History of botany (1530–1860)/Book 2/Chapter 3

by Julius von Sachs

CHAPTER III.

Examination of the Matured Framework of Cell-membrane in Plants.

1800–1840.

There is no sharp line of division between the 18th and the 19th centuries; the phytotomists who appear on the scene during the first years of the new century are scarcely more successful than Hedwig and Wolff; careful and judicious interpretation of their own and others' observations is still rare, and they are often misled by preconceived opinions.

In one respect indeed a very great improvement appeared with the commencement of the 19th century; the number of phytotomists working contemporaneously, checking and criticising one another, became all at once much larger. Hitherto ten or twenty years had intervened between every two works on phytotomy; but in the course of the twelve years after 1800 nearly as many publications followed one another, and scientific discussion enlivened enquiry. Now we meet with a Frenchman for the first time in the field of phytotomy, Brisseau Mirbel, who brought out his 'Traité d' Anatomie et de Physiologic Végétale' in 1802, and raised a series of questions in the discussion of which several German botanists, Kurt Sprengel (1802), Bernhardi (1805), Treviranus (1806), Link and Rudolphi (1807), at once took part. It was a step in advance and one affecting all botanical studies, that with the exception of Rudolphi all these men, like Hedwig before them, were botanists by profession; it was at last felt that the examination of the internal structure of plants, as well as the describing them according to Linnaean patterns, was a part of botanical enquiry; it is at the same time true that the botanical knowledge of these observers was often of service to them in their phytotomical investigations, and directed their work decidedly and from the first towards that which was worth knowing, and towards the objects which claimed the first attention. This remark applies to the younger Moldenhawer even more than to the botanists above-named; his 'Beitrage,' published in 1812, may be taken as closing the first section of this century, during which time he improved the methods of observation, compared his own observations and those of others with great acuteness of judgment, and did all that could be expected with the microscopes of the time.

The period of sixteen years after Moldenhawer, from 1812 to 1828, has nothing of material importance to show in phytotomy. On the other hand, it produced a series of the most important improvements that the compound microscope has undergone since its invention.

As early as 1784 Aepinus had produced objectives of flint and crown glass, and in 1807 Van Deyl^[1] made similar ones with two achromatic lenses, and still the phytotomists complained of the condition of their instruments. Their figures show that they could not see clearly with them, though the magnifying powers were not high; Link says expressly in the preface to his prize-essay of 1807, that he usually observed with a lens that magnified a hundred and eighty times. Moldenhawer in 1812 gives the preference over all the microscopes he had used to one by Wright, which was serviceable with a magnifying power of four hundred times, while the German instruments, especially those by Weickert, could not be used with higher powers than from one hundred and seventy to three hundred.

A certain interval elapsed each time between an improvement in the instrument and the appearance of the advantages which phytotomy derived from it; thus in 1824, Selligue exhibited to the Academy of Paris an excellent microscope with double lenses, several of which could be screwed on one over the other, and which could be used with ordinary daylight and a magnifying power of five hundred times; in 1827 Amici made the first achromatic and aplanatic objectives with three double lenses screwed on one over the other, the flat sides being turned to the object. And yet still in 1836 a practised phytotomist like Meyen spoke with disapproval of the instruments of his time, and gave the preference to an old English microscope by James Man, though he allowed that the newest instruments by Ploessl were a little better. In his work on phytotomy, which appeared in 1830, all the figures were magnified two hundred and twenty times, as were the very beautiful figures in his prize essay of 1836; but in his 'Neues System' (1837), he had already adopted powers that magnified to over five hundred times. How rapid the progress was in the years before and after 1830 is shown by comparing von Mohl's work on climbing plants of 1827 and its antiquated illustrations, with his publications of 1831 and 1833, where the figures have a thoroughly modern appearance.

The art also of preparing anatomical objects rose by degrees with the improvement of the microscope. It was not in a very advanced state at the beginning of the century, if we judge by the language of writers and by their figures. It was a great step in advance when the younger Moldenhawer in 1812 isolated cells by maceration and decay in water, and was thus enabled to view cells and vessels on every side and in a perfect condition, to see their real shape, and to survey the manner of their combination more exactly than had hitherto been done. But even Moldenhawer still made the mistake of submitting delicate microscopic objects to observation in a dry state, though Rudolphi and Link in 1807 had urged the advisability of keeping every part of the preparations moist, especially the surface towards the object-glass, which shows that they did not then use covering glasses. Nor was sufficient attention shown to the use of sharp knives of suitable form, such as the razor, which is now almost exclusively employed, or to practice in making transverse and longitudinal sections of the utmost possible delicacy, two things which, through the example of Meyen's and von Mohl's practice, were afterwards recognised as indispensable helps to phytotomy; even in their time observers were satisfied with crushing and picking their preparations to pieces.

Drawing from the microscope kept even pace on the whole with increasing skill in making preparations, and with the improvement of the instrument. If we compare together the drawings of Mirbel and Kurt Sprengel in the beginning of the century, those of Link and Treviranus in 1807, Moldenhawer's in 1812, and Meyen's and von Mohl's from 1827 to 1840, we shall obtain a rapid and instructive survey of the history of phytotomy during this period of forty years. The figures testify at once to constant increase in the magnifying powers, to the greater clearness of the field of sight, and still more to the constant improvement in the arts of preparing and observing objects. But a curious misconception crept in among the phytotomists at this time; they believed that more correct and trustworthy figures would be obtained, if the observer and writer did not himself make them, but employed other eyes and other hands for that purpose; they imagined that in this way every kind of prejudice,of preconceived opinion would be eliminated from the drawings. Thus both Mirbel and Moldenhawer had their figures drawn by a woman, and many later phytotomists entrusted the execution of their drawings to hired draughtsmen, as Leeuwenhoek had done before them. A drawing from the microscope, like every other copy of an object in natural history, cannot pretend to take the place of the object itself, but is intended to give an exact and clear rendering of what the observer has perceived, and by so doing illustrate the verbal description. The drawing will be perfect in proportion to the practised skill of the eye that observes and of the mind that interprets the forms. The copy should only show to another person what has passed through the mind of the observer, for then only can it serve the purpose of a mutual understanding. There is also another point to be considered; it is exactly in the process of drawing a microscopic object that the eye is compelled to dwell on the individual lines and points and to grasp their true connection in all dimensions of space; it will often happen that in this process relations will be perceived, which previous careful observation had disregarded, and which may be decisive of the question under examination or even open up new ones. As the microscope trains the eye to scientific sight, so the careful drawing of objects makes the educated eye become the watchful adviser of the investigating mind; but this advantage is lost to the observer who has his drawings made by another hand. It is not one of the least of von Mohl's merits, that he practised microscopic drawing under the influence of the views here indicated, and sought to make his figures no mere undigested copies of the objects, but an expression of his own opinions about them.

Enough has been said to show that an important portion of the history of phytotomy lies between the beginning and the end of the period under consideration. The distance between the knowledge of the structure of vegetable tissue which existed at the beginning of the century, and that of Meyen and von Mohl on the same subject in 1840, is wonderfully great; in the one case an uncertain groping about among obscure ideas, in the other a complete exposition of the inner architecture of the mature plant. But in spite of this great difference between beginning and end, it is better to review the efforts of this period of forty years as a connected process of historical development, and, notwithstanding the interval between the appearance of Moldenhawer's contributions in 1812 and Meyen's and von Mohl's labours about 1840, to consider the latter as the settlement of the questions taken up at the commencement of the century. Moreover after 1840, with the appearance of Schleiden and Nägeli on the scene, new points of view were suddenly disclosed, and new aims were proposed in phytotomic investigation; it is no objection to this view of the subject, that the most productive portion of von Mohl's labours falls in the succeeding twenty years, and that during this later period his position is one of equal authority with the new tendency and of participation in it. Up to 1845 his discoveries were the culminating point of the older phytotomy; they put the finishing stroke to the work which Mirbel, Link, Treviranus, and Moldenhawer had begun. The object almost exclusively pursued during all this period was to frame as true a scheme as possible of the inner structure of the mature organs of the plant; it was requisite to gain a right understanding of the diversities of cells and forms of tissues, to classify them and supply them with names, and to secure well-conceived definitions of these names. Hence almost exclusive attention was paid to the configuration of the solid framework of cell-membrane, and of this chiefly in the matured state, to the form of the several elementary organs and their combination in the tissue, to the sculpture of the wall-surfaces, and to the connection of cell-spaces by pores or their separation by closed walls. There was much discussion, especially at first, on the contents of vessels and cells, and on supposed movements of sap in connection with anatomical research, but there was no careful connected investigation of the cell-contents; it was not yet recognised that the true living body of the vegetable cell is only a definite part of the contents inclosed by the cell-wall; the solid walls, the framework of the whole building, were regarded as of primary importance in the structure of the cell. It was not till the following period that in the light of historical development another view asserted itself, namely, that the solid framework of vegetable tissue with all its importance is yet in the genetic sense only a secondary product of the phenomena of vegetative life, that the true cell-body, the cell=protoplasm is prior in time and in conception, and can claim the higher position.

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Mirbel, to whom we now return, had in 1801 laid down a theory of cell-formation which agreed in the main with that of Caspar Friedrich Wolff; he supposed with Wolff that each cell-space was separated from its neighbour by a single wall, and relying on fresh observations asserted the existence of visible pores in the dividing walls of parenchyma and of vessels, and also maintained some new views on the nature and formation of vessels. The essential points of this theory found an opponent in Germany in the person of Kurt Sprengel, the well-known historian of botany, and one of the most variously accomplished botanists of his time, who had published in 1802 a work written in diffuse and familiar style under the title of 'Anleitung zur Kenntniss der Gewächse.' He relied on his own observations, but these were evidently made with small magnifying powers, an obscure field of sight, and indifferent preparations. The cell-tissue, says Sprengel, consists of cavities of very various shape communicating with one another, the dividing walls being in some places broken through and in others wanting. He took the starch-granules which he saw in the seed-leaves of beans and other plants for vesicles, which increase in size by absorption of water and so form new tissue; but he did not explain how we are to conceive of the growth of organs with such a mode of cell-formation. His account of the vessels is extremely obscure, even more obscure than Hedwig's, though he has the merit of refuting the latter's strange theory of reconducting vessels in the epidermis; he also suggested, though only incidentally, the happy idea that spiral passages and even vessels might arise from cell-tissue, since the youngest parts of plants have only the latter; but he did not attempt to explain how or where the process takes place. Like Malpighi and Grew he supposed that the spiral vessels had no wall of their own, but thought that the closely-rolled spiral threads formed a wall; the constrictions in broad short-membered vessels he regarded as real contractions in their substance, caused by the increased tightening of the spiral threads through a sort of peristaltic movement, a mistaken notion often entertained at the beginning of the century, by Goethe among others, and connected with ideas of vital power prevalent at the time. In the stomata, to which he gave the name still in use, Sprengel like Grew, Gleichen, and Hedwig, saw a circular cushion instead of the two guard-cells; but he notices the observation first made by Comparetti, that the orifice closes and opens alternately, being wide open in the morning and closed in the evening. But he considered the stomata to be organs of absorption.

