2. Further Development of Opinion on the Nature of the Solid Framework of Cell-Membrane in Plants after 1845.
Between 1840 and 1850 the most eminent representatives of phytotomy were chiefly engaged, as we have seen, in observing the formation of vegetable cells, and in framing the true theory of the subject by process of induction. It was not to be expected that, while these labours were bringing year by year new things to light and keeping opinion on the formation of cells in a constant state of fluctuation, their results would lead to very important changes in the theory of the solid framework of cell-membrane founded by von Mohl. On the contrary, it was at this time that his views such as we have seen them on the connection of cells one with another, on the configuration of their partition-walls and on their growth in thickness, attained their greatest influence. His theory seemed to stand firm and complete when contrasted with the unsettled state of opinion respecting the origin of cells, and the question, how far it could be made to agree with the new observations on the history of cell-formation, was hardly raised. In the midst of the strife of opinion on the latter subject appeared von Mohl’s ‘Vermischte Schriften’ in 1845, in which his views on the structure of mature vegetable tissue were produced in a series of monographs as the apparently irrefragable result of his observations. And in fact phytotomic research up to 1860 followed the train of thought initiated by von Mohl, till at last the inadequacy of his views was rendered apparent between 1858 and 1863 by Nägeli’s new theory of growth by intussusception, and by the profounder insight obtained into the nature of cell-formation.
A sufficient proof of the correctness of these remarks is to be found in the further development of the views of botanists on the intercellular substance and the cuticle, which might have adapted themselves before 1850 to the new theory of cells, but instead of doing so were moulded by the ideas current before 1845. It has been shown in the preceding chapter how von Mohl gradually restricted the theory of intercellular substance which he had proposed in 1836, and had come in 1850 to regard this substance as only a cement which might in many cases be perceived between the cell-walls. It should be added here, that Schleiden in connection with his theory of cells considered both the intercellular substance and the cuticle to be supplementary secretions from the cells, and made the former fill the intercellular spaces, just as laticiferous and resiniferous passages are filled with secretions from the adjacent cells (1845). Unger too in 1855 (‘Anatomie und Physiologie der Pflanzen’) thought the existence of a cement between the cells necessary to prevent their falling asunder. Schacht, who in his ‘Pflanzenzelle’ of 1852 had followed Schleiden in explaining the intercellular substance and the cuticle as secretions or excreta from the cells of the plant, still kept on the whole to this view in 1858, though he modified it in some important points. This theory of Schleiden and Schacht was first opposed by Wigand in a series of essays (1850-1861), in which in strict adherence to von Mohl’s theory of apposition he sought to prove, that the layers which are visible in wood-cells as intermediate laminae in the partition-walls, and which till then had been regarded as a cement between contiguous cells, an intercellular substance, were nothing else than the thin primary membranous laminae formed in the process of cell-division, and subjected to subsequent chemical change, while the secondary layers of thickening in von Mohl’s sense lie on both sides of them. The cuticle on the epidermis was explained in a corresponding manner. Though Sanio in 1863 raised a variety of objections to Wigand’s view, he still adhered to it in principle, and found a strong confirmation of it in the fact, that he succeeded in producing the well-known cellulose-reaction in the intercellular substance of wood-cells when freed from foreign admixtures.
The researches of Wigand and Sanio were sufficient to overthrow von Mohl’s account of the intercellular substance and the cuticle, but they had not proved that the intermediate laminae are in fact the primary partition-walls on which von Mold’s secondary thickening-layers had been deposited, on both sides in the case of the intercellular substance, on one side in that of the cuticle. The structure of the partition-walls and the existence of the cuticle could be explained in a totally different way from the point of view now opened by Nägeli’s theory of intussusception; there was no need now to see either a secretion or a primary cell-wall in the intermediate lamina of thickened cells or in the cuticle, for it was possible that this lamination might be due to subsequent chemical and physical differentiation of membranes thickened by intussusception. As phytotomists are not yet quite agreed as to the correctness of this view, we must be content with observing here that in the matter of the cuticle and the intercellular substance lies one of the points, the determination of which will involve the question of von Mohl’s earlier theory of apposition. It is not the purpose of this history to give the more modern views that have asserted themselves since 1860, especially where the question is still in debate.
