Caspar Friedrich Wolff[70] in his efforts in phytotomy stands a solitary figure among his contemporaries, not only because he was the first since Malpighi and Grew who devoted labour and perseverance to the study of the anatomy of plants, but still more because at a time, when the structure even of matured vegetable organs was almost a forgotten subject, he endeavoured to penetrate into the history of the development of this structure and the formation of cellular tissue. Unfortunately he was not directed to this by an exclusive interest in phytotomy, but by a more general question which he endeavoured to set at rest in this manner; he wished to refute the prevailing theory of evolution by demonstrating the development of the organs of plants, and to obtain an inductive basis for his doctrine of epigenesis. Though he was often diverted by these means from the pursuit of purely phytotomic questions, yet his famous work, ‘Theoria Generationis’ (1759) is nevertheless important in the history of phytotomy; for though it was disregarded by botanists during the succeeding forty years, or at any rate exercised no noticeable influence, yet it was Wolff’s doctrine of the formation of cellular structure in plants which was in the main adopted by Mirbel at the beginning of the present century, and the opposition which it encountered contributed essentially to the further advance of phytotomy. This late but lasting influence of Caspar Friedrich Wolff’s work was due not to the actual correctness but to the thoughtfulness of his observations, and to the earnest desire which inspired them to search out the true nature of vegetable cell-structure and to explain it on physical and philosophical grounds. The observations themselves on this point are highly inexact, and influenced by preconceived opinions, and his account of them is rendered obscure and often quite intolerable by his eagerness to give an immediate philosophic explanation of objects which he had only imperfectly examined. His efforts to follow the course of development in the first beginnings of the formation of cell-tissue were evidently not seconded by sufficient knowledge of the structure of matured organs, and, to judge by his figures and by his theoretical reflections, his microscope was of insufficient power and its definition imperfect. Notwithstanding all these deficiencies, Wolff’s treatise is doubtless the most important work on phytotomy that appeared in the period between Grew and Mirbel, not, as has been said, on account of any particular excellence of observation, but because its author was able to make some use of what he saw, and to found a theory upon it.
According to that theory all the youngest parts of plants, the punctum vegetationis in the stem, which Wolff first distinguished, the youngest leaves and parts of the flower, consist of a transparent gelatinous substance; this is saturated with nutrient sap, which is secreted at first in very small drops (we might say vacuoles), and these, as they gradually gain in circumference, expand the intermediate substance and so present enlarged cell-spaces. The intermediate substance therefore answers to what we should now call the cell-walls, only these are at first much thicker, and are constantly becoming thinner with the growth of the cell-spaces. We may compare young vegetable tissue, formed as Wolff imagines, with the porosity of fermenting dough, except that the pores are not filled with gas but with a fluid. It is plain from the above description that the vesicles or pores, as Wolff names the cells, are connected together from the first by the intermediate substance, and that one lamina or cell-membrane only lies between each of two adjoining cells, a point which succeeding phytotomists were a long time in determining. As cells are formed by the secretion of drops of sap in the fundamental substance which is at first homogeneous, so vessels, according to Wolff, are produced by longitudinal extension of a drop in the mucilage and formation of a canal; consequently adjoining vessels must be separated from one another by a single lamina of the fundamental substance. Though Wolff expressly mentions the movement of the sap within the firm mucilaginous substance between the cellular cavities and the vascular canals, a movement of diffusion as it might now be termed, he inconsistently enough thinks it necessary to assume the existence of perforations in the bounding-walls of cells and vessels to serve for the movement of sap from cell to cell and vessel to vessel; yet in the single case in which he succeeded in obtaining isolated cells, namely in ripe fruits, he was obliged to allow that the walls were closed.
The growth of the parts of plants, according to Wolff, is effected by expansion of existing cells and vessels, and by the formation of new ones between them in the same way as the first vacuoles were formed in the mucilaginous substance of very young organs; that is to say, the sap which saturates the solid substance between the passages and cavities of the tissue separates in the form of drops, which increase in size and then appear as cells and vessels introduced between the older ones. The substance between the passages and cavities, at first soft and extensible, becomes firmer and harder with increasing age, and at the same time a hardening substance may be deposited on it from the sap which is stagnant in the cell-cavities and in movement in the vascular passages, and this substance in many cases appears as their proper membrane.
