History of botany (1530–1860)/Book 3/Introduction
THIRD BOOK
HISTORY OF VEGETABLE PHYSIOLOGY
(1583-1860)
INTRODUCTION.
All that was known in the 16th and at the beginning of the 17th centuries of the phenomena of life in plants was scarcely more than had been learnt in the earliest times of human civilisation from agriculture, gardening, and other practical dealing with plants. It was known, for instance, that the roots serve to fix plants in the soil and to supply them with food; that certain kinds of manure, such as ashes and, under certain conditions, salt, strengthen vegetation; that buds develope into shoots; and that the blossom precedes the production of seeds and fruits. These and a variety of minor physiological phenomena were disclosed by the art of gardening. On the other hand, the physiological importance of leaves in the nourishment of plants was quite unknown, nor can we discover more than a very indistinct perception of the connection between the stamens and the production of fruitful seeds. That the food-material taken up from the soil must move inside the plant in order to nourish the upper parts was an obvious conclusion, which it was attempted to explain by comparing it with the movement of the blood in animals. Writers on the subject up to the end of the 17th century make very slight mention of the influence of light and warmth on the sustentation and growth of plants, though doubtless the operation of these agencies in the cultivation of plants, as in other matters, must have been early recognised.
So scanty was the stock of knowledge which the founders of vegetable physiology in the latter half of the 17th century found ready to their hand. While the physiological significance of the different organs of the human body and of most animals were known to every one, at least in their more obvious features, the study of vegetable life had to begin with laborious enquiries, whether the different parts of plants are generally necessary to their maintenance and propagation, and what functions must be ascribed to individual parts for the good of the whole. It was no easy matter to make the first step in advance in this subject; something can be learnt of the functions of the parts of animals from direct observation, scarcely anything in the case of plants; and it is only necessary to read Cesalpino and the herbals of the 16th century to see how helpless the botanists were in every case in presence of questions concerning the possible physiological meaning of vegetable organs, when they ventured beyond the conceptions of the root as the organ of nourishment, and of the fruit and seeds as the supposed ultimate object of vegetable life. The physiological arrangements in vegetable organs are not obvious to the eye; they must be concluded from certain incidental circumstances, or logically deduced from the result of experiments. But experiment presupposes the proposing a definite question resting on a hypothesis; and questions and hypotheses can only arise from previous knowledge. An early attempt to connect the subject with existing knowledge was made in the use of the comparison of vegetable with animal life, a comparison which Aristotle had employed with small success. Cesalpino, provided with more botanical and zoological knowledge, endeavoured to arrive at more definite ideas of the movement of the nutrient juices in plants, and when Harvey discovered the circulation of the blood in the beginning of the 17th century, the idea at once arose that there might be a similar circulation of the sap in plants. Thus a first hypothesis, a definite question was framed, and attempts were made to decide it by more exact observation of the ordinary phenomena of vegetation, and still better by experiment; and though a discussion which lasted nearly a hundred years led to the opinion that there is no circulation of sap in plants corresponding to the circulation of blood in animals, the result was obtained by the aid of this hypothesis derived from a comparison between animals and plants. The important discovery that leaves play a considerable part in the nourishment of plants, was to some extent an incidental product of the investigation of the former question, and it preceded that of the decomposition of carbon dioxide by the green parts of plants by more than a hundred years. To give another example; it was obviously a comparison of certain phenomena in vegetable life with the propagation of animals which paved the way for the discovery of sexuality in plants; long before Rudolf Jacob Camerarius made his decisive experiments (1691-1694) on the necessary co-operation of the pollen in the production of seeds capable of germination, the idea had been entertained that there might be an arrangement in plants corresponding to the sexual relation in animals, though that idea was highly indistinct and distorted by various prepossessions. In like manner the interest excited by the discovery of the irritability of the Mimosae in the 17th century, and of similar phenomena of movement in plants at a later time, was mainly due to the striking resemblance suggested between animals and plants; and the first researches into the subject were obviously intended to answer the question whether the movements in plants are due to conditions of organisation similar to those in animals. In all cases of this kind it was matter of indifference whether the analogies presupposed were finally confirmed after prolonged investigation, as in the question of sexuality, or disproved as in that of the circulation of the sap. The