Sprengel in enunciating his own theory of cell-formation accused Mirbel of mistaking the starch-grains in the cells for the pores of the cell-walls. On this point, so important in the doctrine of the cell and in physiology, he was followed by the three candidates for the Gottingen prize, though Bernhardi had in 1805 defended Mirbel's view, and had pointed out how little likely it was, that so skilful an observer as Mirbel should fall into so gross an error. Bernhardi's short treatise, 'Beobachtungen über Pflanzengefässe,' Erfurt (1805^[2]'), was in general distinguished by a variety of new and correct observations, and was the work of a simple and straightforward understanding, which takes things as they are presented to the eye without allowing itself to be led astray by preconceived opinions. His observations are certainly the best in the whole period from Malpighi and Grew to the younger Moldenhawer; his method of dealing with questions of phytotomy is much more to the purpose than that of the three rivals for the Gottingen prize.

In the work just mentioned Bernhardi treats of other forms of tissue as well as vessels, and endeavours to distinguish and classify them more exactly than had hitherto been done. He contrasts favourably with his contemporaries in the fact, that he sought to define the histological terms employed as precisely as possible, a great step in advance at a time when phytotomic conceptions were in a very misty condition. He distinguishes three chief forms of vegetable tissue, pith, bast, and vessels.

By pith he means the tissue which Grew had named parenchyma, and which is still so called; it remained a question with him whether the cells of the pith are pierced by visible pores. By the word bast he understood not only the fibrous elements of the rind, but those of the wood also, and in general what is now known as prosenchyma; this agrees very well with Malpighi's view, which was adopted by Bernhardi and by all his contemporaries, that the inner layers of the bast are changed into the exterior layers of wood to make the increase in thickness of the woody stem; but he did not admit the same origin in the case of the innermost portion of the wood, for this is formed from the first in the young shoots, which alone contain true spiral vessels with threads that may be wound off.

Bernhardi distinguishes vessels into two main groups, air-vessels and vessels properly so called. He calls the first group air-vessels for the same reason that led Grew to give them that name, namely, that they are filled with air during a part at least of the period of vegetation; they are found in the wood, and, where there is no closed woody body, there the woody bundles are formed both of vessels and also of bast strands which enclose vascular canals. These latter he next divides into three chief kinds; annular vessels, which he was the first to discover, true spiral vessels with a band which can be unwound, and scalariform vessels, by which term he understood not only those with broad slits, such as are found in Ferns, but also the pitted vessels in secondary wood. His idea of annular and spiral vessels was perfectly correct, and he mentions Hedwig's notion already described, and shows that its exact opposite is true, namely, that the spiral band is surrounded by a membrane on the outside, a fact which was afterwards denied by Link, Sprengel, and Moldenhawer. On the other hand he did not understand the sculpturing on the scalariform vessels; he took the pits in the dotted vessels for thickenings of the wall, such as are seen in the transverse ridges between the slits in true scalariform vessels, and the slits he thought were closed. If there was much that was erroneous in these views, yet Bernhardi contributed essentially to the clearing up of the subject by his effort to distinguish the different forms of air-vessels, and especially by pointing attention to the fact that neither spiral nor annular vessels are found in secondary wood. The resemblance between different forms of vessels misled many of his contemporaries into supposing that they are due to metamorphosis of true spiral vessels. Bernhardi showed that different forms of wall are found inside one vascular tube, but that this does not depend on modification with age; observation rather teaches that every kind of vessel receives its character in its young state, and especially that the youngest scalariform vessels do not present the form of spiral vessels.

Under the head of vessels proper he reckoned all tubular forms filled with a peculiar juice, milk-cells and true milk-vessels, and also resin-ducts and the like, and he made many good and still valuable observations on their distribution and sap-contents. He could not see the differences of structure in these various fluid-conveying vessels with the low magnifying power of his glass, and therefore attended chiefly to the structure of the large resin-ducts, which on the whole he rightly understood.

The question whether there are any other forms of vessels in the plant beside those already named gave him occasion to define a vessel better than it had yet been defined, namely as an uninterrupted tube or canal, and at the same time he found himself obliged to consider whether his bast-threads are vessels; but he did not give a decided answer to the question. He declared however distinctly against Hedwig's reconducting vessels in the epidermis, as Sprengel had done, and it is worthy of recognition that he understood the true nature of the corners where three longitudinal walls of the parenchyma meet, while later observers found difficulties in them.

Before the appearance of Bernhardi's work the Royal scientific Society of Göttingen proposed a subject for a prize in the year 1804, which shows very plainly what uncertainty was felt at that time on all points of phytotomy. For this reason it will be well to give it at length from the preface to Rudolphi's 'Anatomic der Pflanzen' (1807): 'Since some modern physiologists deny the peculiar construction of vessels in plants which is attributed to them by other and especially the older observers, it would be well to institute new microscopical investigations, which shall either confirm the observations of Malpighi, Grew, Du Hamel, Mustel, and Hedwig, or prove that plants have a special organisation of their own which is more simple than that of animals, whether that organisation is supposed to originate in simple peculiar fibres and threads (Medicus) or with cellular and tubular tissue (tissu tubulaire of Mirbel). Attention should also be given to the following subordinate questions: 1. How many kinds of vessels may certainly be distinguished from the first period of their development? The existence of certain forms having been established; 2. Are the twisted fibres which are called spiral vessels (vasa spiralia) themselves hollow, and do they therefore form vessels, or do they serve by their convolutions for the formation of closed cavities, and how? 3. Do fluids as well as gases move in these cavities? 4. Do the scalariform ducts arise from adherence of the twisted threads (Sprengel), or do the threads owe their origin to the ducts (Mirbel)? Do alburnum and woody fibres originate in the scalariform ducts, or in true vessels, or in tubular tissue?'

We see in this case as in many similar ones, that the subject was proposed by persons who understood little of it, and who were unable to judge of what had been written about it; how else could they have placed the opinions of a Mustel and a Medicus side by side with those of Malpighi and Grew? Had Bernhardi or Mirbel set the question, it would certainly have been better conceived. It was in keeping that the three essays sent in, all inferior to Bernhardi's work already mentioned, though they contradicted one another on the most important points, were nevertheless all accepted; not less so that Treviranus' essay obtained only the second place, though it was decidedly better than the other two, and very much better than Rudolphi's. The best result of the whole affair was that it stirred up the phytotomists of the day, and led Mirbel to submit the three prize treatises to a searching criticism, especially that of Treviranus, which Mirbel with professional acumen recognised as the best. Link's essay appeared in 1807 under the title 'Grundlehren der Anatomie und Physiologie der Pflanzen,' that of Rudolphi as 'Anatomie der Pflanzen,' also in 1807, each forming a handsome octavo volume. The work of Treviranus had already appeared in 1806 with the title, 'Vom inwendigen Bau der Gewächse.'

If we compare the works of Link and Rudolphi^[3], which both received a prize, and which had all the appearance of text-books of general vegetable phytotomy and physiology, we miss in both any clear exposition of the conceptions connected with the words used, and the train of thought therefore is constantly obscure and vacillating. Yet it is easy to see that they are opposed to one another in all essential points, Link^[4] generally hitting on the correct, or at least the correcter view. For instance, Rudolphi denies altogether the vegetable nature of Fungi and Lichens, because he finds no resemblance between their hyphae and vegetable cell-tissue, and he supposes them to arise by spontaneous generation; even of the Confervae he says that the microscope has shown him nothing that agrees with the structure of plants, evidently a sign of bad observation or of incapacity to understand what he saw. Link on the other hand regards all Thallophytes as plants, and sees that the filaments of Lichens and Fungi consist of cells, and that cells occur at least in many Algae. Rudolphi praises in the same breath the views of Wolff and Sprengel on cell-tissue, although they are directly opposed to one another, and although he adopts Sprengel's strange theory of cell-formation without alteration. Link on the contrary declares against Sprengel's theory, and on good grounds, and shows that the vesicles which Sprengel took for young cells are starch-grains; at the same time he makes new cells be formed between the old ones. Rudolphi is of opinion that cells open into one another, as is plainly shown by the passage of coloured fluids. Link maintains that cells are closed bodies, and proves it well by the occurrence of cells with coloured juice in the middle of colourless tissue. Rudolphi represents the orifices of the stomata as encircled by a roundish rim, which he takes without hesitation for a closing muscle because the apertures enlarge and diminish. Link is more happy in taking the part that surrounds the aperture for a cell, or a group of cells. Rudolphi considers the great cavities in hollow stems and in the tissue of water-plants as the only air-passages in plants; Link explains these cavities as gaps caused by the irregular growth of cellular tissue. With Rudolphi the word vessel means not only vascular forms in wood, but milk-vessels and resin-ducts also, and to the former he even transfers Malpighi's view of the structure of spiral vessels. Link designates the tubes of the wood only as vessels, combining the most various forms of them under the term spiral vessels; he excludes milk-vessels, resin-ducts, and the like from the conception of a vessel, and in this he is somewhat inconsistent, since he assumes with Rudolphi that a vessel, in plants as in animals, is a canal for the conveyance of nutrient sap.

With all these contradictions, the two essays agree in adopting the old Malpighian view of the growth in thickness of stems, according to which the new layers of wood are formed from the inner layers of bast, while between the bast-cells, which are here taken to be identical with woody fibre, new spiral vessels arise contemporaneously, and, as Link expressly says, from juices which pour out between the bast-cells.

It is hard to understand how two treatises, so contradictory as they have been shown to be, could have both received a prize at the same time, or how the great difference could have been overlooked between Link's sensible and well-arranged account of his subject, and Rudolphi's uncritical statements, which everywhere rely more on old authority than on his own observation. It is however certain that Link's better production is inferior to Bernhardi's treatise, unless we choose to consider the greater copiousness of detail in Link, the number of his observations, and his aquaintance with the literature of the subject, as giving him the advantage. His figures, as well as Rudolphi's, are not so good as those of Bernhardi.

