Such being the nature of the sources from which we must draw our information, we shall obtain a better general view of the whole subject if we depart from the practice of former chapters, and follow out the more important questions in their historical development instead of connecting them directly with leading persons. Such a treatment of the subject is indeed suggested by the fact that we are now no longer on pure historic ground; for the majority of the men who have developed modern doctrines since 1840 are still alive, and it must be uncertain whether the account here attempted may not be impugned on some ground or other. Owing to the extraordinary diversity of opinion that exists among botanists even on the most general questions in the science, it is extremely difficult to ascertain what can be considered as a common possession,—an unfortunate condition of things, from which no science perhaps suffers so much as botany.
The extent to which individual botanists have contributed to the advance of phytotomy during the period under consideration will appear of itself from the following narrative; and if we speak almost exclusively of Germans, it is for the simple reason that Englishmen from Grew’s time till now can scarcely be said to have added anything to our knowledge of phytotomy; the Italians also, once so gloriously represented by Malpighi, scarcely come under consideration in the questions now to be dealt with, while French botanists, represented by Mirbel in the preceding period, though they have produced many works on phytotomy since his time, have had no important share in deciding the fundamental questions of modern science.
In the preceding period it was necessary to take into consideration the increasing improvement of the microscope, in order to understand the development of opinion on vegetable structure; but it is scarcely needful to do so after 1840. Since that time good and serviceable instruments with strong magnifying powers and clear definition have been within the reach of every phytotomist; and though improvements are still being constantly made, yet the microscopes that were in the hands of skilful observers between 1840 and 1860 were fully adequate to deciding the new questions proposed to them. The chief improvement effected in the microscope during this period was the fitting it with apparatus for the polarisation of light, and for the more convenient measurement of objects; we shall see presently what influence the former improvement had on the perfecting of Nägeli’s molecular theory. As microscopes improved and the questions to be solved grew more difficult, it became necessary to bestow increased care on the preparation of objects; it was no longer sufficient to cut or dissect neatly, and so learn the form of the solid portions of vegetable structure; measures of precaution and auxiliary measures of the most various kinds were needed to obtain a clear view of the soft contents of cells, and to observe the protoplasm as far as possible in a living state and protected from prejudicial influences; all sorts of chemical reagents were applied to make the objects more transparent, or to show their physical and chemical characters. The method invented by Franz Schulze before 1851 deserves to be specially mentioned; it consisted in isolating the cells in a few minutes’ time by boiling them in a mixture of nitric acid and potassium-chlorate, and thus shortening Moldenhawer’s process of maceration or superseding it altogether. In a word, the technicalities of the microscope were perfected in a variety of ways by Schleiden, von Mohl, Nägeli, Unger, Schacht, Hofmeister, Pringsheim, De Bary, Sanio, and others, and raised to an art which must be learnt and practised like any other art. Young microscopists were able after 1850 to learn this art in the laboratories of their elders, and to profit by their technical experience and scientific counsels; schools of phytotomy were formed at least in the German universities; elsewhere, it is true, the old condition of things remained in which everyone had to trust to himself from the beginning.
The general dissemination of good microscopes was accompanied by a higher standard of requirement in the execution of drawings from the instrument, especially after von Mohl had shown the way; and the invention of lithography and the revival of wood-engraving ministered to the needs of science, supplying the place of the old costly copper-plate printing. Hence we find an increasing number of beautiful drawings in scientific monographs; the text-books also could now be supplied with an abundance of figures, and this greatly promoted the general understanding of things which could otherwise be seen only under the glass of each observer. From the close of the 16th century wood-cuts had fallen more and more into disuse, and had been replaced by copper-plates; after 1840 wood-engraving was restored to its old rights and was found to be a more convenient method of pictorial illustration, especially for text-books; thus Schleiden’s ‘Grundzüge’ of 1842, von Mohl’s ‘Vegetabilische Zelle’ of 1851, Unger’s and Schacht’s text-books were enriched with many and sometimes very beautiful wood-cuts. Lithographs were generally preferred for periodicals and monographs; the ‘Botanische Zeitung,’ founded by Mohl and Schlechtendal in 1843, and till after 1860 the chief organ for shorter phytotomic communications, was illustrated by a large number of beautiful prints from the establishment of the Berlin lithographer, Schmidt.
1. Development of the Theory of Cell-formation
from 1838 to 1851.
