[THIRD BOOK]
HISTORY OF VEGETABLE PHYSIOLOGY
(1583-1860)
[INTRODUCTION.]
All that was known in the 16th and at the beginning of the 17th centuries of the phenomena of life in plants was scarcely more than had been learnt in the earliest times of human civilisation from agriculture, gardening, and other practical dealing with plants. It was known, for instance, that the roots serve to fix plants in the soil and to supply them with food; that certain kinds of manure, such as ashes and, under certain conditions, salt, strengthen vegetation; that buds develope into shoots; and that the blossom precedes the production of seeds and fruits. These and a variety of minor physiological phenomena were disclosed by the art of gardening. On the other hand, the physiological importance of leaves in the nourishment of plants was quite unknown, nor can we discover more than a very indistinct perception of the connection between the stamens and the production of fruitful seeds. That the food-material taken up from the soil must move inside the plant in order to nourish the upper parts was an obvious conclusion, which it was attempted to explain by comparing it with the movement of the blood in animals. Writers on the subject up to the end of the 17th century make very slight mention of the influence of light and warmth on the sustentation and growth of plants, though doubtless the operation of these agencies in the cultivation of plants, as in other matters, must have been early recognised.
So scanty was the stock of knowledge which the founders of vegetable physiology in the latter half of the 17th century found ready to their hand. While the physiological significance of the different organs of the human body and of most animals were known to every one, at least in their more obvious features, the study of vegetable life had to begin with laborious enquiries, whether the different parts of plants are generally necessary to their maintenance and propagation, and what functions must be ascribed to individual parts for the good of the whole. It was no easy matter to make the first step in advance in this subject; something can be learnt of the functions of the parts of animals from direct observation, scarcely anything in the case of plants; and it is only necessary to read Cesalpino and the herbals of the 16th century to see how helpless the botanists were in every case in presence of questions concerning the possible physiological meaning of vegetable organs, when they ventured beyond the conceptions of the root as the organ of nourishment, and of the fruit and seeds as the supposed ultimate object of vegetable life. The physiological arrangements in vegetable organs are not obvious to the eye; they must be concluded from certain incidental circumstances, or logically deduced from the result of experiments. But experiment presupposes the proposing a definite question resting on a hypothesis; and questions and hypotheses can only arise from previous knowledge. An early attempt to connect the subject with existing knowledge was made in the use of the comparison of vegetable with animal life, a comparison which Aristotle had employed with small success. Cesalpino, provided with more botanical and zoological knowledge, endeavoured to arrive at more definite ideas of the movement of the nutrient juices in plants, and when Harvey discovered the circulation of the blood in the beginning of the 17th century, the idea at once arose that there might be a similar circulation of the sap in plants. Thus a first hypothesis, a definite question was framed, and attempts were made to decide it by more exact observation of the ordinary phenomena of vegetation, and still better by experiment; and though a discussion which lasted nearly a hundred years led to the opinion that there is no circulation of sap in plants corresponding to the circulation of blood in animals, the result was obtained by the aid of this hypothesis derived from a comparison between animals and plants. The important discovery that leaves play a considerable part in the nourishment of plants, was to some extent an incidental product of the investigation of the former question, and it preceded that of the decomposition of carbon dioxide by the green parts of plants by more than a hundred years. To give another example; it was obviously a comparison of certain phenomena in vegetable life with the propagation of animals which paved the way for the discovery of sexuality in plants; long before Rudolf Jacob Camerarius made his decisive experiments (1691-1694) on the necessary co-operation of the pollen in the production of seeds capable of germination, the idea had been entertained that there might be an arrangement in plants corresponding to the sexual relation in animals, though that idea was highly indistinct and distorted by various prepossessions. In like manner the interest excited by the discovery of the irritability of the Mimosae in the 17th century, and of similar phenomena of movement in plants at a later time, was mainly due to the striking resemblance suggested between animals and plants; and the first researches into the subject were obviously intended to answer the question whether the movements in plants are due to conditions of organisation similar to those in animals. In all cases of this kind it was matter of indifference whether the analogies presupposed were finally confirmed after prolonged investigation, as in the question of sexuality, or disproved as in that of the circulation of the sap. The result was of less importance than the obtaining points of departure for the investigation. It answered this purpose to adopt certain actual or only apparent analogies between plants and animals, and to assume, to some extent to invent, certain functions for the apparently inactive organs of plants, and to interrogate them upon the point. Scientific activity was set in motion, and it mattered not what the result might be. In all questions connected with the phenomena of life, our own life is not only the starting-point but also the standard of our conceptions; what animate nature is as opposed to inanimate we discern first by comparing our own being with that of other objects. From our own vital motions we argue to those of the higher animals, which we comprehend immediately and instinctively from their conduct; by aid of these the motions of the lower animals also become intelligible to us, and further conclusions from analogy lead us finally to plants, whose vitality is only in this way made known to us. While plants were thus even in ancient times regarded as living creatures and allied to animals, further reflection naturally suggested the idea that the phenomena of animal life would be reproduced in plants even in details. We learn from the botanical fragments of Aristotle that this was in fact the way in which the first questions in vegetable physiology arose; they assumed a more definite form with Cesalpino, and later physiologists repeatedly made use of similar conclusions from analogy. The historian of this branch of botanical science must seek no other beginning of it, for it had no other and could have no other from the nature of the case. And if preconceived analogies between plants and animals often proved deceptive and mischievous, yet continued investigation gradually brought to light more important and more essential points of agreement between the two kingdoms; it has become more and more evident in our own days, that the material foundations of vegetable and animal life are in the main identical,—that the processes connected with nourishment, movement of juices, sexual and asexual propagation present the most remarkable similarities in both kingdoms.