The work of Treviranus^[5] to which the judges at Göttingen awarded the second place, is much less comprehensive than those of his competitors; the style is inferior to Link's, and may even be called clumsy. But the much better figures show at once that Treviranus was the more accurate observer, and his work, in spite of the inferiority of its style, is of far higher value on account of the attention paid in it to the history of development; Treviranus laid greater stress on this method than either Link or Rudolphi, and it led him to form views on some of the fundamental questions of phytotomy, in which we see the germs of theories afterwards perfected by von Mohl. His account of the formation of cell-tissue is mainly that of Sprengel, and therefore an unfortunate one; but nevertheless his observations on the composition of wood and the nature of vessels were as good and correct as could be expected from the condition of the microscope at the time. He made one discovery of considerable value, that of the intercellular spaces in parenchyma, but he lessened its merit by filling these passages with sap, and even describing its movement. Woody fibres are due, he thinks, to strong longitudinal extension of vesicles. He supported Bernhardi's view of the nature of vessels, that the separable spiral threads of spiral vessels are not wound round a membranous tube but are surrounded by one. He maintains against Bernhardi the distinctness of punctated vessels or porous woody tubes from false tracheae or scalariform vessels, while he gave a more correct description of the latter as they occur in Ferns. He rejected Mirbel's view that the pits in dotted vessels are depressions surrounded by a raised glandular edge, and explained them as grains or little spheres. Against this mistake we may set off the very important step which he made in advance, when he not only conjectured that the pitted vessels of the wood are formed from cells previously divided off from one another, but proved by observation that the members composing such vessels are at first actually separated by oblique cross-walls, which afterwards disappear. But this correct observation was impaired by the mistaken idea, which Treviranus shared with his predecessors, that the wood is the result of transformation of the bast, and consequently that the vessels of the wood are bast-fibres, which elongate considerably after they are arranged in a direct chain one after the other ; the unevennesses caused by the oblique junctions of the tissue gradually disappear, the boundaries of each member of a vessel being still for some time indicated by oblique transverse markings. The dividing walls originally existing at these points disappear by widening of the cavities, so that the different parts come to form a continuous canal. To illustrate the disappearance of a parting wall between two adjoining cells Treviranus aptly points, somewhat to our surprise, to the formation of the conjugating tube in Spirogyra. He rejects with Bernhardi the view represented by Sprengel, Link, and Rudolphi, that the different kinds of vessels are formed from true spiral vessels; he says that he had found the scalariform ducts in Ferns so formed in their earliest stage and not as spiral vessels; he thinks it highly probable that the distinct transverse bands on false spiral vessels (scalariform ducts) and the pits of dotted vessels are formed on the walls of membranous fibre-tubes; in like manner he derives true spiral vessels from long thin-walled cells, on whose inner surface the spiral band is formed, and well compares the members of young spiral vessels with the elaters of the Jungermannieae. Here then we find the first more definite indications of a theory of growth in thickness of cell-walls, which, like the theory of the origin of vessels from rows of cells, was afterwards developed by von Mohl and laid on better foundations. At the close of the essay the histology of the Cryptogams, Monocotyledons and Dicotyledons is compared, and the subject is better and more perspicuously handled than in the corresponding chapters of his competitors.

Though Treviranus' account of vegetable tissues was on the whole weak as far as concerns the history of development, yet Mirbel^[6] recognised in him the most dangerous opponent of his own theory, and addressed a public letter to him and not to his other German antagonists, Sprengel, Link and Rudolphi, in defence of the views he had formerly expressed. This letter is the first part of a larger work which appeared in 1808, 'Exposition et défense de ma théorie de l'organisation végétale,' in which Mirbel endeavours to meet the objections of his opponents with great adroitness of style and with the results of varied rather than profound observation, and to find new arguments for his theory of vegetable tissue; he admits that his former treatises were in many respects faulty, but demands that his critics should discuss his system as a whole and not take offence at single expressions. Mirbel's idea of the inner structure of plants is essentially the same as that broached by Caspar Friedrich Wolff. The first and fundamental idea is that all vegetable organisation is formed from one and the same tissue differently modified. The cell-cavities are only hollow spaces of varying form and extension in homogeneous original matter, and have no need therefore of a system of filaments, as Grew supposed, to hold them together. The tracheae only are an exception, which Mirbel, in striking opposition to the much more correct view of Treviranus, considers to be narrow spirally wound laminae, inserted into the tissue and connected with it only at the two ends. If it is asked how interchange of sap is effected in such a cellular tissue as this, it becomes necessary to assume that the membranous substance of plants is pierced by countless invisible pores, through which fluids find their way. But nature has a speedier and more powerful instrument in the larger pores, which the microscope reveals. Mirbel does not discuss the question how the fluids are set in motion, easily disregarding such mechanical difficulties, as was usual in those days, when vital power was always in reserve to be the moving agent. He warmly repels the imputation, which Sprengel had made against him, of having confounded pores and granules, by appealing to his figures; he says that he has depicted prominences on the outside of the walls of the dotted vessels, and an orifice in each of them, which his opponents simply never saw. The question whether these prominences lie on the inside or the outside of the walls of the vessel has no meaning, if we suppose with Mirbel that the dividing wall is single; he is only concerned to enquire whether the perforated projections lie on the one or on the other side of the wall. He refers Treviranus, who had denied the presence of the pores, to his description of scalariform vessels, in which he had himself seen the slits which correspond to the pores.

In comparison with these fundamental questions Mirbel's further account of matters of detail does not concern us here. He gave a connected view of the whole of his doctrine of tissues in the form of aphorisms, which occupy the second part of his book. Of all that he says on the five kinds into which he distinguishes vessels the most interesting is the statement, that diaphragms pierced like a sieve separate the different members of his 'beaded' vessels. We find that the weakest part of Mirbel's phytotomy, as of that of his opponents, is his description of the true vessels (vasa propria), with which he classes the milk-cells of the Euphorbiae and the resin-ducts of Coniferae, but he saw clearly enough that the latter were canals inclosed in a layer of tissue of their own. The third part of the book is devoted to these forms of tissue, and we learn that he classes not only many kinds of sieve-cell-bundles, but also true bast-fibres, as those of nettle and hemp, with his bundles of true vessels. Like his opponents he makes the growth in thickness in woody stems to be due to transformation of the inner layer of bast; but he gives a new turn to this view, which brings it nearer to the modern theory ; during the period of vegetation a delicate tissue with large vessels is developed in Dicotyledons on the confines of the wood and the bark, and these augment the mass of the woody body, while a loose cellular tissue is formed on the other side, destined to replace the constant losses of the outer rind. To later phytotomists, who understood by the word cambium a thin layer of tissue constantly engaged in producing wood and rind, Mirbel's otherwise indistinct conception of growth in thickness must have become more indistinct from his using the word cambium not for the layer of tissue afterwards so called, but for a highly 'elaborated and purified sap' which is intended for the food of the plant and makes its way through all membranes; we see this cambium-sap appear at the spots where it produces new tubes and cells after the manner of the Wolffian theory. The cells appear at first as minute spheres, the tubes are very fine lines; both enlarge and gradually show pores, clefts, etc. This is essentially Wolffs doctrine, which Mirbel afterwards endeavoured to confirm against his German opponents from the germination of the date-palm with the help of a more powerful microscope.

Mirbel insisted more than the German phytotomists of his day on the idea, that all forms of vegetable tissue are developed originally from young cell-tissue, an idea suggested by Sprengel and following naturally with Mirbel from Wolffs theory. Both Mirbel and Wolff were hasty in observation and too much under the influence of theory in giving reasons for what they observed, and therefore too ready with far-reaching explanations of phenomena which only long-continued observation could decide.

Treviranus replied, though after some delay, to Mirbel's polemics by incorporating into his 'Beiträge zur Pflanzenphysiologie,' Göttingen (1811), an essay entitled 'Beobachtungen im Betreff einiger streitigen Puncte der Pflanzenphysiologie,' in which he again took up the questions in dispute between himself, Mirbel, Link and others, and supported his own views by fresh investigations. It cannot be denied that in this short treatise Treviranus brought some important questions nearer to a decision; he added materially to the knowledge of bordered pits, on which subject his views now approximated more nearly to those of Mirbel; he drew attention to the vesicular nature of vegetable cells, which are often separable from one another, and to the occurrence of true spiral vessels in the neighbourhood of the pith in Conifers also, and among other things discovered the stomata on the capsule of Mosses. On the subject of the theory of cell-formation which he had borrowed from Sprengel, he endeavoured to extricate himself from his difficulty by ingeniously pointing out that though the starch-grains in the seed-leaves of the bean disappear without producing new cells in them, they are dissolved and then serve as fluid material for new cell-formation in other parts of the germinating plant, which however was giving up Sprengel's theory; yet he cited as a direct proof of that theory the origination of gonidia in the cells of Hydrodictyon, and their development into new nets.

Mirbel and his German opponents moved for the most part in a circle of ideas which had been formed by the speculations of Malpighi, Grew, Hedwig and Wolff, though it must be allowed that the observations of Treviranus did open new points of view. But Johann Jalob Paul Moldenhawer^[7] travelled far beyond these older views as early as 1812 in his important work, 'Beitrage zur Anatomic der Pflanzen.' He took up from the first a more independent position as regards former opinions than either of the writers hitherto considered. He relied on very detailed, varied, and systematic observations evidently made with a better instrument, abided by what he himself saw, and chose his point of view in accordance with it, while he criticised the views of his predecessors in detail with an unmistakable superiority, and in so doing displayed minute acquaintance with the literature of the subject and varied phytotomical experience. He fixed his eye firmly on the points in question, and made each one the subject of earnest investigation and copious and perspicuous discussion. His figures prove the carefulness of his examination and the greater excellence of his instrument; they are undoubtedly the best that were produced up to 1812. His mode of dealing with his subject and his figures, though they were not executed by himself, remind us in many respects of von Mohl, though it would be more correct to say that von Mohl's manner reminds us of Moldenhawer, for from the great respect which von Mohl displays for him, especially in his earlier writings, it can scarcely be doubted that he formed himself on Moldenhawer's 'Beitrage,' and first learnt from them the earnestness and carefulness demanded by phytotomic work.

It has been already mentioned that the study of vegetable physiology is indebted to Moldenhawer for one important practical improvement. He was the first who isolated cells and vessels by allowing parts of plants to decay in water and afterwards crushing and dissecting them, a process not much used in modern times, though it may still be applied with advantage in conjunction with what is known as Schulze's solution, especially if it is carried out with Moldenhawer's carefulness and circumspection. The isolation of the elementary organs of plants by maceration in water necessarily brought Moldenhawer into direct antagonism with Mirbel, who with Wolff assumed that the partition between any two cells was a single wall; whereas Moldenhawer found that the cells and vessels were closed tubes and sacs after isolation, and must necessarily, as it would seem, so lie one against another in the living plant, that the wall between every two cell-spaces is formed of two membranous laminae, and he expressly says that this is the case even in very thin-walled parenchyma. This result remained unassailable, so long as no one was in a position to conclude from the history of the development of cell-tissue that the partitions are originally single, or by aid of strong magnifying power to prove the true structure of the walls and their later separation, and the differentiation of the once single wall into two separable laminae. If the view based on the results of maceration was still not the true view, yet it was nearer the truth as regards the matured state of the cell-wall than that of Wolff and Mirbel, and the important advantage was gained of being able to study the form of elementary organs and the sculpture on their walls more accurately than before. It is true that Link had occasionally isolated cells by boiling in 1809, and Treviranus had drawn attention in 181 1 to the fact that it was possible to isolate parenchyma-cells in their natural condition ; but neither of them made systematic use of these observations, and to Moldenhawer belongs the exclusive merit of having first isolated vessels and woody cells ; but as usually happens, he did not himself obtain all the possible results from his method of preparation. In his work which indeed embraces the whole of phytotomy, he is continually recurring to one species, maize. This supplies the starting-point in every question to be discussed. The results obtained there are the firm supports on which he leans in the examination of a great variety of plants, which he then compares together at length. This mode of treatment was well chosen both for investigation and instruction in the existing state of the science ; it was a particularly happy idea that of choosing the maize-plant for his purpose; former phytotomists had generally had recourse to dicotyledonous stems, and preferred those that had compact wood and complex rind, but the examination of these plants presents difficulties at the present day to a practised observer with a good microscope. Occasionally observers had taken the stem of the gourd, where the large cells and vessels suited small magnifying power, but where many abnormal conditions occurred to interfere with their conclusions. The Monocotyledons, like the Vascular Cryptogams, had hitherto been comparatively neglected. When then Moldenhawer made a monocotyledonous and rapidly growing plant, with very large-celled tissue and comparatively very simple structure, the chief subject of his investigations, he was sure to succeed in making out many things more clearly than his predecessors. It was an important point that he found the fibrous elementary organs in this plant united with the vessels into bundles, which are separated by a strict line of demarcation from the large