It was a part of von Mohl’s idea of the cell-tissue and one to which he had firmly adhered since 1828, that except in the cross walls of genuine wood-vessels and some very isolated cases the partition-walls in cellular tissue are never perforated; that both simple and bordered pits always remain closed by the very thin primary lamina of cellulose. But between 1850 and 1860 several cases were discovered which were at once exceptions to von Mohl’s rule, and of great importance to physiology. Theodor Hartig, in his ‘Naturgeschichte der forstlichen Kulturpflanzen Deutschlands’ (1851), described peculiar rows of cells in the bast-system, in which the transverse and sometimes the longitudinal walls appear to be pierced like a sieve by numerous minute holes, and to these cells he gave the name of sieve-tubes. Von Mohl (1855), while in other points confirming and extending Hartig’s discovery, declared against the perforation of the walls, believing that the appearances were due to lattice-like thickenings of the cell-walls; he proposed therefore to call Hartig’s sieve-tubes latticed cells. Then Nägeli showed in 1861 that in some cases at least there can be no doubt that the walls are actually perforated, and that the sieve-plates serve for the passage of mucilaginous matter in bast-tissue, and the author of this history, it may be remarked in passing in 1863, and Hanstein in 1864, suggested means by which it may be ascertained with certainty that Hartig’s sieve-plates are perforated. Meanwhile a number of laticiferous organs had been recognised as forms of vessels in von Mohl’s sense, and it was found that such canals are produced by dissolution of the septa of adjacent cells. But the knowledge of the laticiferous organs continued till towards 1865 to be very unsettled and defective, and the examination of resin-passages, and the discovery that they are formed by simple parting of cells from one another, belong to modern phytotomy; Hanstein, Dippel, N. J. C. Müller, Frank, and others have since 1860 enlarged our knowledge of these forms of tissue. Schacht in 1860 established one of the most important exceptions to von Mohl’s view above-mentioned, by demonstrating the formation and true form of bordered pits in the wood of Conifers and in dotted vessels in Angiosperms from the history of their development, and by showing moreover that in all cases where bordered pits are formed on both sides of a partition-wall and the adjacent cells afterwards convey air, there the original very thin partition-wall in the bordered pit disappears, and that consequently in such cases the bordered pits represent so many open holes, through which adjacent cells and vessels communicate. At the same time another hitherto inexplicable phenomenon received its explanation. Malpighi, and after him the phytotomists at the beginning of the present century had remarked, that the large vessels in the wood are not unfrequently filled with parenchymatous cell-tissue, for the origin of which no one could account. The phenomenon, however, could now be explained quite simply after Schacht’s discovery; the formation of thylosis in vessels only takes place when these border on closed parenchyma-cells in the wood; when this is the case, the very thin membrane which separates the bordered pits from the contiguous cells is not absorbed, but it bulges inwards into the cavity of the vessel under the pressure of the sap of the neighbouring parenchyma-cell, there swells up like a bladder, and may by the formation of partition-walls give rise to parenchymatous tissue; this, if proceeding from a number of pits, fills up the cavity of the vessel.
3. History of Development and Classification
of Tissues.
It has been already stated, that the first step to a real understanding of the structure as a whole of the higher plants was made by Moldenhawer, who beginning with the study of the Monocotyledons, first formed an idea of the vascular bundles as a distinct whole, a system composed of various forms of tissue, and applied this idea to explain the construction of the stems of Dicotyledons, upsetting thereby Malpighi’s earlier theory of the growth in thickness of stems. It was also observed, that von Mohl, advancing further in the same direction, gave a more exact description of the epidermis and of the tissues connected with it, and classified them, that is, introduced a terminology founded on real investigation, but did not succeed in bringing the subject to an entirely satisfactory conclusion; this could in fact be reached only by the study of the history of development, the only decisive method of investigation, whether the object be to determine the true nature of cells and their subordinate forms, or the solid fabric of vegetable structure, or as in the present case to distinguish and classify forms of tissue; it is this method which supplies the morphological points of view necessary for the understanding of the inner structure of the plant by investigating tissues in those states of development, in which they are not yet adapted to subsequent physiological functions. The combination of morphological and physiological points of view in the determination of facts has maintained itself longer in this part of botanical study than in any other; but here too ideas and opinions were gradually sifted and cleared up under the influence of the modern study of the history of development, though it was not till after 1850 that the determination of the chief points in the theory of cell-formation left the leading phytotomists at liberty to devote themselves to histological questions.
