History of Botany (1530-1860)

by Julius Sachs

Thallophytes.

The method of investigation which starts from the first steps towards the formation of the embryo before and after fertilisation, and follows the advancing segmentation and growth through all the stages of development up to the final completion of the embryo-plant, has led since 1850 in the case of the Muscineae, Vascular Cryptogams, and Phanerogams to great certainty in the morphological explanation of the organs, while the determination of affinities has ceased to be arbitrary and insecure; the way was now known which would lead to the desired end, whenever it was sought to establish the affinities of a genus of Cryptogams or of the larger groups of Phanerogams; the day of ingenious guessing and trying was over; the only plan was patient investigation, and this always yielded a result of lasting value.

The case was quite different with the Thallophytes still in 1850; what was certainly known about them only showed how uncertain the rest was; the Algae, Fungi, and Lichens presented a chaotic mass of obscure forms in contrast with the well-ordered knowledge of the Muscineae and Vascular plants. In the Mosses and Ferns the series of developments within the limits of the species was so set forth in its several stages, that all the important points in the advancing growth were clearly ascertained, while the alternation of generations at once sharply distinguished and connected together the chief sections in the development; on the other hand the development of the Algae and Fungi seemed to break up into a disorderly and motley throng of forms that appeared and disappeared, and it seemed scarcely possible to discover their regular genetic connection. Here the important point was to determine which of the known forms belonged to one and the same cycle of development, for these plants go back at the most various stages of development to the segregation of single cells, which are the beginning of a new development either repeating or carrying on the old one. The beginnings of the most different species of Algae lay mixed up together in the same drop of water, those of quite different Fungi grew together and even upon one another on the same substratum; in the Lichens, Fungus and Alga were united together. Such was the case with the small and microscopic species; the large Seaweeds, the Mushrooms, and the large Lichens were easier to distinguish specifically, but less if possible was known of their development than of that of the microscopic Thallophytes.

Nevertheless the knowledge of individual forms in these organisms had been considerably extended before 1850. Collectors and amateurs, intent only on determining what is immediately presented to the eye and making little enquiry into origin and affinities, were indefatigable in adding to their collections, and made catalogues and proposed various systems founded on external marks taken at pleasure. The names of species were counted by thousands, their characters filled thick volumes and the figures large folios; the abundance of forms in the Thallophytes proved to be so great that many botanists devoted their whole attention to them, many collected and described only the Algae, others only the Fungi and Lichens. It is true that a deeper insight into the connection of these forms of life with one another and with other plants was not to be obtained in this way; still an empirical basis was formed for a knowledge of the Cryptogams, such as had been established for the Phanerogams by the herbals of the 17th century. All forms open to observation were named and arranged in one way or another; and there was no difficulty in understanding what form was meant, when names, or tables and figures, were cited from the various books. Of such works, those of Agardh[56], Harvey, and Kützing on the Algae, those of Nees von Esenbeck[57], Elias Fries, Léveillé, and Berkeley on the Fungi, and especially Corda’s elaborate work on the latter plants are the most valuable.

The views entertained on the subject of the development and propagation of the lower Cryptogams down to the year 1850 were very uncertain and fluctuating. In some Algae, Fungi, and Lichens certain organs of multiplication and propagation were known, in others they were quite unknown; some forms appeared in places and under circumstances which seemed to necessitate the assumption of spontaneous generation; in 1827 Meyen declared that the small Algae, known as ‘Priestley’s matter,’ which are formed in stagnant water and even in closed vessels, are produced by free generation, and Kützing endeavoured to show this by experiment in 1833; some Fungi were regarded as diseased growths from other organisms, many were supposed to spring up spontaneously, though they might be capable at the same time of propagating themselves by spores; this view was shared by even the best botanists with regard to the most simple Fungi up to 1850. But the systematic investigation of the Algae and Fungi was as little hindered by the notion of spontaneous generation after 1850 as that of Phanerogams had been in the 17th century by the same notion; it was however at first affected by the view put forth by Hornschuch in 1821 and by Kützing in 1833, that the simplest of all Alga-cells (Protococcus and Palmella), once produced spontaneously, could develop according to circumstances into a variety of Algae, and even of Lichens and Mosses; as some observers even now consider Penicillium and Micrococcus to be the starting-points of very different Fungi. There was a difficulty also in drawing the boundary-line between the lower animals and plants; the difficulty was solved by classing all objects capable of independent movement with animals; thus whole families of Algae (the Volvocineae, Bacillariaceae, and others) were claimed by the zoologists, and when the swarmspores of a genuine Alga were seen for the first time in the act of escaping, the phenomenon was described as the changing of the plant into an animal. Trentepohl in 1807, and Unger in 1830, explained in this way the escape of the zoospores of Vaucheria. The remarkable thing is, not that such views were entertained, but that the majority of botanists combined with them a belief in the constancy of species. But this dogma rendered good service to the science in this instance, for the botanists, who at a later time applied themselves to the systematic examination of the Algae and Fungi, confided in the constancy of the processes of development in each species, which they expected would assert itself in these forms as in the Mosses and higher plants.