This is in all essential points Wolff’s theory. We may omit his statements on the subject of the first formation of leaves at the growing point and of the development of the parts of the flower, as well as his physiological views on food and sexuality, which remained for a long time without influence on the growth of opinion, and mention only his doctrine of the growth of thickness of the stem. The stem is originally the prolongation of all the leaf-stalks united together. As many bundles of vessels are formed in the developed stem as there are leaves springing from the vegetative axis; each leaf has a single vascular bundle belonging to it in the stem, in modern phraseology an inner leaf-trace. The union of these bundles from the different leaves forms the rind of the stem; but if the leaves are very numerous, their descending bundles form a closed cylinder, and if the stem is perennial, the fresh production of leaves every year produces new zones of wood of this kind every year, which are the yearly rings. This view of Wolff’s on the growth of the stem in thickness bears an unmistakable resemblance to the theory afterwards suggested by Du Petit-Thouars, according to which the roots which descend from the buds are supposed to effect the thickening of the stem.
The contests between Mirbel and his German antagonists at the beginning of the present century will bring us back again to the more important points in Wolff’s theory of the cell. Contemporary botanists paid less attention to the ‘Theoria Generationis’ than they did to Hedwig’s[71] phytotomic views, not on the formation of cells, but on the structure of mature tissue. Hedwig had given various figures and descriptions of phytotomic subjects in his ‘Fundamentum Historiae Muscorum’ (1782) and afterwards in his ‘Theoria Generationis’ (1784); but he treats these topics at greater length in his treatise ‘De fibrae vegetabilis et animalis ortu,’ published in 1789, and known to the author of this work only imperfectly from quotations in later writers. Hedwig’s figures of histological objects appear to be better than those of any of his predecessors; they show that he used strong magnifying powers, and that his glass had a clear field of sight. His defect lay in preconceived opinions and hasty interpretation of what he observed. In order to refute Gleichen’s view of the stomata in ferns, he demonstrated the existence of these organs in many phanerogams, and observed the opening of the slits, which he named ‘spiracula.’ On the epidermis which he had removed for the purpose of these observations he saw plainly the double contour lines bounding the epidermis-cells, and therefore the cell-walls, which are at right angles to the surface. These he took for a particular form of vessel, and called them ‘vasa reducentia’ or ‘lymphatica,’ and afterwards ‘vasa exhalantia,’ and he thought that he had found them again in the interior of parenchymatous tissue, evidently taking the places where three wall-surfaces meet for vessels; such vessels he also saw in the milk-cells of Asclepias, described in 1779 by the elder Moldenhawer, who seems himself to have regarded even the intercellular spaces in the pith of the rose as equivalent to these milk-cells. The word vessel even in the 18th century was used in such an indefinite manner, that the broad air-tubes of the wood and the finest fibres were called vessels. Hedwig’s idea of the construction of spiral vessels was strange enough; he took the spiral band itself for the vessel, and supposed it to be hollow because it is coloured by absorption of coloured fluids; in those spiral vessels in which the turns of the spiral band are distant he saw, it is true, the delicate original membrane which lies between the turns, but he supposed that it lay inside the spiral band, which was wound round it on the outside. On the second plate of the first part of the ‘Historia Muscorum’ he even figures the network of ridges which the adjoining cells have left on the wall of the spiral vessel, but explains it as wrinkles caused by desiccation.
Hedwig was without doubt a very practised microscopist, and he constantly recommended the extremest care in the interpretation of all that the instrument reveals; but if an observer so careful and practised, who moreover was provided with a glass of tolerably strong magnifying power, fell into such gross mistakes, it cannot surprise us if others, as P. Schrank, Medicus, Brunn, and Senebier, accomplished still less. These highly unimportant achievements are all that mark the close of the 18th century.
[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 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.