How little advance had been made towards the true understanding of the varieties of forms of tissue in the higher plants before 1850 is shown, for instance, by Schleiden’s account of tissues on page 232 of his ‘Grundzüge’ of 1845, where parenchyma, intercellular substance, vessels, vascular bundles, bast-tissue, bast-cells in Apocyneae and Asclepiadeae, laticiferous vessels, felted tissue, epidermal tissue, are discussed in this succession in co-ordinated sections of the text. It is obvious that no well-ordered view of the whole cellular structure of a plant of the higher order could be obtained in this way. Further on in the same work, where Schleiden attempts a classification of vascular bundles, which he distinguishes into closed and open, and assigns the latter to Dicotyledons, we find the cambium-layer named as the outer boundary of these open vascular bundles; the bast which lies outside the cambium was therefore not considered to be a part of the open vascular bundles, and this necessarily excluded any profitable comparison of the circumstances in Monocotyledons and Dicotyledons. The case is still worse in many respects in Schacht’s work already mentioned, ‘Die Pflanzenzelle’ of 1852, where under the heading ‘Kinds of vegetable cells’ the histology is discussed in the following co-ordinated sections; the swarm-filaments of Cryptogams, the spores of the same, pollen-grains, cells and tissue of Fungi and Lichens, cells and tissue of Algae, parenchyma and its cells, vessels of the plant, wood and its cells, bast-cells, stomata, appendicular organs of the epidermis, cork; then follows a paragraph on the thickening-ring, and then to the no small astonishment of the reader comes an account of the vascular bundles, after the vessels, the wood, and the bast-cells have been already dismissed. That such a mode of presenting the subject is due to the little insight possessed by the writer into the structure of the plant as a whole is apparent from simply reading the book, and a similar confusion of ideas is found in his text-book of 1856.
We find a much better classification of tissues in 1855 in Unger’s ‘Anatomie und Physiologie der Pflanzen’; an account of cells is followed by a description of cell-complexes, as one of the chief divisions of the book, and herein of cell-families, cell-tissues, and cell-fusions. Another chief section is occupied with cell-groups, and here epidermal formations, air-spaces, sap-receptacles, glands and vascular bundles are noticed; here certainly the fact has been overlooked that vascular bundles may be co-ordinated with epidermal formations, but not air-spaces, sap-receptacles and glands. His last chief division gives an account of tissue-systems and of the way in which the vascular bundles are united together in different plants, and secondary growth in thickness and the activity of the cambium-layer are described quite in the right connection.
In this branch of the science, as in every case where it is a question of establishing fundamental conceptions, of surveying facts from extensive points of view, and of seeking the requisite principles by means of the history of development, we find that it is Nägeli who opens the way and lays the foundation. In his ‘Beiträge zur wissenschaftlichen Botanik’ of 1858, he proposed a classification of tissues from purely morphological points of view. His first division was into generating and permanent tissue; in each section he distinguished two forms, prosenchymatous and parenchymatous tissue. Parenchymatous generating tissue, the original component of every young organ, he named primary meristem as distinguished from prosenchymatous generating tissue, which is differentiated in the form of strands and layers, and received from him the general name of cambium; this was certainly not a happy distinction, because Nägeli’s cambium by no means consists entirely of prosenchymatous tissue. By the term secondary meristem Nägeli designated the tissue-strands and tissue-layers which are formed between the permanent tissue of older parts. The cambium he regards as the first product of the primary meristem. The second chief form, permanent tissue, he divides into two classes, not according to the form of the cells or physiological relations, but according to its origin; all permanent tissue, which is derived immediately from primary meristem, is protenchyma, all that comes directly or indirectly from cambium is epenchyma. And since the tissue-strands, till then known as vascular bundles, do not contain vessels only but always fibrous elements also, as Bernhardi had shown in 1805, Nägeli thought that they should therefore be called fibrovascular strands. If it cannot be denied that the obvious distinction between epidermal and other tissue did not find suitable expression in this classification, and though other points of view may at the present day be proposed for the genetic arrangement of tissues, yet Nägeli’s classification and terminology have the merit of having for the first time exhibited the general histology of plants on comprehensive and genetic principles. It contributed materially to impart a better understanding of the collective structure of plants.