History of botany (1530–1860)/Book 2/Introduction
SECOND BOOK
HISTORY OF VEGETABLE ANATOMY
(1671-1860)
INTRODUCTION.
That the substance of the more perfect plants consists of layers of different constitution was a fact that could not escape the most untutored observation in primitive times; ancient languages had still words to designate the most obvious anatomical components of plants, rind, wood and pith. It was also easy to perceive that the pith consists of an apparently homogeneous succulent mass, the wood of a fibrous substance, while the rind of woody plants is composed partly of membranous layers, partly of fibrous and pith-like tissue. The obtaining of threads for spinning from the rind of the flax-plant, for instance, must have suggested some idea, if only a vague one, in the earliest ages of the way, in which the fibrous could be separated from the pulpy part of the bark by decay and mechanical treatment. Neither Aristotle nor Theophrastus failed to compare these components of vegetable substance with corresponding ones in animal bodies, and it has been already shown in the first book how Cesalpino, following his masters, took the pith for the truly living part of the plant and the seat of the vegetable soul, and applied this idea in his morphology and physiology. He remarked that the root generally has no pith, and that the part of the root which answers to the wood of the stem is often soft and fleshy; the composition of the leaves from a green and succulent substance and strands of fibres at once suggested a certain resemblance to the green rind of the stem; and it was evidently this which led him to consider that not only the leaves, but also the leaf-forms of the flower-envelopes had their origin in the rind of the stem, while the soft, pulpy, succulent condition of the unripe seeds and seed-vessels seemed to point to their identity with the pith. That not only are juices contained in plants, but that they must move in them, could not escape the simplest reflection; and further, the bleeding of the vine, the flow of gum from resiniferous trees, the gushing of a milky juice from the wounds of certain plants, exhibited so striking a resemblance to the bleeding of a wound in the body of an animal, that the idea of canals inside the plant, which, like the veins in animals, contain those juices and set them in motion, appeared quite natural, as we see plainly from Cesalpino's reflections on these structural conditions. If we add that it was known that the seeds are enclosed in the fruits, and that the embryo, together with a pulpy mass (cotyledons and endosperm), are in their turn enclosed in the seed, we have pretty well the whole inventory of phytotomic knowledge up to about the middle of the seventeenth century.
With careful preparation and skilful dissection of suitable parts of plants, and attentive consideration of the changes produced by decay and corruption, anatomical knowledge might have been considerably enlarged at an earlier time; but seeing is an art that must be learnt and cultivated; a definite aim and end must stimulate the observer into willingness to see exactly, and to distinguish and connect together correctly what he sees. But this art of seeing was not far advanced in the middle of the 17th century. All that was achieved in this direction did not go beyond the distinguishing the outer organs of leaf-forms and stem-forms, and we have seen in the first book how unsuccessful was the attempt to distinguish the minuter parts of the flower and fruit.
The invention of the microscope made small things seem large, and revealed to sight what was too small to be seen without it; but the use of magnifying glasses brought an advantage with it of a different kind—it taught those who used them to see scientifically and exactly. In arming the eye with these increased powers the attention was concentrated on definite points in the object; what was seen was to some extent indistinct, and always only a small part of the whole object; perception by means of the optic nerve had to be accompanied by conscious and intense reflection, in order to make the object, which is observed in part only with the magnifying glass, clear to the mental eye in all the relations of the parts to one another and to the whole. Thus the eye armed with the microscope became itself a scientific instrument, which no longer hurried lightly over the object, but was subjected to severe discipline by the mind of the observer and kept to methodical work. The philosopher Christian Wolff observed very truly in 1721, that an object once seen with the microscope can often be distinguished afterwards with the naked eye; and this, which is the experience of every microscopist, is sufficient evidence of the effect of the instrument in educating and training the eye. This remarkable fact appears also in another way. We saw in the history of morphology and systematic botany that botanists for a hundred years scarcely attempted to make themselves masters in a scientific sense of the external and obvious relations of form in plants, and to consider them from more general points of view. Jung was the first who applied systematic reflection to the morphological relations of plants which lay open before his eyes, and it was not till late in our own century that this part of botany was again handled in a scientific and methodical manner. This extremely slow progress in obtaining a mental mastery over external form in plants on the part of those who are continually occupied with them appears to be due chiefly to the fact, that the unassisted eye glances too impatiently over the form of the object, and the attention of the observer is disturbed by its hasty movements. In direct contrast to this customary want of thoughtfulness in contemplating the external form of plants, we find the first observers with the microscope, Robert Hooke, Malpighi, Grew, and Leeuwenhoek in the latter half of the seventeenth century, endeavouring by earnest reflection to apply the powers of the mind to the objects seen with the assisted eye, to clear up the true nature of microscopic objects, and to explain the secrets of their constitution. If we compare the works of these men with the utterances of the systematists of the same period on the relations of form in plants, we cannot fail to see how superior the matter of the former is in intellectual value. This appears most strikingly when we put what Malpighi and Grew tell us of the construction of the flower and fruit side by side with the knowledge of Tournefort, Bachmann, and Linnaeus on the same subject.