With much that was obscure and doubtful, the result of occasional observation accompanied by uncritical interpretation, the literature of the subject had contained for some time a certain number of single well-established facts of real importance, which were well adapted to serve as starting-points for earnest and exact investigation. Among the Algae the genera Spirogyra and Vaucheria especially had supplied remarkable phenomena; Joseph Gärtner observed the formation of zygospores in Spirogyra in 1788, Hedwig saw in the mode of their production at least a suggestion of sexuality (1798), and Vaucher[58], in his ‘Histoire de Conferves d’eau douce,’ which appeared in 1803 and was far in advance of its time, called conjugation distinctly a sexual process; the optical means at his disposal did not enable him to observe the fertilisation in Vaucheria (Ectosperma), which was named after him, though he described the sexual organs accurately; the movement also of the zoospores in this genus escaped him, and Trentepohl first observed their escape and swarming in 1807[59]. Vaucher had also observed the formation of new nets in the old cells of Hydrodictyon, and Areschoug repeated the observation in 1842, when he saw the swarming of young cells in the old ones. Bischoff, as early as 1828, saw the spermatozoids of Chara, though without understanding them. Observations on conjugating Algae were multiplied; Ehrenberg in 1834 saw corresponding phenomena in Closterium, and Morren described them more exactly in 1836. The formation of swarmspores in fresh-water and salt-water Algae was frequently observed between 1820 and 1830, and in his ‘Neues System,’ iii, which appeared in 1839, Meyen gave a summary of all that was known up to that time of the propagation of the Algae. But a new aspect was given to the knowledge of the Algae by those researches of Nägeli between the years 1844 and 1849, which have been already mentioned, and which are the first since Vaucher’s time that can be regarded as systematic. Nägeli studied especially the laws of cell-division in sexual multiplication and growth, but he considered the Florideae to be the only Algae that were sexually differentiated, and distinguished the rest as being without sexuality. Braun in his ‘Verjüngung’ (1850) made numerous contributions to the biology of the fresh-water Algae, affording many and most interesting glimpses into a connection still little understood between these forms; and in 1852 he gave an account of the history of growth in the Characeae, a work conceived in Nägeli’s spirit and a model of scientific research, in which the mode of derivation of every cell from the apical cell of the stem was shown, the sexual organs were minutely examined, and the relation established between the direction of the ‘streaming’ of the cell-contents and the morphology of the organs. Gustav Thuret had already made the zoospores of the Algae the subject of detailed examination.

Such was the condition of affairs with respect to the Algae about the year 1850, when Hofmeister made the formation of the embryo in the Phanerogams, the Vascular Cryptogams, and the Muscineae the central point of investigation in morphology and systematic botany. He made it clear that a perfect insight into the whole cycle of development in the plant and into its affinities can only be obtained, if we succeed in making its sexual propagation, the first commencement of the embryo, the starting-point of the investigation. It was natural to expect as happy results from the embryology of the Algae, as had been obtained in the case of the higher plants; it was important therefore, that the observer should no longer rest satisfied with a knowledge of the sexual multiplication of the Algae; he must enquire into their asexual propagation, and by its aid discover the complete history of their development. Former observations suggested the probability that here too sexual propagation is the prevailing rule; but it was easy to foresee that it would be a task of great labour to make out a connected history of development, a task of which the collectors who liked to call themselves systematists had never formed a conception; but Nägeli’s and Hofmeister’s researches had made botanists familiar with the highest demands of this kind, and the men who were to gain new conquests for genuine science were already engaged in the work in 1850. A splendid result appeared in 1853, in Thuret’s account of the fertilisation of the genus Fucus; this was a simple process as a matter of embryology; but the sexual act was so clear, and even open to experimental treatment, that it threw light at once upon other cases more difficult to observe. Then followed discoveries of sexual processes in rapid succession; Pringsheim solved the old enigma in Vaucheria in 1855, and between 1856 and 1858 in the Oedogonieae, Saprolegnieae and Coleochaetae; in 1855 Cohn observed the sexual formation of spores in Sphaeroplea. Pringsheim however was not content with carefully observing the sexual act; he gave detailed descriptions of growth in the same families in its progress cell by cell, of the formation of the sexual organs, and the development of the sexual product. The asexual propagations which are intercalated into the vegetation and embryology were shown in their true connection. Processes were recognised which often recalled the alternation of generations in the Muscineae; it was shown that very different forms of sexuality and of general development occur in the Algae, and these led to the formation of systematic groups, quite different from those founded on the superficial observation of collectors. It soon appeared in the Algae, and later in the Fungi and Lichens, that special investigation must lay new foundations for the system. From the confused mass of forms not before understood, Pringsheim brought out a series of characteristic groups, which, thoroughly examined and skilfully described in words and by figures, stood out as islands in the chaotic sea of still unexamined forms, and threw light in many ways on all around them. In like manner the morphology of the Conjugatae was thoroughly examined by De Bary before 1860; fragments of the history of development in the Algae were added by Thuret, and he and Bornet cleared up the remarkable embryology of the Florideae in 1867, while Pringsheim established the pairing of the swarmspores in the Volvocineae in 1869. The Algae offer at present a greater variety in the processes of development than any other class of plants; sexual and asexual propagation and growth work one into the other in a way which opens entirely new glimpses into the nature of the vegetable world.