celled parenchyma that surrounds them. Thus the peculiar character, the idea, of the vascular bundle was brought prominently into contrast with that of other forms of tissue. This took the place of the distinction between rind, wood, and pith, which had served former phytotomists as the basis of their histological survey, but which is in itself only a secondary result of the later elaboration of certain parts of the plant. Moldenhawer, in laying the chief stress from the first on the contrast between vascular bundles and parenchyma, hit upon a histological fact of more fundamental importance, the right appreciation of which has since enabled the phytotomist to find his way through the histology of the higher plants. For while the construction of Monocotyledons and Ferns must seem abnormal and quite peculiar to any one who starts with examining the rind, wood, and pith of old dicotyledonous stems, those on the contrary who, with Moldenhawer, have recognised a special histological system in the vascular bundles of Monocotyledons, have the way opened to them to seek for a similar one in the Dicotyledons, and to refer the secondary phenomenon of wood and rind to the primary existence of vascular bundles. Moldenhawer did in fact open this way, when he showed how the growth of a dicotyledonous stem may be understood from the structure and position of the originally isolated vascular bundles (Beitrage, p. 49, etc.). But he was thus of necessity led to the rejection of Malpighi's theory of the growth in thickness of woody stems, which all vegetable anatomists from Grew to Mirbel had adopted. Though Bernhardi and Treviranus made weak attempts to discredit it, Moldenhawer was the first who distinctly rejected the origin of the external layers of wood from the inner bast, and proposed the first really practical basis for the later and correct theory of secondary growth in thickness (p. 35). The removal of this ancient error is in itself a very important result, and one which, apart from all other services, must secure him an honourable place in the history of botany.

But the light must have its attendant shadow, and all his carefulness in observation and cautiousness in judgment did not protect him from one prejudice and its evil consequences. After Moldenhawer had isolated the elementary organs by maceration, he had to answer the question how we are to conceive of their firm coherence in the living plant. He came to the conclusion, as did von Mohl, Schacht, and others after him, that there must be some special connecting medium; but he did not hit upon their idea of a matrix, in which the cells are imbedded, or of a cement which holds them together, but on a much stranger theory, which reminds us at once of Grew's thread-tissue, and like that rests partly on incorrect observations. These were too hastily accepted as the basis of a theory which in its turn interfered with after observations. He thought that the cells and vessels were surrounded and held together by an extremely delicate net-work of fine fibres; in some cases he really believed that he saw these fibres, and interpreted in this way the thickened bands in the well-known cells of Sphagnum, and still more strangely he appears to have taken the thickened longitudinal and transverse edges of cells and vessels for such threads. The unfavourable impression produced by this theory is necessarily heightened by the fact that he gave the name of cell-tissue, a term long used in a different sense, to his fancy-structure of reticulated threads which were to hold the cells and vessels together, while he called the parenchyma itself cellular substance, an expression which fortunately no one copied, and which certainly contributed at a later time to discredit the great services which Moldenhawer rendered to phytotomy.

His 'Beitrage zur Anatomic der Pflanzen' are divided into two portions; the first treats of the parts surrounding the spiral vessels ; the second of the spiral vessels themselves.

The position and collective form of the component parts of the vascular bundle in the stem of the maize-plant are well described in the first section of the work. It is correctly stated that there is a sheath to the whole bundle composed of strongly thickened fibrous cells, that each of these cells has its own membrane and is entirely closed, and that they resemble the bast and the fibrous elements of the wood of Dicotyledons. The segmented wood-cells and the parenchyma-cells of the wood arranged in rows are incidentally noticed. Under the name of fibrous tubes he included the cells of the sclerenchyma-sheath of many vascular bundles and the true bast and wood-fibres, which latter he says are wanting in the Coniferae. He explained the secondary growth in thickness of rind and bast by the example of the shoot of the vine, in which he correctly distinguished the medullary sheath and the spiral vessels. In herbaceous dicotyledons he found the bundles of vessels to consist of a bast portion and a woody portion, and he attributed the formation of the compact wood of true woody plants to the blending together of the woody portions of these separate bundles.

In discussing the parenchymatous cell-tissue he rejects emphatically and on good grounds the origin of new cells from the granular contents of older ones, which had been the view of Sprengel and Treviranus, as also the theory of Wolff and Mirbel, while he maintains against Mirbel especially, that the separation of fibrous tubes is possible even where no dividing line can be seen between them in the cross section. He considers that both in thin-walled and thick-walled parenchyma the dividing wall is double and the cell-membrane entirely closed. 'It appears,' he continues on p. 86, 'from these observations that cellular substance consists of separate closed tubes, which may be round or oval, or more or less elongated, or almost cylindrical in shape, and these by mutual pressure assume an angular and flattened form, which is either regular like the cells of the comb of bees or more or less irregular. Such an aggregate of separate cells (and here he is certainly quite right) has nothing in common with a tissue, and the word cell-tissue seems therefore less suitable than the term cellular substance, composed of cell-like tubes.' Further on he rejects Mirbel's idea of the existence of visible holes in the walls of cells, and points out that they are not necessary for the movement of sap. The dispute between Mirbel and his opponents respecting the porousness of cell-walls was extended at the same time to the stomata of the epidermis^[8], the slits in them being supposed to be apertures in the epidermis regarded as a simple membrane. Moldenhawer took occasion to examine the anatomy of stomata more closely, and produced the first accurate descriptions and figures of these organs, showing especially that the apertures are not surrounded by a simple border, as most previous observers believed, but lie between two cells, and that therefore they are not examples of the existence of pores in cell-walls, as Mirbel imagined. It may be observed here by the way, that Mirbel afterwards considered stomata to be short broad hairs ; Amici in 1824, Treviranus in 1821, demonstrated their true structure by cross sections, and von Mohl at a later period investigated it thoroughly. Moldenhawer on the present occasion also enquired into the faculty attributed to stomata of opening and closing alternately, which, first observed by Comparetti, was then much discussed by the German phytotomists, and has been made the subject of repeated investigation in modern times. The whole of this discussion was in connection with the question of the pitting of cell-walls, the true nature of which Moldenhawer however never clearly understood.

The peculiar vessels, known as 'vasa propria,' were a stone of stumbling to Moldenhawer, as they were to his predecessors and to many of his successors, because misled by the resemblance in their contents he included under this name forms of very different kinds. A very good description of the soft bast in the vascular bundle of the maize-plant is followed by a notice of the milk-tubes of Musa, the milk-cells of Asclepias which he explains incorrectly, and the milk-vessels of Chelidonium which he understood better. All these 'vasa propria' he took for cellular vessels, formed of tubes opening into one another; but he clearly distinguished the turpentine-ducts from them, and has given a correct figure of such a duct from the pine, though he assumes the existence of a special membrane lying inside the cell-rows which surround it, and lining the passage. Finally he passes on to the intercellular spaces, which he considers to be gaps in the cellular substance, and illustrates by Musa and Nymphaea. He does not notice particularly the narrow interstices which Treviranus had observed traversing the parenchyma.

In the second portion of his work he includes all the vessels found in the vascular bundle of the maize-plant under the term spiral vessels, but he distinguishes the different forms of them well, and especially points out that rings and spirals appear on one and the same vascular tube in different parts of its course, as Bernhardi had already shown. The isolating of the vessels gave him a better opportunity of seeing how they are made up of portions of different lengths than his predecessors had enjoyed, and he proves at some length the existence of a thin closed membrane forming the vessel, but like Hedwig he places the thickenings on the outside. He as little overcame the difficulties of bordered pits as did von Mohl and Schleiden after him. In this case as in others, it was the history of development which first taught the true nature of these formations (Schacht, 1860).

It was mentioned in the Introduction that Moldenhawer may be said to close the first portion of the period from 1800 to 1840, not only because the majority of the questions ventilated up to that time were to a certain extent settled by him, but also because there is no material advance in phytotomy to be recorded for several years after the publication of his work in 1812. It is true that Kieser in his 'Grundzüge der Anatomic der Pflanzen' (1815) attempted a connected exposition of the whole subject, but his book offers nothing really new, being merely a playing with the unmeaning phrases of the current nature-philosophy, while it revived gross errors like Hedwig's doctrine of the presence of lymphatic vessels in the tissue of the epidermis, and made the Mosses consist of conferva-threads. Phytotomy was on the contrary really enriched by the miscellaneous works of Treviranus published in 1821, especially in respect to questions connected with the epidermis, and by Amici's discovery in 1823, that the inter-cellular spaces in plants contain not sap but air, and that the vessels too chiefly convey air. We may quietly pass over the later writings of Mirbel, Schulze, Link, Turpin and others, which appeared after 1812 and before 1830, as our business is not so much with an account of the literature of the subject as with evidence of real advance.

Meyen and von Mohl may be said to have commenced their labours with 1830, and in the course of the succeeding ten years they became the chief authorities on phytotomy, though a highly meritorious work of Mirbel's on Marchantia polymorpha and the formation of pollen in Cucurbita falls as late as 1835. We may even pass over so elaborate a work as the 'Physiologie der Gewachse' of Treviranus (1835-1838), which embraces also the whole of phytotomy, because though its treatment of some of the details is good, it presents its subject virtually from the points of view opened before 1812. This work, though it neglects no part of its subject and contains much useful reference to the works of other observers, was unfortunately out of date at the time of its appearance, for owing to von Mohl's labours an entirely new spirit had entered since 1828 into the treatment of phytotomy.

Though Meyen and von Mohl must be regarded as the chief representatives of phytotomy from 1830 to 1840, yet they are men of very different importance in the science. The essential difference between them cannot perhaps be better shown than by pointing to the fact, that Meyen's labours cannot at present claim more than a historical interest, while von Mohl's earliest investigations between 1828 and 1840, so far from being obsolete, are the sources of our present knowledge, and from them every one must still draw who proposes to cultivate any portion of phytotomy. Meyen's views, in spite of the many investigations which he made himself, are entirely confined within the circle of thought represented by the Gottingen essayists, though in his observations he went beyond them, and even beyond Moldenhawer; but the phytotomical views of these men were from the first no law to von Mohl; he took up an entirely independent position at once with respect even to Moldenhawer and Treviranus, though a longer time certainly elapsed, before he succeeded in freeing himself wholly from Mirbel's authority. For these reasons, and because Meyen's work was interrupted by his death so early as 1840, while von Mohl aided to advance phytotomy for another thirty years, we will speak first of Meyen's labours in that department.