If the first founders of scientific vegetable physiology surrendered themselves thoroughly to teleological views, this was owing to the circumstances of the time, and it served indeed to promote the first advances of the science. There was no need in the 17th and 18th centuries that a man should be an Aristotelian to presuppose design and arrangements in conformity with design in all parts of physiological investigation. This is everywhere and always the original point of view which precedes all philosophy; but it is the part of advanced science to abandon this position; and as early as the 17th century philosophers recognised the fact that the teleological mode of proceeding is unscientific. But the first vegetable physiologists were not philosophers in the stricter sense of the word, and in their investigations they accepted the teleological conception of organic nature without question, because they regarded it as a self-evident fact, that every organ must be purposely and exactly so made as to be in a condition to perform the functions necessary for the permanence of the whole organism. This conception was in accordance with views then prevailing, and was even useful; it was no disadvantage in the first beginnings of the science, that it should be supposed that every, even the minutest, part of a plant was expressly contrived and made for maintaining its life, for this was a strong motive for carefully examining the organs of plants, which was the first thing requisite. This is exemplified in Malpighi, Grew, and Hales, and we shall see that even towards the end of the 17th century Konrad Sprengel made splendid discoveries respecting the relations of the structure of the flower to the insect world, while strictly carrying out his teleological principles. The teleological view was injurious to the progress of morphology from the first, though the history of systematic botany shows how hard it was for botanists to free themselves from such notions. The case was different with physiology; so long as it was a question of discovering the functions of organs, and learning the connection between the phenomena of life, teleology proved highly useful if only as a principle of research. But it was another matter when it became requisite to investigate causes, and to grasp the phenomena of vegetation in their causal connection. To this the teleological mode of view was inadequate, and it became necessary indeed to discard it as a hindrance, in spite of the difficulty of explaining adaptation in the arrangements of organisms from any other than the teleological point of view. It is sufficient here to say that this difficulty is satisfactorily removed by the theory of selection. This theory is become as important in this respect to physiology, as the theory of descent is to systematic botany and morphology. If the theory of descent finally liberated the morphological treatment of organisms from the influence of scholasticism, it is the theory of selection which has made it possible for physiology to set herself free from teleological explanations. Only an entire misunderstanding of the Darwinian doctrine can allow anyone to reproach it with falling back into teleology; its greatest merit is to have made teleology appear superfluous, where it seemed to naturalists in former times, in spite of all philosophical objections, to be indispensable.
If the comparison of plants with animals as well as the teleological conception of organisms promoted the first attempts at the physiological investigation of plants, other influences of decisive importance came into play when the time came for endeavouring to conceive and explain the causes and conditions of the functions, which had then been ascertained at least in their most obvious features. Phytotomy was here the chief resource. In proportion as the inner structure of plants was better known and the different kinds of tissue better distinguished, it became possible to bring the functions of organs, as made known by experiment, into connection with their microscopic structure; phytotomy dissected the living machine into its component parts, and could then leave it to physiology to discover from the structure and contents of the tissues, how far they were adapted to perform definite functions. Obviously this only became possible when the phenomena of vegetation had been previously studied in the living plant. For example, the microscopic examination of the processes which take place in fertilisation could first be made to yield further conclusions, after sexuality itself, the necessity of the pollen to the production of fruitful seeds, had been proved by experiment; in the same way the anatomical investigation of wood could only supply material for explaining the mode in which water rises in it, when it had first been ascertained by experiment that this happens only in the wood, and so in other cases.