The old conceptions of the nature of plants had been greatly modified by Hofmeister’s discovery of the alternation of generations, and the reduction to it of the formation of the seed in Phanerogams; in like manner the first beginnings of plant-life, the simplest forms of Algae, exhibit phenomena, which compel us to revise our fundamental conceptions of morphology, if we are ever to be able to give a systematic view of the whole vegetable kingdom.

The methodical examination of the Fungi after 1850 led to similar but still more comprehensive results. From earliest times the Fungi had been objects of wonder and superstition; what Hieronymus Bock said of them has been told in the first chapter; this was repeated by Kaspar Bauhin, and similar notions existed till late into our own century; about the middle of the 17th century Otto Von Münchausen thought that mushrooms were the habitations of Polypes, and Linnaeus assented to that view. What the nature-philosophers, as Nees von Esenbeck for instance, had to say on the nature of Fungi, need not be reproduced here.

Still some useful observations had been accumulating for some time on this subject; as early as 1729 Micheli[60] had collected the spores of numerous Fungi, had sown them and obtained not only mycelia but also sporophores (fructifications), and Gleditsch confirmed these observations in 1753; Jacob Christian Schaeffer[61] about the year 1762 published very good figures of all the Fungi of Bavaria and the Palatinate, and collected the spores of many species. Yet Rudolphi and Link at the beginning of the present century ventured to deny the germination of the spores of Fungi; Persoon in 1818 thought that some Fungi grow from spores, others from spontaneous generation. A decided improvement appears after 1820 in the views of botanists with respect to Fungi, and to this Ehrenberg’s elaborate essay, ‘De Mycetogenesi,’ published in that year in the Leopoldina, contributed greatly. In that work he collected together all that was then known on the nature and propagation of the Fungi, and communicated observations of his own on spores and their germination; he gave figures also of the course of the hyphae in large sporophores and in other parts, but his most important service was a description of the first observed case of sexuality in a Mould, namely, the conjugation of the branches of Syzygites. In the same year Nees von Esenbeck sowed Mucor stolonifer on bread, and obtained ripe sporangia in three days (Flora, 1820, p. 528); Dutrochet proved in 1834 (Mém. ii. p. 173) that the larger Fungi are only the sporophores of a filiform branching plant, which spreads usually under ground or in the interstices of organic substances, and had been till that time regarded as a peculiar form of Fungus under the name of Byssus. Soon after, Trog (Flora, 1837, p. 609) carried these observations further; he distinguished the mycelium from the sporophore, and pointed out that the former is often perennial and is the first product of the germinating spores. He made an attempt to examine the morphology of the larger sporophores, and showed that it was possible to collect the spores of mushrooms on paper, and that those of Peziza and Helvella are forcibly ejected in little clouds of dust; he also produced new proofs of Gleditsch’s statement, that the spores of Fungi are disseminated everywhere by the air. Schmitz published in ‘Linnaea,’ between the years 1842 and 1845 excellent observations on the growth and mode of life of several of the larger Fungi. It was not unnecessary at that time to make it clearly understood that the spores of Fungi reproduce their species exactly.