Meyen^[9] is remarkable for the extraordinary number of his written productions. In 1826, at the early age of twenty-two, he wrote his treatise 'De primis vitae phenomenis in fluidis' two years later he published researches anatomical and physiological into the contents of vegetable cells, and in 1830 appeared his 'Lehrbuch der Phytotomie,' founded on his own investigations in every branch of the subject, with many figures on thirteen copper plates very beautifully executed for the time. His industry as a writer was then interrupted by a voyage round the world made in the years 1830-1832, but was again marvellously productive during the last four years of his life (1836-1840); it is difficult to conceive how he found time even for the mechanical part of his work, for in 1836 he published his treatise on the latest advances in vegetable anatomy and physiology, a quarto volume of 319 pages with twenty-two plates, which gained the prize from the Teyler society in Haarlem; the figures are well drawn, the style is that of a practised writer, but the matter of the work is somewhat superficially handled. A year later (1837) appeared the first volume of his 'Neues System der Pflanzenphysiologie,' and two more volumes by the year 1839,—a work also rich in new observations and figures. In the course of the same years (1836-39) he wrote detailed annual reports of the results of investigations in the field of physiological botany, which fill a portly volume, and published in 1837 a prize-essay on the organs of secretion, and in 1836 a sketch of the geography of plants; in 1840 appeared a treatise on fructification and polyembryony, and a posthumous work on vegetable pathology in 1841. The number of works thus given to the world between the years 1836 and 1840, though partly prepared before that period, is so unprecedented, that it is impossible for the composer to have maturely meditated his facts or their inner connection, and the study of his writings shows that he was often too hasty in propounding new views, and in rejecting or accepting the statements of others. The style is perspicuous and flowing, and animated by a genuine scientific spirit; but the expressions are often inexact, the ideas not unfrequently immature, and points of fundamental importance are sometimes neglected for unimportant and secondary matters. These faults are the result of hasty production ; we must set against them conspicuous merits; Meyen had an eye open to every question in phytotomy and left nothing unnoticed, while he made it his constant aim to give clear general views of his subject as a connected whole, and enable his reader to see his way in every direction, in order to make phytotomy and vegetable physiology accessible to wider circles of scientific men; the like praise is due to his drawings from the microscope which are beautifully executed; they present to the reader not the small fragments of earlier phytotomic works but whole masses of tissue so connected together, that it is possible to gain some insight into the disposition of the different systems of tissue and their mutual relations. The superiority of Meyen's drawings of 1836 as compared with those of 1830 is very striking, though he used the same microscope in both cases and the same magnifying power of two hundred and twenty times.

To learn what were Meyen's independent contributions to the advance of phytotomy, we must turn to his 'Phytotomie' of 1830; for in his later works and especially in the 'Neues System der Physiologic' of 1837 he was able to avail himself of von Mohl’s earliest and searching investigations; these necessarily influenced his views, though he always assumed the character of a rival and opponent of von Mohl, and treated not only Treviranus and Link, but even Kieser and men of his stamp, as entitled to equal rank with him. And as in his later writings he was reluctant to acknowledge von Mohl's services to science and overlooked their fundamental importance, so in his earlier work in 1830 he often appears as an assailant of Moldenhawer and tries to set up Link's authority against him; we find to our astonishment in the first volume of the 'Neues System' a dedication to Link as the 'founder of German vegetable physiology.' The position of a scientific man in relation to his science as a whole is certainly most simply and clearly defined by his judgment on the merits of his contemporaries and predecessors, and we may conclude from what has now been said that Meyen moved within the circle of ideas of the Gottingen prize-essays, and did not clearly see the importance of the points of view opened by Moldenhawer and von Mohl; though it must always be allowed that Meyen working independently far outstripped Link on his own path.

If it was our purpose to write a biography of Meyen, we should have to go through his works, and show the steps by which his views arrived at clearness and precision; it is sufficient in this history to show what was peculiar and original in his general conception of the problems of phytotomy. This appears most plainly in the 'Phytotomie' of 1830; and we may base our historical survey on that work because its views are in the main those of the first volume of the 'Neues System' which appeared seven years later, and still more because a detailed examination of the later publication would involve us in a lengthy discussion on Meyen's scientific relation to von Mohl. It is less important in this place to give an estimate of Meyen's character as a man of science than to show, how in the year 1830, when Mohl was beginning to apply himself to phytotomy but as yet exercised no important influence on opinion, views on the structure of plants were formed by one who gave himself up to its study with decided ability and great zeal ; in this way we shall gain a standard by which to judge of the advance made chiefly by von Mohl and in part by Mirbel during the succeeding ten years. In judging of Meyen's book, we must not forget that it was written when he was only twenty-five or twenty-six years old, and that it is under any view of it a remarkable performance for so young a man.

Meyen adopted three fundamental forms of elementary organs in plants; cells, spiral tubes, and sap-vessels ; systems, he says, are formed by union of similar elementary organs; hence there is a cell-system, a spiral tube-system, and a system of sap-vessels (vascular system). We see at once by this classification how closely he follows the ideas formed before Moldenhawer. The establishment of these three systems is a retrograde step, since Moldenhawer had already clearly distinguished between vascular bundles and cell-tissue. Meyen then discusses each system at length and shows how they are grouped together. He lays great stress, as he did also at a later period, on the difference in the characteristic forms of cell-tissue, for which he introduced the names merenchyma, parenchyma, prosenchyma and pleurenchyma. These he calls regular cell-tissue, the shapes of the cells being like geometrical bodies, in opposition to the irregular tissue of Fuci, Lichens and Fungi. It is a decided improvement on former practice, and one that marks his later works also, that in connection with the structure of the solid cell-fabric he discusses the contents of cells in a special chapter, in which first the matter in solution, then the granular bodies with organized structure are considered, though with the latter he classes not only starch-grains, chlorophyll-corpuscles and the like, but also the spermatozoa in pollen-grains and layers of thickening matter projecting on the inside of cell-walls, such as the spiral bands in the elaters of Jungermannieae and several similar formations. He describes the crystals in vegetable cells at some length, and finally discusses the movement of the cell-contents ('sap'), not omitting that of rotation in the Characeae as observed by Corti, and in other water-plants. The chapter on intercellular spaces also shows considerable advance on the views which obtained in 1812; Meyen calls it an account of the spaces produced in cell-tissue by the union of the cells; the true intercellular passages filled with air are here distinguished from receptacles of secretions, resin- passages, gum-passages, oil- passages, and secretion-receptacles of the nature of cavities. The large air-passages and gaps, such as occur in water-plants, are a third form of intercellular space; his air-canals in the wood of oak filled with cell-tissue are obviously vessels filled with the substance known as thylosis. The form of the cells in the tissue he thinks is not due to mutual pressure, and he rejects Kieser's view that the ideal fundamental form of cells must be a rhombododecahedron; but he thinks there is a significant resemblance between the shape of cells and that of basaltic columns.

In dealing with the spiral tube-system he first discusses the spiral fibre, which appears, he says, either detached between the cells or inside them as well,—an account of the matter decidedly inferior to those of Bernhardi and Treviranus. The spiral tubes are, he says on page 225, cylindrical or conical bodies formed of spiral fibres which are afterwards surrounded by a delicate membrane. He puts annular, reticulated, and pitted vessels together as metamorphosed spiral tubes. His explanation of these forms cannot well be understood except by supposing that he assumed an actual metamorphosis in time in accordance with the view of Rudolphi and Link ; but he afterwards in his 'Neues System,' i. p. 140 declares this to be a misunderstanding, though his real meaning is still doubtful; the obscurity attending the doctrine of metamorphosis did not fail to cause misunderstandings in phytotomy, as it did in the morphology of organs. Meyen makes only the striated and pitted vessels in the wood convey air, the true spiral vessels sap. That vessels are formed from cells, as Mirbel had already maintained and Treviranus had partly observed, Meyen intimates indeed, but not with an air of entire conviction.

The different forms of laticiferous organs are examined under the head of the ' system of circulation in plants.' Meyen sees in this system the highest product of the plant, being fully persuaded with Schulz, that the latex (milk), or as he also terms it the life-sap, is in constant circulation, like the blood in the veins. He gives a more summary account than is his wont of the course of the laticiferous organs, but bestows more care on the nature of the latex, and on the structure of the receptacles that contain it. That some of these are produced by cell-fusion, that others represent intercellular spaces, while others again are long branched cells, was not known to Meyen or even to later phytotomists before 1860.

This condensed account of the contents of Meyen's 'Phytotomie' shows a striking mixture of advance and retrogression, when compared with what had been achieved before his time; by the side of the fact established by Treviranus that the epidermis does not consist of a single membrane but of a layer of cells, to which Meyen assents, we find the gross mistake of taking the guard-cells of stomata for cuticular glands, the apertures in which he considers as of secondary importance. It is still more striking that Meyen expressly rejects on page 120 the fact established two years before by von Mohl that the pits of parenchyma are thinner spots, and treats the various pit-formations of the cell-wall as raised portions of the surface.

In the first volume of his later work the 'Neues System,' Meyen gives a detailed account of phytotomy, which accords on the whole with the scheme developed in the book we have been examining, and as might be expected he corrects many errors, adduces many new observations, and introduces us to many steps in advance of former knowledge ; we shall recur to some of his later views in ensuing portions of this history with which they are more in connection, remarking only here, that Meyen paid more attention to the contents of the cell than his contemporaries, and especially made a number of observations on the streaming movement, without however recognising the peculiar nature of the protoplasm which is its substratum. The cell-wall, which he had once considered to be homogeneous, he afterwards believed to be composed of fine fibres, a view resting on correct but insufficient observation and afterwards set right by von Mohl and Nageli.

It is scarcely possible to imagine a more striking contrast between two men pursuing the same science than that between Meyen and his much more important contemporary Hugo von Mohl; Meyen was more a writer than an investigator; von Mohl wrote comparatively little in a long time, and only after most careful investigation; Meyen attended more to the habit, the collective impression produced by objects seen with the microscope, von Mohl troubled himself little about this, and always went back to the foundation and true inner connection of the structural relations; Meyen quickly formed his judgment, von Mohl often delayed his even after long investigation; Meyen was not critical, though always prone to opposition, in von Mohl the critical power much overweighed that of constructive thought. Meyen has not so much contributed to the definitive settlement of important questions, as brought to light manifold phenomena, and so to speak accumulated the raw material; von Mohl on the other hand aimed from the first at penetrating as deeply as possible into vegetable cell-structure, and employing all the anatomical facts in framing a coherent scheme.

