung,' 1863, Beilage, p. 7; Schleiden, 'Ueber die Bildung des Lichens und Entstchung des Embryos,' in 'Nova Acta Academiae Leopoldinensis,' 1839, vol. xi, Abtheilung, i; Hofmeister, 'Zur Uebersicht der Geschichte von der Lehre der Pflanzenbefrachtting,' in 'Flora' of 1867, p. 119.
25. The authorities for these statements are collected by Hofmeister in 'Flora,' 1857, p. 120, etc.
26. W. P. Schimper, in his 'Recherches anatomiques et morphologiques sur les Mousses' of 1850, had made some important statements respecting the sterility of female moss-plants growing at a distance from male specimens, and proved that the presence of male plants among females that are otherwise barren renders them fruitful.