This enhancement of the mental capacity of the observer by the microscope is however the result of long practice; the best microscope in unpractised hands is apt soon to become a tiresome toy. It would be a great mistake to suppose that progress in the study of the anatomy of plants has simply depended on the perfecting of the microscope. It is obvious that the perception of anatomical objects must grow more distinct as the magnifying power of the instrument is increased, and the field of sight is made brighter and clearer, but these things by themselves would not add much to real knowledge. In examining the structure of plants, as in every science, it is necessary to work with the mind upon the object seen with the eye of sense, to separate the important from the unimportant, to discover the logical connection between the several perceptions, and to have a special aim in the examination; but the aim of the phytotomist can only be to obtain so clear an idea of the whole inner structure of the plant in all its connections, that it can be reproduced by the imagination at any moment in full detail with the perfect distinctness of sense-perception. It is not easy to attain this end because the more the microscope magnifies, the smaller is the part of the whole object which it shows; skilful and well-considered preparation is required, careful combination of different objects and long practice. The history of phytotomy shows how difficult a task it is to combine the separate observations and to fashion what has been seen bit by bit into a clear and connected representation.
It appears then that progressive improvement of the microscope was not in itself sufficient to ensure the advance of phytotomy. It would not indeed be too much to say, that the progress which microscopic anatomy made step by step with the aid of imperfect instruments repeatedly gave the impulse to energetic efforts to improve them. Only practical microscopists could tell where the real defects of existing instruments lay; it was their anxiety to make them more manageable, their constant complaints of the poor performance of the optical part—complaints loudly expressed, especially at the end of the previous and the beginning of the present century, which urged the opticians to turn their attention to the microscope and to endeavour to make it more perfect. Moreover, essential improvements in the instrument were made by microscopists themselves. Thus Robert Hooke was the first who in 1760 gave the compound microscope a form convenient for scientific observation, and Leeuwenhoek developed the powers of the simple microscope to their highest point. The modern microscope is greatly indebted for its perfectness to Amici; nor ought the name of von Mohl to be omitted here, who invented improved methods for microscopic measurement, and in his work 'Mikrographie' (1846) on the construction of the microscope gave many practical hints to the opticians.
We shall not then make the most important advances in the anatomy of plants depend as a matter of course and quite passively on the history of the microscope; they were determined here as in other parts of botany by a logical necessity of their own; here as elsewhere we have to fix our eye on the objects pursued by successive enquirers. If for this purpose we cast a glance over the history of the subject, it will appear that its founders in the latter half of the seventeenth century, Malpighi and Grew, were chiefly bent on determining the connection between the cellular and fibrous elements in the structure of plants. Two fundamental forms of tissue were assumed from the first, the succulent cellular tissue composed of chambers or tubes, and, in contrast to this, the elongated usually fibrous or tubular elementary organs, the distinction of which into open canals or vessels and fibres with closed ends continued to be doubtful. The characteristic feature of this period is, that the investigation of the more delicate structure is everywhere closely interwoven with reflections on the function of the elementary organs, and that thus anatomy and physiology support each other, but not without mutual injury through the imperfections of both. But the physiological interest far outweighed the anatomical with the first phytotomists, who used anatomical research for the purposes of physiology.
The imperfectness of the microscope during the whole of the eighteenth century produced a certain disinclination to anatomical studies, which were after all only regarded as auxiliary to physiology. The latter had made very important progress without the help of anatomy in the hands of Hales, and later on towards the end of the 18th century in those of Ingen-Houss and Senebier, and thus the interest in phytotomy was almost extinguished. Not only was very little addition made to the contributions of Malpighi and Grew during the 18th century, but they had to some extent ceased to be understood.