The relation between physiology and physics and chemistry suggests similar considerations; it is necessary to make some preliminary remarks in explanation of this relation, because we often meet with the view, especially in modern times, that vegetable physiology is virtually only applied physics and chemistry, as though the phenomena of life could be simply deduced from physical and chemical doctrines. This might perhaps be possible, if physics and chemistry had no further questions to solve in their own domains; but in fact both are still as far distant from this goal, as physiology is from hers. It is true indeed, that modern vegetable physiology would be impossible without modern physics and chemistry, as the earlier science had to rely on the aid of the physics and chemistry of the day, when she was engaged in forming a conception of ascertained vital phenomena as operations of known causes. But it is equally true, that no advance which physics and chemistry have made up to the present time would have produced any system of vegetable physiology, even with the aid of phytotomy; history shows that a series of vital phenomena in plants had been recognised in the 17th and 18th century, at a time when physics and chemistry had little to offer, and were in no condition to supply explanations of any kind to the physiologist. The true foundation of all physiology is the direct observation of vital phenomena; these must be evoked or altered by experiment, and studied in their connection, before they can be referred to physical and chemical causes. It is therefore quite possible for vegetable physiology to have reached a certain stage of development without any explanation of the phenomena of vegetation from physics or chemistry, and even in spite of erroneous theories on those subjects. What Malpighi, Hales, and to some extent Du Hamel produced, was really vegetable physiology, and of a better kind than some moderns are inclined to believe; and their knowledge was derived from observations on living plants, and not from the chemical and physical theories of their time. The discovery even of important facts, for example, that green leaves only can form the food suitable to effect the growth and formation of new organs, was made a hundred years before that of the decomposition of carbon dioxide by the green parts of plants, at a time indeed when chemistry knew nothing of carbon dioxide and oxygen. A whole series of physiological discoveries might be mentioned, which were distinctly opposed to chemical and physical theories, and even served to correct them. We may give as examples, the establishment of the facts that roots absorb water and the materials of food without giving up anything in return, which seemed quite unintelligible on the earlier physical theory of the endosmotic equivalent; and that the so-called chemical rays of the physicists are of subordinate importance in vegetable assimilation, while contrary to the prevailing notions of physicists and chemists the yellow portions of the spectrum and those adjacent to it actively promote the decomposition of carbon dioxide. From what doctrines of the physicists could it have been concluded, that the downward growth of roots and the upward growth of stems was due to gravitation, as Knight proved in 1806 by experiments on living plants; or could optics have foreseen that the growth of plants is retarded by light, and that growing parts are curved under its influence. Our best knowledge of the life of plants has been obtained by direct observation, not deduced from chemical and physical theories. After these preliminary remarks we may proceed to give a rapid survey of the progress of vegetable physiology.
1. That the first beginnings of vegetable physiology were made about the time that chemistry and physics began to take their place among the true natural sciences, is no proof that they called vegetable physiology into existence. She, like general physiology, mineralogy, astronomy, geography, owed her origin to the outburst of the spirit of enquiry in the 16th and 17th centuries, which feeling the emptiness of the scholastic philosophy set itself to gather valuable knowledge by observation in every direction. It was in the second half of the 17th century that societies or academies for the study of the natural sciences were founded in Italy, England, Germany, and France under the influence of this feeling; the first works on vegetable physiology play a very prominent part in their transactions; not to speak of less important cases, it was the Royal Society of London which published between 1660 and 1690 the memorable works of Malpighi and Grew; the first communications of Camerarius, which form an epoch in the history of the doctrine of sexuality, appeared in the journals of the German Academia Naturae Curiosorum, and the French Academy undertook about the same time to organise methodical researches in vegetable physiology under Dodart’s direction, though the results it is true did not answer to the goodness of the intention. This period of movement in all branches of science, when the greatest discoveries followed one another with marvellous rapidity, witnessed also the first important advances in vegetable physiology; such were the first investigations into the ascending and descending sap, especially those made in England, Malpighi’s theory which assigned to leaves the functions of organs of nutriment, Ray’s first communications on the influence of light on the colours of plants, and above all the experiments of Camerarius, which proved the fertilising power of the pollen. It was the period of first discoveries; the attempts at explanation were certainly weak; but phytotomy which was just commencing its own work lent aid from the first to physiology, while physics and chemistry could do but little for her. On the other hand, the predilection for mechanics and mechanical explanation of organic processes in Newton’s age bore fair fruit in Hales’ enquiries into the movement of sap in plants; his ‘Statical Essays’ of 1727 connect closely with the works before mentioned which had laid the foundations of the science, and with this important performance the first period of its history reaches a distinctly marked conclusion.