result was of less importance than the obtaining points of departure for the investigation. It answered this purpose to adopt certain actual or only apparent analogies between plants and animals, and to assume, to some extent to invent, certain functions for the apparently inactive organs of plants, and to interrogate them upon the point. Scientific activity was set in motion, and it mattered not what the result might be. In all questions connected with the phenomena of life, our own life is not only the starting-point but also the standard of our conceptions; what animate nature is as opposed to inanimate we discern first by comparing our own being with that of other objects. From our own vital motions we argue to those of the higher animals, which we comprehend immediately and instinctively from their conduct; by aid of these the motions of the lower animals also become intelligible to us, and further conclusions from analogy lead us finally to plants, whose vitality is only in this way made known to us. While plants were thus even in ancient times regarded as living creatures and allied to animals, further reflection naturally-suggested the idea that the phenomena of animal life would be reproduced in plants even in details. We learn from the botanical fragments of Aristotle that this was in fact the way in which the first questions in vegetable physiology arose; they assumed a more definite form with Cesalpino, and later physiologists repeatedly made use of similar conclusions from analogy. The historian of this branch of botanical science must seek no other beginning of it, for it had no other and could have no other from the nature of the case. And if preconceived analogies between plants and animals often proved deceptive and mischievous, yet continued investigation gradually brought to light more important and more essential points of agreement between the two kingdoms; it has become more and more evident in our own days, that the material foundations of vegetable and animal life are in the main identical, that the processes connected with nourishment, movement of juices, sexual and asexual propagation present the most remarkable similarities in both kingdoms.
If the first founders of scientific vegetable physiology sur- rendered themselves thoroughly to teleological views, this was owing to the circumstances of the time, and it served indeed to promote the first advances of the science. There was no need in the 17th and 18th centuries that a man should be an Aristotelian to presuppose design and arrangements in conformity with design in all parts of physiological investigation. This is everywhere and always the original point of view which precedes all philosophy; but it is the part of advanced science to abandon this position; and as early as the 17th century philosophers recognised the fact that the teleological mode of proceeding is unscientific. But the first vegetable physiologists were not philosophers in the stricter sense of the word, and in their investigations they accepted the teleological conception of organic nature without question, because they regarded it as a self-evident fact, that every organ must be purposely and exactly so made as to be in a condition to perform the functions necessary for the permanence of the whole organism. This conception was in accordance with views then prevailing, and was even useful; it was no disadvantage in the first beginnings of the science, that it should be supposed that every, even the minutest, part of a plant was expressly contrived and made for maintaining its life, for this was a strong motive for carefully examining the organs of plants, which was the first thing requisite. This is exemplified in Malpighi, Grew, and Hales, and we shall see that even towards the end of the 17th century Konrad Sprengel made splendid discoveries respecting the relations of the structure of the flower to the insect world, while strictly carrying out his teleological principles. The teleological view was injurious to the progress of morphology from the first, though the history of systematic botany shows how hard it was for botanists to free themselves from such notions. The case was different with physiology; so long as it was a question of discovering the functions of organs, and learning the connection between the phenomena of life, teleology proved highly useful if only as a principle of research. But it was another matter when it became requisite to investigate causes, and to grasp the phenomena of vegetation in their causal connection. To this the Ideological mode of view was inadequate, and it became necessary indeed to discard it as a hindrance, in spite of the difficulty of explaining adaptation in the arrangements of organisms from any other than the teleological point of view. It is sufficient here to say that this difficulty is satisfactorily removed by the theory of selection. This theory is become as important in this respect to physiology, as the theory of descent is to systematic botany and morphology. If the theory of descent finally liberated the morphological treatment of organisms from the influence of scholasticism, it is the theory of selection which has made it possible for physiology to set herself free from teleological explanations. Only an entire misunderstanding of the Darwinian doctrine can allow anyone to reproach it with falling back into teleology; its greatest merit is to have made teleology appear superfluous, where it seemed to naturalists in former times, in spite of all philosophical objections, to be indispensable.