We have already called attention to Hugo von Mohl's ^[10] pre-eminent position in the history both of this and also of the succeeding period. Occupying himself for the most part with phytotomical questions which had been already investigated, he made the solid framework of cellulose the object of special and searching examination, and completed the work of his predecessors on this subject; he thus laid a firm foundation for the researches into the history of development afterwards undertaken by Nageli. Von Mohl, like former phytotomists, generally connected his researches into structural relations with physiological questions; but there was one great and unmistakable difference; he never forgot that the interpreta tion of visible structure must not be disturbed by physiological views; he used therefore his thorough physiological knowledge chiefly to give a more definite direction to his anatomical researches, and to illustrate the connection between structure and function in organs. By scarcely any other phytotomist was the true relation between physiological and anatomical research so well understood and turned to such practical account as by von Mohl, who was equally averse to the entire separation of phytotomy from physiology, and to the undue mixing up of the one with the other, which has led his predecessors, Meyen especially, into misconceptions.

His anatomical researches profited by his extraordinary technical knowledge of the microscope; he could himself polish and set lenses, which would bear comparison with the best of their time. As the majority of botanists from 1830 to 1850 had little knowledge of the kind, there was no one so well qualified as von Mohl to give instruction in short treatises on the practical advantages of a particular instrument, to remove prejudices and finally as in his 'Mikrographie' (1846) to give detailed directions for the management of the instrument.

But his mental endowments were of course of the higher importance, and it is difficult to imagine any more happily suited to the requirements of vegetable anatomy during the period from 1830 to 1850. At a time when men were building fanciful theories on inexact observations, when Gaudichaud was once more explaining the growth in thickness of the woody portions of the plant after the manner of Wolff and Du Petit-Thouars, when Desfontaines' account of the endogenous and exogenous growth of stems was still accepted, when Mirbel was endeavouring to support his old theory of the formation of cells by new observations and beautiful figures, when Schulz Schulzenstein's wildest notions respecting laticiferous vessels were being rewarded with a prize by the Paris Academy, when Schleiden's hastily adopted views respecting cells and fertilisation appeared on the scene with great external success, von Mohl, for ever going back to exact observation, was cutting away the ground from under ill-considered theories in careful monographs, and at the same time bringing to light a mass of well-established facts leading to further and serious investigation. These theories have now only a certain historical interest, while von Mohl's contemporaneous works are still a rich repertory of useful observations, and true models of clear exposition.

His written productions were preceded by a careful study of all branches of botanical knowledge and the auxiliary sciences. That he not merely acquired knowledge in this way, but trained the powers of his understanding also, is shown by the striking precision and clearness of his account of his first investigations. At a time when the nature-philosophy and Goethe's doctrine of metamorphosis in a distorted form were still flourishing, von Mohl in spite of his youth approached the subjects of his investigation with a calmness and a freedom from prepossessions, which are the more remarkable when we observe that his friend Unger was at first quite carried away by the stream, and only slowly managed to reach the firm ground of genuine inductive enquiry.

Owing to the extravagances and aberrations with which he made acquaintance as a young man in the nature-philosophy, von Mohl contracted an aversion to all philosophy, evidently taking the formless outgrowths from the doctrines of Schelling and Hegel for something inseparable from it, as we may gather from his address at the opening of the faculty of natural history in Tübingen, which had been separated at his instance from that of philosophy. His dislike to the abstractions of philosophy was evidently connected with his distaste for far-reaching combinations and comprehensive theories, even where they are the result of careful conclusions from exact observations. Von Mohl was usually satisfied with the establishment of separate facts, and in his speculative conclusions he kept as closely as possible to what he had actually seen, for instance, in his theory of the thickening of cell-walls; and where new views opened before him as a result of his exact observation, he cautiously restrained himself and was generally content to hint at matters which bolder thinkers afterwards proceeded to investigate; such a case occurred in his examination of cell-membranes by polarised light. Hence we miss to some extent the freer flight of imaginative genius in von Mohl's scientific labours; but there is more than sufficient compensation for this want in the sure and firm footing which he offers to the reader of his works; if we pass from the study of the writings of phytotomists before 1844 to those of von Mohl, we are sensible of one predominant impression, that of security ; we have the feeling that the observer must have seen correctly because the account which he gives of the matter before us seems so thoroughly natural and almost necessarily true, and all the more because he himself notices all possible doubts, and lets those which he cannot remove remain as doubts. In these points von Mohl's style resembles that of Moldenhawer, but in von Mohl it attains to a mastery which is wanting in the other.

There is an evident connection between von Mohl's dislike of far-reaching abstractions and philosophic speculation on the results of observation and the fact, that in the course of more than forty years' unintermitted application to phytotomy he never composed a connected general account of his subject. His efforts as a writer were confined to monographs usually connected with questions of the day or suggested by the state of the literature. In these he collected all that had been published on some point, examined it critically, and ended by- getting at the heart of the question, which he then endeavoured to answer from his own observations.

For the purpose of these observations he looked about in every case for the most suitable objects for examination, a point to which former phytotomists, with the exception of Moldenhawer, had paid little attention; he then studied these objects thoroughly, and thus prepared the way for the examination of others, which presented greater difficulties. Every monograph of this kind was a nucleus, round which a larger number of observations might afterwards gather. In a long series of such solid productions he treated conclusively all the more important questions of phytotomy.

Von Mohl's extraordinary carefulness was not however able to guard him, calm observer though he was, from some serious mistakes, at least in his earlier years, such as those which occur in his first theory of intercellular substance (1836), and in his earliest views on the nature of the cell-membrane of the pollen-grain (1834). These and some other errors on the part of a gifted and truly inductive enquirer are instructive, since they show that observation without any ground-work of theory is psychologically impossible; it is a delusion to suppose that an observer can take the phenomena into himself as photographic paper takes the picture; the sense-perception encounters views already formed by the observer, preconceived opinions with which the perception involuntarily associates itself. The only means of escaping errors thus produced lies in having a distinct consciousness of these prepossessions, testing their logical applicability and distinctly defining them. When von Mohl laid down his theory of intercellular substance, there evidently floated before his mind indistinct, half-conscious ideas of the kind that Wolff and Mirbel entertained of the structure of the vegetable cell; and as he considered the cell-membrane of the pollen-grain to consist of a cell-layer, he summarised its obscure structural relations under the then very obscure conception of the cell. As a true investigator of nature, who adheres always and firmly to the results of further observation, and endeavours to clear his ideas by their aid, conceding only a relative value to every view, von Mohl soon escaped from these errors, and him- self supplied proofs of the incorrectness of his former opinion. The number of really erroneous statements in his works is wonderfully small considering the very large number of investigations in which he engaged.

In examining the part which von Mohl played in the general development of phytotomy we can distinguish satisfactorily two periods in his scientific career, the first of which extends from 1827 to about 1845. Before 1845 he was acknowledged to be the first of phytotomists, decidedly superior to all rivals; his authority, though often attacked by unimportant persons, grew from year to year. This period may be said to close with the publication of his 'Vermischte Schriften' in 1845. Up to that time investigations into the form of the solid framework of cell-membrane had chiefly attracted the interest of phytotomists, and in this subject there was no one who could measure himself with von Mohl. Yet he began soon after 1830 to study the history of development in plants; in 1833 he described the development of spores in a great variety of Cryptogams, in 1835 the multiplication of cells by division in an alga, and the cell-division in the formation of stomata in 1838; in this period appeared Mirbel's first observations on the formation of pollen-cells (1833). Von Mohl too was the first, if we disregard Treviranus' somewhat imperfect notices of the origin of vessels in 1806 and 1811, who explained the history of the development of those organs; and his theory of the thickening of cell-membranes, the principles of which are to be found in his treatise on the pores in cellular tissue (1828), may also be regarded as a mode of conceiving the sculpture of the cell-membrane from the point of view of the history of development.

Ever since 1838 Schleiden had raised the history of development to the first rank in botanical investigation, but he had proposed a thoroughly faulty theory of cell-formation, to which von Mohl at first at least did not withhold his assent in spite of previous and much better observations; but after 1842 Niigeli devoted himself still more thoroughly and with more lasting results to the study of the development both of vegetable cells and tissue-systems, and of the external organs. He introduced new elements into phytotomic research, and it soon became apparent that even the questions hitherto examined must be grappled with in a different fashion. Von Mohl did not hold aloof from the new direction, but completed a series of excellent investigations connected with the new questions in the theory of cell-formation. The most important of these was his enquiry into the nature of protoplasm, to which he gave the name still in use. In his treatise, 'Die Vegetabilische Zelle,' which came out in 1851 in Wagner's Dictionary of Physiology, he even gave an excellent account of the modern theory of cell-formation; but notwithstanding all this, and the great authority which he rightly continued to enjoy, he was no longer the guide who led the way in the domain of phytotomy, as he had been before 1845.

His zeal as an observer had at all times been chiefly attracted to the solid framework of vegetable structure in its matured condition, though a number of his most important works were devoted to the study of cell-contents.

Except in his 'Anatomic der Palmen' (1831), where he ex- pended much and to some extent even unnecessary labour on figures representing the general appearance of the tissue (histologic habit), von Mohl's microscopic drawings do not aim at giving the collective impression, but at facilitating the understanding of the delicate structure of single cells and their combinations by aid of the simplest possible lines. He always despised pictures from the microscope, such as were introduced at a later time by Schacht, a kind of artistic restoration of the originals and to some extent a playing with science; and in his later publications he was more sparing of illustrations or omitted them altogether, in proportion as he acquired the power of giving clear verbal explanations of even difficult structural conditions.

Von Mohl's scientific activity was so wonderfully productive thatjt is not easy to present the reader with a clear account of it; but we must endeavour at least to furnish such a summary of its chief results as may serve to give a general idea of his importance in the history of our science. We may here pass over such of his treatises as do not bear on the main questions of phytotomy, and notice only those that relate to the structure of the solid framework of plants, because the historical significance of his investigations into the history of development can only be understood in connection with the questions to be treated in the following chapter. But we shall not limit our- selves to publications which appeared before 1845, though we may be thus compelled to notice researches which in succession of time belong to the next period, and indeed almost to the present moment.

