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 Physiologie 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 ‘Beiträge,’ 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[72] 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.
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 Göttingen 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[73]), 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 Göttingen 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 ‘Anatomie 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[74], 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[75] 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 acquaintance 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[76], 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[77] 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 Wolff’s 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 Wolff’s 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 Jakob Paul Moldenhawer[78] travelled far beyond these older views as early as 1812 in his important work, ‘Beiträge zur Anatomie 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 ‘Beiträge,’ 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 1811 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 (Beiträge, 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 network 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 ‘Beiträge zur Anatomie 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[79], 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 Anatomie 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 intercellular 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 Gewächse’ 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 Göttingen 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[80] 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 Physiologie’ 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 Göttingen 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 Nägeli.
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[81] 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 Nägeli. 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 interpretation 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.