If the comparison of plants with animals as well as the teleological conception of organisms promoted the first attempts at the physiological investigation of plants, other influences of decisive importance came into play when the time came for endeavouring to conceive and explain the causes and conditions of the functions, which had then been ascertained at least in their most obvious features. Phytotomy was here the chief resource. In proportion as the inner structure of plants was better known and the different kinds of tissue better distinguished, it became possible to bring the functions of organs, as made known by experiment, into connection with their microscopic structure; phytotomy dissected the living machine into its component parts, and could then leave it to physiology to discover from the structure and contents of the tissues, how far they were adapted to perform definite functions. Obviously this only became possible when the phenomena of vegetation had been previously studied in the living plant. For example, the microscopic examination of the processes which take place in fertilisation could first be made to yield further conclusions, after sexuality itself, the necessity of the pollen to the production of fruitful seeds, had been proved by experiment; in the same way the anatomical investigation of wood could only supply material for explaining the mode in which water rises in it, when it had first been ascertained by experiment that this happens only in the wood, and so in other cases.
The relation between physiology and physics and chemistry suggests similar considerations; it is necessary to make some preliminary remarks in explanation of this relation, because we often meet with the view, especially in modern times, that vegetable physiology is virtually only applied physics and chemistry, as though the phenomena of life could be simply deduced from physical and chemical doctrines. This might perhaps be possible, if physics and chemistry had no further questions to solve in their own domains; but in fact both are still as far distant from this goal, as physiology is from hers. It is true indeed, that modern vegetable physiology would be impossible without modern physics and chemistry, as the earlier science had to rely on the aid of the physics and chemistry of the day, when she was engaged in forming a conception of ascertained vital phenomena as operations of known causes. But it is equally true, that no advance which physics and chemistry have made up to the present time would have produced any system of vegetable physiology, even with the aid of phytotomy; history shows that a series of vital phenomena in plants had been recognised in the 17th and 18th century, at a time when physics and chemistry had little to offer, and were in no condition to supply explanations of any kind to the physiologist. The true foundation of all physiology is the direct observation of vital phenomena; these must be evoked or altered by experiment, and studied in their connection, before they can be referred to physical and chemical causes. It is therefore quite possible for vegetable physiology to have
reached a certain stage of development without any explanation of the phenomena of vegetation from physics or chemistry, and even in spite of erroneous theories on those subjects. What Malpighi, Hales, and to some extent Du Hamel produced, was really vegetable physiology, and of a better kind than some moderns are inclined to believe; and their knowledge was derived from observations on living plants, and not from the chemical and physical theories of their time. The discovery even of important facts, for example, that green leaves only can form the food suitable to effect the growth and formation of new organs, was made a hundred years before that of the decomposition of carbon dioxide by the green parts of plants, at a time indeed when chemistry knew nothing of carbon dioxide and oxygen. A whole series of physiological discoveries might be mentioned, which were distinctly opposed to chemical and physical theories, and even served to correct them. We may give as examples, the establishment of the facts that roots absorb water and the materials of food without giving up anything in return, which seemed quite unintelligible on the earlier physical theory of the endosmotic equivalent; and that the so-called chemical rays of the physicists are of subordinate importance in vegetable assimilation, while contrary to the prevailing notions of physicists and chemists the yellow portions of the spectrum and those adjacent to it actively promote the decomposition of carbon dioxide. From what doctrines of the physicists could it have been concluded, that the downward growth of roots and the upward growth of stems was due to gravitation, as Knight proved in 1806 by experiments on living plants; or could optics have foreseen that the growth of plants is retarded by light, and that growing parts are curved under its influence. Our best knowledge of the life of plants has been obtained by direct observation, not deduced from chemical and physical theories. After these preliminary remarks we may proceed to give a rapid survey of the progress of vegetable physiology. 1. That the first beginnings of vegetable physiology were made about the time that chemistry and physics began to take their place among the true natural sciences, is no proof that they called vegetable physiology into existence. She, like general physiology, mineralogy, astronomy, geography, owed her origin to the outburst of the spirit of enquiry in the 16th and 17th centuries, which feeling the emptiness of the scholastic philosophy set itself to gather valuable knowledge by observation in every direction. It was in the second half of the 17th century that societies or academies for the study of the natural sciences were founded in Italy, England, Germany, and France under the influence of this feeling; the first works on vegetable physiology play a very prominent part in their transactions; not to speak of less important cases, it was the Royal Society of London which published between 1660 and 1690 the memorable works of Malpighi and Grew; the first communications of Camerarius, which form an epoch in the history of the doctrine of sexuality, appeared in the journals of the German Academia Naturae Curiosorum, and the French Academy undertook about the same time to organise methodical researches in vegetable physiology under Dodart's direction, though the results it is true did not answer to the goodness of the intention. This period of movement in all branches of science, when the greatest discoveries followed one another with marvellous rapidity, witnessed also the first important advances in vegetable physiology; such were the first investigations into the ascending and descending sap, especially those made in England, Malpighi's theory which assigned to leaves the functions of organs of nutriment, Ray's first communications on the influence of light on the colours of plants, and above all the experiments of Camerarius, which proved the fertilising power of the pollen. It was the period of first discoveries; the attempts at explanation were certainly weak ; but phytotomy which was just commencing its own work lent aid from the first to physiology, while physics and chemistry could do but little for her. On the other hand, the predilection for mechanics and mechanical explanation of organic processes in Newton's age bore fair fruit in Hales' enquiries into the movement of sap in plants; his 'Statical Essays' of 1727 connect closely with the works before mentioned which had laid the foundations of the science, and with this important performance the first period of its history reaches a distinctly marked conclusion.