1. The view that the cell is the sole and fundamental element in vegetable structure had been already maintained by Sprengel and Mirbel, but not supported by exact observations. Treviranus too had shown that the vessels in wood are formed by the union of rows of cell-like tubes, but he had never arrived at a thoroughly clear conception of the matter. On the one side was the theory that the plant consists entirely of cells, on the other, and for long the old and strange view, that the spiral thread was an independent elementary organ of vegetable structure, a view which Meyen still maintained in 1830. Von Mohl must be regarded as the first who took up the all-important position, that not only the fibrous elements of bast and wood, which had long been considered to be elon- gated cells, but the vessels of the wood also are formed from cells ; and we may on this point give great weight to his own assertion that he was the first who observed the formation of vessels from rows of closed cells. This discovery happened in the year 1831, and he describes distinctly, though briefly, the decisive observations in his treatise on the structure of the palm-stem. At the points of constriction in the vessels he saw the dividing walls, the existence of which had been denied by all former phytotomists; 'these dividing walls,' he says, 'are entirely different from the rest of the membranes of the plant, being formed of a network of thick fibres with openings between them.' He studied the history of the development of these vessels both in palms and in dicotyledonous plants. ' In the young shoot,' he says, 'are found at the spots, where afterwards there are large vessels, perfectly closed large cylindrical tubes with a transparent and very delicate membrane.' He then shows how by degrees the sculpture peculiar to the walls of vessels is formed on the inside of these tubes, and he takes the opportunity of saying that a metamorphosis in time from one form of vessel into another is entirely out of the question, as Treviranus also and Bernhardi had maintained. 'The dividing walls (transverse septa),' he continues, 'are formed in a precisely similar manner to the side-walls of vessels; only the original tender membrane of the septa is usually lost in the meshes of the network of fibres.' From that time no phytoto-mist capable of an independent judgment has had any doubt with regard to this view of the formation of the vessels in wood. It is however striking enough that von Mohl, who thought it so important to show that the cell is the sole foundation of vegetable structure, never extended the proof to milk-vessels and other secretion-canals in order to show whether and how these also are formed from the cells. In his treatise on the vegetable cell (1851) he still expressed doubt about Unger's assertion, that the milk-vessels are also formed from rows of cells that coalesce with one another, and held rather to the view of an anonymous writer in the ' Botanische Zeitung ' of 1846, page 833, that these vessels are membranous linings of gaps in the cell-tissue. He might well lose his taste for the exam- ination of these and similar organs after Schultz Schultzenstein had by his various treatises, written after 1824, on the so-called vital sap and the circulation which he attributed to it, made this part of phytotomy a very quagmire of error, and had not refrained from replying in an unbecoming manner to von Mohl, who repeatedly opposed his views; moreover Schultz's essay 'Ueber die Circulation des Lebenssafter' (1833), which teems with absurdities, had received a prize from the Academy of Paris.

2. The growth in thickness of the cell-membrane, and the sculpture caused by it was a subject that is more or less connected with most of von Mohl's writings. He developed the chief features of his view in 1828 in his first work, 'Die Poren des Pflanzengewebes!' The way in which he represented to himself the growth in thickness of cell-membranes at a later time may be expressed as follows. All elementary organs of a plant are originally very thin-walled perfectly closed cells, which in the tissue are separated by walls formed of two laminae^[11]; on the inside of these primary cell-membranes, after they have ceased to increase in circumference, new layers of membranous substance are formed, which lying one upon another adhere closely together, and represent the whole amount of secondary thickening layers; on the inner side of the membrane thus thickened by apposition there may usually^[12] be perceived a tertiary layer of a different character.

But there are certain sharply defined spots on the original cell-wall, where this thickening does not take place; in such spots the cell is still bounded only by the primary membrane; it is these thin spots which bear the name of pits, and which Mirbel, and in some cases Moldenhawer, took for holes, but von Mohl considered that it was only in very exceptional cases that they were really changed into holes by resorption of the thin primary wall. In accordance with this theory, the spiral, annular, and reticulated vessels are produced by deposition of thickening matter in the form suitable to each case on the inside of the originally smooth thin cell-wall. But like Schleiden and other phytotomists, von Mohl was not quite clear in his views either of the origin or mode of formation of matured bordered pits; it was supposed that the two laminae of the dividing wall parted from one another at certain spots in such a manner that a lenticular hollow space was formed between them, and that this space answered to the outer border of the pit, while the inner border was the result of ordinary pit-formation. This view, which could be shown to be incorrect by the history of development, arose in fact from inexact observation,—a rare case with von Mohl; the true course of events in the formation of bordered pits was first described by Schacht in 1860.

It was mentioned above, that Meyen in his 'Neues System der Physiologic' of 1837, i. p. 45, made cell-membranes consist of spirally wound fibres; von Mohl had described in 1836 the structural relations of certain long fibrous cells of Vincaand Nerium, which might be provisionally explained in this way; he was led by Meyen's ideas on the subject to a renewed and minute examination of the more delicate structure of the cell-membrane in 1837; he first of all cleared the ground round the question, by distinguishing the cases in which real spiral thickenings lie on the inner side of the membrane, from those in which the membrane is smooth on the outside, but shows an inner structure of fine spiral lines; in these cases he assumed a peculiar arrangement of the molecules of cellulose, and endeavoured to illustrate the possibility of such a disposition by the phenomena of cleavage in crystals ('Vermischte Schriften,' p. 329); but he did not succeed in explaining these very delicate conditions of structure, which we now call the striation of the cell-membrane, so clearly as Nägeli afterwards did in connection with his molecular theory.

3. The question of the substance and chemical nature of cell-membranes was intimately connected with von Mohl's theory of its growth in thickness ; he was engaged in 1840 in minutely studying the reactions which various cell-membranes exhibit with iodine solution under different conditions, a question on which Schleiden and Meyen had recently disagreed; von Mohl arrived at the result, that iodine imparts very various colours to vegetable cell-membrane, according to the quantity in which it is absorbed; a small amount produces a yellow or brown, a larger a violet, a still larger a blue tint; this depends partly on the extent to which the membrane is capable of distcntion; the blue colour especially depends on the absorption of a sufficient quantity of iodine. Greater interest, excited at first by a very important work by Payen^[13] in 1844, was taken in the question of the chemical nature of the solid framework of the vegetable body, in which it was shown that the substance of all cell-membranes exhibits a similar chemical composition when freed from foreign elements. Payen considers that this material, cellulose, is present in a tolerably pure form in the membranes of young cells, but is rendered less pure in older ones by 'incrusting substances,' whose presence changes the physical and chemical characters of cell-membranes in various ways. These incrusting substances may be more or less removed by treating the membranes with acids, alkalies, alcohol, and ether, while other inorganic matters remain behind after combustion as an ash-skeleton. This theory, which has been more perfectly worked out in modern times, was soon afterwards met by Mulder with the assertion, that a large part of the layers composing the walls of cells consist from the first of other combinations and not of cellulose; he at the same time deduced from this view certain conclusions respecting the growth in thickness of cell-walls. He and Harting, relying on microscopic examination, maintained that the innermost tertiary layer in thickened membranes is the oldest, and that the other layers are deposited on the outside of this, and are not composed of cellulose. Von Mohl opposed this view decidedly and successfully in the Botanische Zeitung of 1847; he likewise in his work on the vegetable cell (p. 192), refuted the view of the varying substance of cell-membrane, which Schleiden had founded on some obscure chemical grounds.

It would carry us much too far to enter into the details of this scientific dispute; Payen's view of the chemical nature of the vegetable cell-wall, which von Mohl adopted and elaborated, has maintained itself to the present day, and is generally considered to be the true one; on the other hand, the foundations of von Mohl's theory of growth in thickness were shaken in 1858 by Nägeli's observations, and we may say that on the whole it has been for ever superseded. It has been nevertheless of great service in the development of our views on cell-structure in plants; keeping closely to the facts directly observed, it served to bring almost all the conditions of the sculpture of cell-walls under one point of view, and to refer their formation to one general and very simple scheme; every such theory helps to advance science, because it facilitates mutual understanding; in this case, when Nägeli proposed his more profound theory of intussusception, the understanding of it was essentially assisted by a previous exact knowledge of von Mohl's theory in its principles and results. In conclusion it may be mentioned here that von Mohl afterwards in his investigation into the occurrence of silica in cell-membranes made a large and important addition to the knowledge of their more delicate structure, and of the way in which incrusting substances are deposited in them (Botanische Zeitung, 1861).

4. The views of phytotomists on the so-called intercellular substance during the twenty years from 1836 to 1856 were closely connected with the older theories of cell-formation, but were opposed to the modern doctrine of the cell founded by Nägeli in 1846. Von Mohl himself had introduced this idea for the first time into the science in 1836 in one of his earlier and inferior essays, 'Erläuterung meiner Ansicht von der Structur der Pflanzensubstanz,' rather in opposition to than in connection with his own theory of the growth and structure of cell-walls. Setting out from modes of formation of cell-membranes in some Algae, difficult to understand and in some respects quite peculiar, von Mohl believed that he saw in many cases in the higher plants also between the sharply-defined membranes, which bound the cell-spaces and which he regarded as the entire cell-membranes, a substance in which the cells are imbedded, for such is its appearance when it is largely developed; when it lies in small quantity only between cells in close apposition, it looks like a thin layer or cement. After Meyen in his 'Neues System,' pp. 162, 174 had declared against this view in 1837, von Mohl too abandoned it more and more, and afterwards limited the occurrence of intercellular substance to certain cases, being convinced that much that he had before taken for it consisted only of layers of secondary thickening, between which he still saw the primary lamina of the cell-membrane. The theory of intercellular substance was taken up and further developed by other phytotomists, by Unger especially in the Botanische Zeitung for 1847, p. 289, and afterwards chiefly by Schacht; Wigand came forward as an opponent of it in 1854 in his 'Botanische Untersuchungen,' p. 65, and logically following out von Mohl's theory of the cell-membrane declared the thin layers of intercellular substance as well as the cuticle, which had been first correctly distinguished by von Mohl, to be laminae of primary cell-membrane, the substance of which had undergone profound chemical change. These ideas also of the intercellular substance and the cuticle assumed an entirely different aspect when Nägeli introduced his theory of intussusception.

The limits imposed on this history render it necessary to be content with these indications of von Mohl's share in the working out of the theory of cells in its connection with the structure of the solid framework of cell-membrane; we shall return again to his observations on the formation of individual cells.

5. Forms of tissue and comparative anatomy. Phytotomy up to 1830 had been weak in its classification of tissues, in its ideas as to their arrangement, and consequently in its histological terminology; the inconvenience arising from this state of things was most distinctly felt when it became necessary to compare the structure of different classes of plants, Cryptogams, Conifers, Monocotyledons and Dicotyledons, and to establish their true differences and actual agreements. How little phytotomy had advanced in this respect is shown plainly in the account of tissues given by Meyen in his 'Neues System' in 1837. To von Mohl belongs the merit of having perceived at an early period in his scientific career, and more clearly than his contemporaries, the value of a natural and sufficient discrimination of the various forms of tissue, and the necessity of obtaining a correct view of their relative disposition ; he thus showed the way to an understanding of the general structure of the higher plants, and rendered it possible to make a scientific comparison of the structure of different classes of plants.