Since we are here dealing with questions of fundamental importance not only to one branch of botanical study but to the whole science of botany, and even to the rest of the natural sciences, it seems imperative that we should follow step by step the founding and perfecting of the theory of the cell, as far as is possible in the limited space at our command; we shall deal with the sexual theory further on in a similar manner.
As usually happens in the inductive sciences, the period of strict inductive investigation into cell-formation was preceded by a still longer time, during which botanists ventured to put forward general theories in reliance on highly imperfect observations. We have already seen how Caspar Friedrich Wolff in 1759 made cells originate as vacuoles in a homogeneous jelly, and how this view was adopted in all essential points by Mirbel at a late period in the 18th century; how Kurt Sprengel, and with him a number of phytotomists, among them Treviranus as late as 1830, supposed cells to be formed from granules and vesicles in the cell-contents, an idea which Link it is true opposed in 1807, but afterwards accepted to a great extent. Though Moldenhawer as early as 1812 (‘Beiträge,’ p. 70) distinctly rejected these theories, and published observations which if followed up would have led to the right path, yet the botanists above-named and others with them, long continued to adhere to the earlier views. Kieser, for example (‘Mémoire sur l’organisation des plantes,’ 1812) further developed Treviranus’ theory, that the fine granules in the latex of plants are cell-germs which are afterwards hatched in the intercellular spaces. Schultz-Schultzenstein in his work ‘Die Natur der lebenden Pflanze,’ 1823-28, i, p. 607 rejected this view and adopted that of Wolff and Mirbel. Scarcely better than the notion of cell-germs represented by Sprengel, Treviranus, and Kieser was the theory propounded by Karsten soon after 1840; that of the French botanists Raspail and Turpin[85] (1820-1830), though conveyed in a different terminology, corresponded in its main points with the views of Sprengel.
It had been the good fortune of Mirbel at the beginning of the century, and again thirty years later, to promote the advance of phytotomy by means of important observations, though he may have interpreted some of them incorrectly; the same thing happened again thirty years later, and it was a German enquirer, von Mohl, who corrected his observations and views on both occasions.
In his famous treatise on Marchantia polymorpha, which appeared in 1835 in the Memoirs of the French Institute, the first part having been laid before the Paris Academy in 1831-32, Mirbel distinguished three modes of cell-formation; in the germination of the spores of Marchantia new cells are formed from the germ-tube and new cells again from these by a similar process, much in the same way therefore as that which actually occurs in the germination of Yeast-fungi; he found a second kind of cell-formation in the production of the gemmae of Marchantia, where he distinctly observed the successive appearance of the dividing walls, but formed an erroneous idea of the proceeding on the whole; in the further development of the gemmae and in other cases of growth he considered that new cells are formed between those that are already present in the manner supposed in his earlier theory.
Von Mohl’s dissertation on the multiplication of vegetable cells by division, published in 1835 and reprinted in ‘Flora’ of 1837, shows how strange these processes even then appeared; in this work he expresses some doubts about Mirbel’s statements, but he accepts them on the whole, and only makes incidental mention of his own more numerous and better observations on the development of spores (‘Flora,’ 1833), though he had there seen several cases of cell-division and free cell-formation with tolerable distinctness. Adolph Brongniart (‘Annales des sciences naturelles,’ 1827) also had observed, though imperfectly, the formation of pollen-grains in their mother-cells in Cobaea scandens, and Mirbel, in the appendix to the work mentioned above, had given a correct description and good figures of the formation of pollen-cells; and yet von Mohl neglected to compare these important observations of cases of cell-division with his own; even in 1845, when he published the latter in a revised form in his ‘Vermischte Schriften,’ he overlooked the close relation between the formation of those pollen-grains and spores, and the cell-division in Cladophora. Still this treatise of von Mohl’s is of great importance in the history of the theory of cell-formation, because it described a case of cell-division for the first time step by step and brought all the salient points into relief. Dumortier had observed the division of cells as early as 1832[86], and Morren had seen it in Closterium in 1836, but had not given the needful details. Finally, von Mohl applied the experience which he had gained from Cladophora to other filamentous Algae, and pointed out the similarity between these processes and the division of Diatoms, which he consequently claimed as plants in opposition to Ehrenberg, who considered them to be animals (‘Flora,’ 1836, p. 492).