This time of vigorous advance was followed by many years, in which no notable work was done and no great discovery effected ; there was active disputation on what had been already ascertained, but it did not lead to any deeper conception of the questions or to new experimental determinations.
2. About the year 1760 new life was infused into the consideration of various branches of vegetable physiology. Du Hamel's 'Physique des arbres' (1758) gave a summary of former knowledge and added a number of new observations, and from that time till the beginning of the present century a series of important discoveries was made. The doctrine of sexual propagation, which had scarcely been advanced since the time of Camerarius, and was disfigured by the theory of evolution, found an observer of the first rank in Koelreuter (1760-1770), who threw new light upon the nature of sexuality by his experiments on the artificial production of hybrids; he was the first who carefully studied the arrangements for pollination, and pointed out the- remarkable connection between them and insect-life. These relations were afterwards (1793) examined in greater detail by Konrad Sprengel, who arrived at such astonishing and far-reaching results, that they were not even understood by his contemporaries, nor was their significance fully appreciated till quite modern times and in connection with the theory of descent.
No less important was the advance made in the doctrine of the nourishment of plants. Between 1780 and 1790 Ingen Houss proved, that the green parts of plants absorb carbon dioxide under the influence of light and eliminate the oxygen, and thus obtain the carbon which plants accumulate in organic combinations, but that all parts of plants also absorb at all times smaller quantities of oxygen, and exhale carbon dioxide, and so perform a process of respiration exactly corresponding to that of animals. He was soon followed by Theodore de Saussure with more thorough investigation of these processes, and with proofs that the ash-constituents of a plant are no chance or unimportant addition to its food, as had been hitherto commonly supposed (1804). The influence also of general physical forces on vegetation was established in some important points, though not yet submitted to searching examination. Thus Senebier showed in the period between 1780 and 1790 the great effect which light exercises on the growth and green colour of plants, and De Candolle at a later date discovered its operation in the case of leaves and flowers that show periodic movements. Still more important was Knight's discovery in 1806 that the upright growth of stems and the downward direction of the main roots are determined by gravitation.
3. This second period of important discoveries was also followed by a relapse, and again doubts were raised as to the correctness of the very facts which had been best established; attempts were made under the influence of preconceived opinions to invalidate or ignore these facts, and to substitute for them theories that wore the guise of philosophy. The so-called nature-philosophy, which had long been a great hindrance to morphology, proved in like manner injurious to vegetable physiology; the doctrine of the vital force especially stood in the way of every attempt to resolve the phenomena of life into their elementary processes, to discern them as a chain of causes and effects. The ash-constituents of plants, and even their carbon, were traced to this vital force, and misty notions connected with the word polarity were used to explain the direction of growth and much beside. In like manner the influence of the nature-philosophy was brought to bear on the established results of the sexual theory to the destruction of all sound logic, and the sexuality of plants was once more openly impugned in the face of Koelreuter's investigations. This state of things continued till some time after 1820, but then it began to improve once more. L. C. Treviranus examined and refuted the errors of Schelwer and Henschel in 1822; in England Herbert conducted new and very valuable investigations into the question of hybridisation; and it was in this period that Carl Friedrich Gärtner studied and experimented on normal fertilisation and the production of hybrids during more than twenty years; his conclusions, published in exhaustive works in 1844 and in 1849, finally settled the more important questions connected with the sexual theory about the same time that Hofmeister established the microscopic embryology of Phanerogams on a firm foundation.