Von Mohl, like Moldenhawer long before, showed from the first a correct apprehension of the peculiar character of the vascular bundles as compared with other masses of tissue. He, too, examined them first in Monocotyledons, and gave an account of them in his treatise on the structure of Palms (1831), and also in his later essays on the stems of Tree-ferns, Cycads, and Conifers and on the peculiar form of stem in Isoetes and Tamus elephantipes, to be found in his 'Vermischte Schriften' of 1845. His just conception of them as special systems composed of various forms of tissue has made his account clear and intelligible, and his whole treatment of the subject appears new in comparison with that of every previous writer except Moldenhawer. If these labours of von Mohl are surpassed in value by later studies of the history of development, they served for the time as a nucleus for further investigations, especially into the nature of stems. It contributed in a high degree to a correct insight into the structure of the stem, that von Mohl, agreeing in this with Moldenhawer, distinguished the portion belonging to the wood from the portion belonging to the bast in the vascular bundles, and regarded both as essential constituents of a true vascular bundle. Not less important were his enquiries into the longitudinal course of the vascular bundle in the stem and leaf, which showed that in the Phanerogams the bundles in the stem are only the lower extremities of the bundles, the upper extremities of which bend outwards into the leaves, and that the Monocotyledons and Dicotyledons agree in this particular, though the course of the bundle differs considerably in the two cases. He obtained an important result in this respect in his researches on palm-stems in 1831, when he proved the incorrectness of the distinction between endogenous and exogenous growth in thickness, which had been laid down by Desfontaines, and even employed by De Candolle in framing his system. According to Desfontaines, the wood of Monocotyledons appears as a collection of scattered bundles, of which those that run out above into the leaves come from the centre of the stem. From this very imperfect observation he deduced the view, that the bundles of vessels in Monocotyledons originate in the centre of the stem, and that they continue to be formed there, until the older hardened bundles in the circumference form so solid a sheath that they withstand the pressure of the younger; then all further growth in thickness must cease, and hence the columnar form of the monocotyledonous stem. This doctrine found general acceptance, and was employed by De Candolle to divide vascular plants into Endogens and Exogens, in accordance with the very general inclination felt in the first half of the present century to distinguish the great groups of the vegetable kingdom by anatomical characters. It is true that Du Petit-Thouars had already shown that some monocotyledonous stems have unlimited growth in thickness; neither his nor Mirbel's later observations succeeded in shaking the theory, the adherents of which met such cases by assuming a peripherical as well as a central growth. Then von Mohl in the treatise above-mentioned demonstrated the true course of the vascular bundles in the stem of Monocotyledons, and at once did away with the whole theory of endogenous growth in the opinion of all who were capable of judging, though some even eminent systematists for a long time maintained the old error. The results which von Mohl obtained from his study of the comparative anatomy of the stem, rested mainly on careful observation of the mature tissue-masses, and when he studied the history of development, he was not in the habit of going back to the very earliest and most instructive stages. Hence he failed to explain fully the real points of agreement and difference of structure between Tree-ferns and other Vascular Cryptogams and Phanerogams, and in like manner he stopped half-way when engaged in explaining the secondary growth in thickness of dicotyledonous stems from the nature of their vascular bundles, and the formation of cambium. The account of growth in thickness which he still gave in 1845 ('Vermischte Schriften,' p. 153), and which rests less on observation than on an ideal scheme, is highly obscure, and even in the treatise which he published in the Botanische Zeitung in 1858 on the cambium-layer of the stem of Phanerogams, and in which he criticises the newer doctrines of Schleiden and Schacht, the subject is far from being fully cleared up, though the views there advocated are decidedly superior to his former ones. A satisfactory conclusion with respect to growth in thickness of the woody body and of the rind was not reached till the history of development in vegetable histology began to be more thoroughly studied.

As von Mohl had from the first laid special stress on the peculiar character of the vascular bundles as compared with other tissue-masses, so he perceived that the structure of the epidermis and of the different forms of exterior tissue was thoroughly characteristic, and he succeeded in arriving at a clearer understanding of the matter in this case than in the other. Very confused ideas had prevailed on the subject before he took it up, and we owe to him the best and most important knowledge which we at present possess. Especially important were his researches into the origination and true form of stomata (1838 and 1856), and into the cuticle and its relation to the epidermis (1842 and 1845). He brought entirely new facts to light by his study of the development of cork and the outer bark in 1836; these tissues had scarcely been examined with care till then, and their formation and relation to the epidermis and the cortical tissue were quite unknown. In the latter treatise, one of his best, the difference between the suberous periderm and the true epidermis was first shown, the various forms of the periderm were described, and the remarkable fact established that the scaling of the bark was due to the formation of fine laminae of cork, which, penetrating gradually into the substance of the cortex, withdraw more and more of it from its connection with the rest of the living tissue, and as they die off form by their accumulation a rugged crust, which is the outer bark surrounding most thick-stemmed trees. The investigation was so thorough and comprehensive, that later observers, Sanio especially in 1860, could only add to it some more delicate features in the history of the process. In the same year appeared his enquiry into the lenticels, where von Mohl however overlooked what Unger discovered at the same time ('Flora,' 1836), namely, that these forms arise beneath the stomata; but he at once corrected Unger's hazardous supposition that the lenticels are similar forms to the heaps of gemmae on the leaves of the Jungermannieae. Unger, for his part, was not long in adopting von Mohl's explanation of the lenticels as local cork-formations.

Since von Mohl thus distinctly brought out the special character of the vascular bundles and of the different forms of epidermal tissues, it must excite surprise that he, like former phytotomists, did not find himself under the necessity of framing some conception of the rest of the tissue-masses in their peculiar grouping as a whole, as a special system, and of classifying and suitably naming the different forms that compose them, though his examination of Tree-ferns would seem to have offered him an occasion for doing so. Von Mohl, like his contemporaries, was satisfied with calling everything that is neither epidermis, cork or vascular bundle, parenchyma, without distinctly defining the expression.

Here we leave von Mohl and his labours for the present, to return once more in the following chapter to the share which he took in the further progress of phytotomy. We shall perhaps best realise his importance in the history of the science, if we try to think of all that we have now seen him doing for it as still undone. There would then be a huge gap in modern phytotomic literature, which must have been filled up by others before there could be any further addition to the knowledge of cells and tissues founded on the history of their development; for it can hardly be conceived that the advance to which we owe the present condition of vegetable anatomy, could have been based upon ideas such as those of Meyen, Link, and Treviranus, without von Mohl's preliminary discoveries.

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1. See P. Harting, 'Das Mikroskop,' § 433 and 434.
2. Johann Jakob Bernhardi, born in 1774, was Professor of Botany in Erfurt, and died there in 1850.
3. Karl Asmus Rudolphi, born at Stockholm in 1771, was Professor of Anatomy and Physiology in Berlin, and died there in 1832.
4. Heinrich Friedrich Link was born at Hildesheim in 1767, and became Doctor of Medicine of Göttingen in 1788. In 1792 he became Professor of Zoology, Botany, and Chemistry in Rostock, Professor of Botany in 1811 in Breslau, and in 1815 in Berlin, where he died in 1851. He was a clever man of very varied accomplishment, but not a very accurate observer of details, and was held in estimation by many chiefly as a good teacher and philosophic author of popular works on natural science. He was one of the few German botanists in the early part of the present century who aimed at a general knowledge of plants, and combined anatomical and physiological enquiries with solid researches in systematic botany. Of his many treatises on all branches of botanical science, zoology, physics, chemistry, and other subjects, his Göttingen prize essay must be considered to have contributed most to the advancement of science. Von Martius somewhat overrates his scientific importance in his 'Denkrede auf H. F. Link' in the 'Gelehrte Anzeigen,' Munchen (1851), 58-69.
5. Ludolf Christian Treviranus, born at Bremen in 1779, became Doctor of Medicine of Jena in 1801, and practised at first in his native town, where he became a teacher at the Lyceum in 1807. In 1812 he accepted the professorship in Rostock vacated by Link, and was again his successor in Breslau. In 1830 he exchanged posts with C. G. Nees von Esenbeck, who was a professor in Bonn; he died in that town in 1864. In the first part of his life he occupied himself chiefly with vegetable anatomy and physiology, afterwards with the determination and correction of species. His first works, which are noticed in the text, and the treatises on sexuality and the embryology of the Phanerogams, published between 1815 and 1828, are the most important in a historical point of view. His 'Physiologie der Gewächse' in two volumes (1835-1838) is still of value for its accurate information on the literature of the subject; but it can scarcely be said to have contributed to the advance of physiology, for its author adhered in it to the old views, and especially to the notion of the vital force, at a time when new ideas were already asserting themselves. The 'Botanische Zeitung' for 1864, p. 176, contains a notice of his life.
6. Charles François Mirbel (Brisseau-Mirbel) was born at Paris in 1776, and died in 1854. He began life as a painter, but having been introduced by Desfontaines to the study of botany, he became Member of the Institute in 1808, and soon after Professor in the University of Paris. From 1816 to 1825 the cares of administration withdrew him from his botanical studies, but he resumed them and became in 1829 Professeur des cultures in the Museum of Natural History. Mirbel was the founder of microscopic vegetable anatomy in France. All that had been accomplished there before his time was still more unimportant than the work done in Germany. His writings involved him in many controversies, and he made enemies by refusing in his capacity of teacher to allow the importance at that time attributed to systematic botany, but directed his pupils to the study of structure and the phenomena of life in plants. We are told by Milne-Edwards that he suffered much from the fierce attacks which were made upon him; he sank at last into a weak and apathetic state, and was for some time before his death unable to continue his studies or official duties ('Botanische Zeitung' for 1855, p. 343).
7. Johann Jakob Paul Moldenhawer was Professor of Botany in Kiel; he was born at Hamburg in 1/66, and died in 1827.
8. On the doubts which were entertained till after 1812 on the subject of stomata, see Mohl's 'Ranken und Schlingpflanzen' (1827), p. 9.
9. Franz Julius Ferdinand Meyen was born at Tilsit in 1804, and died as Professor in Berlin in 1840. He applied himself at first to pharmacy and afterwards to medicine, and having taken a degree in 1826 he practised for some years as a physician. In 1830 he set out on a voyage round the world under instructions from A. von Humboldt, and returned in 1832 with large collections. He was made Professor in Berlin in 1834. There is a notice of his life in 'Flora' of 1845, p. 618.
10. Hugo Mohl (afterwards von Mohl) was born at Stuttgart in 1805, died as Professor of Botany in Tübingen in 1872. His father held an important civil office under the Government of Würtemberg. Robert Mohl, also in the service of the Government, Julius Mohl, the Oriental scholar, and Moritz Mohl, the political economist, were his brothers. The instruction at the Gymnasium at Stuttgart, which he attended for twelve years, was confined to the study of the ancient languages; but Mohl early evinced a preference for natural history, physics, and mechanics, and devoted himself in private to these subjects. He became a student of medicine in Tübingen in 1823, and took his degree in 1828. He then spent several years in Munich in intercourse with Schrank, Martins, Zuccharini and Steinheil and obtained abundant material for his researches into Palms, Ferns, and Cycads. He became Professor of Physiology in Berne in 1832, and Professor of Botany in Tübingen after Schübler's death in 1835, and there he remained till his death, refusing various invitations to other spheres of work. He was never married, and his somewhat solitary life of devotion to his science was of the simplest and most uneventful kind. He was intimately acquainted with all parts of botanical science, and possessed a thorough knowledge of many other subjects; he was in fact a true and accomplished investigator of nature. A very pleasing sketch of his life from the pen of De Bary is to be found in the 'Botanische Zeitung' of 1872, No. 31.
11. But von Mohl expressed some doubts on this point in 1844 ('Botanische Zeitung,' p. 340).
12. This tertiary layer was at first supposed by Theodor Hartig to be of general occurrence; von Mohl in 1844 considered it to be present only in certain cases.
13. Anselm Payen (1795-1871) was born at Paris and was Professor of Industrial Chemistry in the École des Arts et Métiers in that city. His most important botanical works were his 'Mémoire stir l'amidon,’ etc., Paris (18391, and his 'Mémoire sur le développement des Végétaux,' published in the Memoirs of the Academy of Paris.