However towards the end of that time the microscope came again into fashion; in the compound form it had become somewhat more convenient and manageable; Hedwig showed how it revealed the organisation of the smallest plants, and especially of the Mosses, and he examined also the construction of cell-tissue and vascular bundles in the higher plants. But with the beginning of the present century the interest in phytotomy suddenly rose high again; Mirbel in France, Kurt Sprengel in Germany made the microscopic structure of plants once more the subject of serious investigation. The performances of both men were at first extremely weak and contradicted one another; a lively dispute on the nature of cells, fibres, and vessels grew up during the succeeding years, and many German botanists soon took part in it; life was once more infused into the whole subject, especially when the academy of Göttingen in 1804 offered a prize for the best essay on the disputed points, for which Link, Rudolphi and Treviranus contended, while Bernhardi occupied himself with private researches into the nature of vessels in plants. It was not much that was attained in this way; men began once more from the beginning, and after 130 years Malpighi and Grew were still the authorities to whom everybody appealed. Yet the questions now discussed were in the main different from the old ones; Malpighi, Grew and Leeuwenhoek had chiefly set themselves the task of studying the different tissues in their mutual connection; the moderns were chiefly concerned to get a clearer understanding of the more delicate construction of the various tissues themselves, to know what was the true account of cell-structure in parenchymatous tissue, and the real nature of vessels and fibres. That very slow progress was made at first in this direction was due partly to the imperfectness of the microscope, and still more to very unskilful preparations, to the influence of various prejudices, and to too slight exertion of the mind. But a comprehensive work by the younger Moldenhawer in 1812 was a considerable step in advance. It is marked by careful and suitable preparation of the objects, and by critical examination of what was observed by the writer himself and of what had been written by others; in fact it is a fresh commencement of a strict scientific treatment of phytotomy. Hugo von Mohl continued Moldenhawer's work after 1828, and Meyen was a contemporary and a zealous student of phytotomy; but the period in the study of vegetable anatomy which reaches to 1840 may be said to have been brought to a conclusion chiefly by von Mohl's contributions. Weak as the beginnings were at the commencement of this period (1800–1840), and important as was the advance made by von Mohl towards the end of it, yet we may include all that was done during that time in one view, since the questions examined were essentially the same; like Mirbel and Treviranus, Moldenhawer and Meyen, von Mohl was chiefly occupied up to the year 1840 in deciding the questions, what is the nature of the solid framework of cellulose in the plant in its matured state, whether a single or double wall of membrane lies between two cell-spaces, what is the true account of pits and pores, and of the various forms of fibres and vessels; one great result of these efforts must be mentioned, namely, the establishment of the fact that all the elementary organs of plants may be referred to one fundamental form, the closed cell; that the fibres are only elongated cells, but that true vessels are formed by cells which are arranged in rows, and have entered into free communication with one another.
Phytotomists before 1840, and von Mohl especially, had occasionally paid attention among other things to circumstances connected with the history of development, and single cases of the formation of various cells had been described by von Mohl and Mirbel between 1830 and 1840, but greater interest was taken in the right understanding of the structure of mature tissues; physiological questions also, though no longer of the first importance in anatomical investigations, were still of weight, so far as the enquiry was influenced by the relation of anatomical structure to the functions of elementary organs. But with Schleiden and Nägeli the question of historical development and the purely morphological examination of interior structure assumed an exclusive prominence in phytotomy. The first commencement of vegetable cells especially and their growth were the subjects now discussed. Schleiden had proposed a theory of cell-formation before 1840, which, resting on too few and inexact observations, referred all processes of cell-formation in the vegetable kingdom to a single form; it attracted great attention in the botanical world, but could not easily be reconciled with what was already known; and in 1846 it was completely refuted by Nägeli, who substituted for it the history of the formation of the various kinds of vegetable cells in their main features, based on profound and extensive investigations. It was natural that these researches into the formation of cells should turn the attention of observers, which had hitherto been almost exclusively devoted to the solid framework of cell-tissue, to the juicy contents of cells. Robert Brown had already discovered the cell-nucleus; Schleiden recognised its more constant presence, but misunderstood its relation to cell-formation; Nägeli and von Mohl next demonstrated the peculiar nature of protoplasm, the most important component of vegetable cells, and especially the weighty part which it plays in their origination. Unger in 1855 called attention to the great resemblance which exists between the protoplasm of the vegetable cell and the sarkode of the more simple animals,—a discovery which was subsequently brought into prominence by observations on the behaviour of the Myxomycetes, and after 1860 finally led zootomists as well as phytotomists to the conclusion, that protoplasm is the foundation of all organic development, vegetable and animal. But there is yet another direction in which the study of the history of development by the phytotomist led to new points of view and to new results; we have already pointed in the end of the first book to the way in which Nägeli after 1844 made the sequences of cell-division in the growth of organs the basis for his morphology, and how in this way the Cryptogams especially revealed their inner structure; we also noticed the splendid results which Hofmeister achieved by his study of the development of the embryo; here we have further to show, how after 1850 the various forms of tissue, especially the vascular bundles, were examined by observation of the history of their development, and how in this way botanical science has succeeded in explaining the inner histological connection between leaves and axes, shoots and parent-shoots, primary and secondary roots, and above all in gaining a correct insight into subsequent growth in thickness and so learning to understand the true mode of formation of a woody body and of the secondary rind. It is then the task of the following chapter to give a more detailed account of the history of phytotomy, the salient points in which have now been indicated.