Other parts also of vegetable physiology had been considerably advanced before 1840; Theodore de Saussure observed in 1822 the production of heat in flowers and its dependence on respiration ten years later Goeppert proved the rise of temperature in germinating and vegetating organs. Dutrochet stimulated enquiry by his researches in various branches of the science between 1820 and 1840; he was the first to apply the phenomena of diosmosis to the explanation of the movement of sap in plants with a lasting influence on the further progress of physiology. Chemical investigations were less fruitful in results, though they served to collect a considerable material of single facts, which could afterwards be turned to theoretical account.
The close of this period, which began with unprofitable doubts, but in which much was set in a train for further development after 1840, is marked by the publication of some important compilations, in which all that had as yet been done in vegetable physiology was presented in a connected form. In addition to Dutrochet's collected works (1837) three comprehensive compendia of vegetable physiology made their appearance, one by De Candolle, which was translated into German by Roeper and published with many improvements and additions in 1833 and 1835; this was followed by a work on vegetable physiology by L. C. Treviranus, 1835–1838, and lastly by Meyen's 'Neues System der Pflanzenphysiologie,' 1837–1839. These works exhibit the characteristic features of the period chiefly in this, that physiology finds as yet no strong support in phytotomy, while the old views of vital force are brought face to face with more exact physico-chemical explanations of processes of vegetation.
4. We have already pointed out the wonderful impulse given to the study of morphology and phytotomy, of embryology and cells about the year 1840; it was shown also that this was due in a great measure to discarding the errors of the nature-philosophy and the idea of vital force, and requiring in the place of such speculations exact observation and systematic induction, and how Schleiden's 'Grundzüge' soon after 1840 vigorously met the demands of the newer time in these respects, but without satisfying them by the positive results obtained. The rapid progress made by phytotomy and the doctrine of cells in the hands of von Mohl and Nägeli proved specially favourable to vegetable physiology, by making it possible to follow the processes of fertilisation in the interior of the ovule. The formation of the pollen-tube from the pollen-grain had been observed long before 1840, and Schleiden in 1837 had proposed the view that the embryo of Phanerogams was formed at the end of the pollen-tube by free cell-formation after it had entered the embryo-sac. But Amici in 1846 and Hofmeister in 1849 showed that this notion was erroneous, and that the germ-primordium is in existence in the embryo-sac before the arrival of the pollen-tube and is excited by it to further development, to the forming the embryo. Similarly Hofmeister's further observations on the embryology of Vascular Cryptogams and Mosses left no doubt, that the spermatozoids of these groups of plants discovered by Unger and Nägeli serve to fertilise the germ-cell or egg-cell previously formed in the female organ and to excite it to further development (1849, 1851). Soon after the sexual act was observed in various Algae, and these afforded the best opportunity for solving by the aid of the microscope the questions which experiment had still left open. Thuret showed in 1854, how the large egg-cells in species of Fucus are surrounded and fertilised by spermatozoids, and he even succeeded in producing hybrids by fertilising the egg-cells of one species with the spermatozoids of another; but it was still uncertain whether simple contact of the male and female organs was sufficient, or whether fertilisation is due to the mingling of the substance of the spermatozoid and the germ-cell; the question was settled by Pringsheim in 1855; he saw the male organ of fertilisation of a fresh-water alga penetrate into the substance of the egg-cell and be dissolved in it, and this proceeding was afterwards observed in higher Cryptogams and is represented in its simplest form in the sexual act of the Conjugatae, which De Bary described at length in 1858 and like Vaucher regarded as a sexual process.
When we consider to what an extent the time and power of work of the most eminent botanists was devoted after 1840 to long and difficult observations on the minute anatomy of plants, on cell-formation, embryology and the history of the development of organs, we cannot wonder if other parts of vegetable physiology, which require experiments on vegetation in plants, were cultivated but little and by the way only; but these studies also gained firmer footing in the advance of phytotomy, which supplied the physiologist with a more definite idea of the organism in which the phenonema of vegetative life are produced.
The chemistry of the food of plants was one of the strictly physiological subjects, which like the sexual theory was studied without intermission and with considerable success in the period from 1840 to 1860, but chiefly or entirely by chemists, who connected their investigations into the processes of nutrition in plants with Saussure's results. Agricultural chemists were chiefly engaged till nearly 1860 with the questions, whether all or certain constituents of the ash of a plant are indispensable parts of its food, and whence these constituents are derived, and with cognate considerations on the exhaustion of the soil by cultivation and its remedy by suitable manuring. In France Boussingault had undertaken experimental and analytical investigations on these subjects before 1840, and it was he who in the course of the next twenty years made the most valuable physiological discoveries; of these the most important was the fact that plants do not make use of free atmospheric nitrogen as food, but take up compounds of nitrogen for the purpose. In Germany the interest in such questions was increased by the instrumentality of Justus Liebig, who gathered from the knowledge that had been accumulated up to 1840 all that was fundamental and of real importance, and drew attention to the great practical value of the theory of the nutrition of plants in agriculture and in the management of woods and forests; considerable state provision was soon made for investigations of the kind, but these often wandered from the right path for the reason, that being designed to promote practical interests they lost sight of the inner connection between all vital phenomena. Still a great mass of facts was accumulated, which careful sifting might afterwards render serviceable to pure science. Some of the best agricultural chemists deserve the credit of vindicating purely scientific as well as practical points of view, and explained in comprehensive works the general subject of the nutrition of plants, so far as it was possible to do so without going deeply into their organisation; among these were Boussingault and the Germans Emil Wolff and Franz Schulze. But the questions of the nutrition of plants, which are connected with the chemical processes of assimilation and metabolism within them, remained still undecided, though some valuable preliminary work on these points dates from this time. In comparison with this important advance in the sexual theory and the doctrine of the nutrition of plants little was done in the branches of vegetable physiology which remain to be mentioned, and that little appeared in an unconnected and fragmentary state; different observers established the connection between the temperature of plants and oxygen-respiration; some new single facts were discovered in connection with the downward curvature of roots, Brücke published in 1848 an excellent enquiry into the movements of Mimosa-leaves, and Hofmeister showed in 1857 that the phenomenon, then known as bleeding in the vine and some other trees, takes place in all woody plants, and not in spring only but in every period of the year, if the requisite conditions are present. These and many other isolated observations were very valuable for the future, but were not used at the time to frame comprehensive theories, because no one devoted himself exclusively to questions of the kind with the perseverance, which in these difficult subjects can alone lead to certain results and to a deeper insight into the inner connection of the phenomena. Surprisingly small was the addition to the knowledge of the movement of sap in plants, and still less was discovered respecting the external conditions of processes of growth and the movements connected with them. The important question of the dependence of the phenomena of vegetation on temperature, was it is true not wholly neglected; but the mistake was made of attempting a short cut by multiplying the total period of vegetation of a plant by the mean daily temperature, in the hope of finding in this product an expression for the total warmth required by a given plant; this mistake was especially misleading in the geography of plants.
The more valuable knowledge which had been gathered up to 1851 was brought together by von Mohl in his often-mentioned work on the vegetable cell with equal perspicuity and conciseness, and current views were critically examined; vegetable physiology generally was expounded at greater length but with less critical sifting in Unger's text-book of 1855; these were the two books which did most to disseminate a knowledge of the subject up to 1860, and they performed their task with credit; that which appears in Schacht's books after 1852 under the head of vegetable physiology rests on such imperfect acquaintance with this branch of science, as to diminish rather than increase its reputation.
Passing from this preliminary survey to a more detailed account of the subject, it will be found necessary to keep the history of the sexual theory distinct from other questions in vegetable physiology. This mode of proceeding is required by the fact, that the establishment and further elucidation of the decisive points in the sexual theory were made independently of the rest of physiology, so that the historical continuity would be interrupted and the account rendered obscure by any attempt to connect the development of the theory chronologically with other topics. In like manner the doctrine of the nutrition of plants and of the movement of the sap was developed uninterruptedly and in independence of other physiological matters; it will be advisable therefore to devote a separate chapter to those subjects also. Earlier discoveries respecting the movements of the parts of plants and the mechanics of growth will be briefly recounted in a third chapter.