History of Botany (1530-1860)

by Julius Sachs

These arguments like those of Jung, and in the main also those of Malpighi, rested on facts which were on the whole as well known in ancient times as in the 17th century; but no one had before given heed to considerations, which were in themselves quite sufficient to do away with the Aristotelian teaching on the subject of the nutrition of plants.

In the second part of his letter Mariotte discusses the phenomena of vegetation which depend on nutrition; he compares the endosperm in the seed with the yolk of the egg in animals, and the entrance of the water into the roots with its rising in capillary tubes; he takes the milky juice to be the nutrient sap and compares it with arterial blood, the other watery juices answering to venous blood. He says something quite new about the pressure of the sap; he notices the high pressure at which the sap stands in plants, and concludes from it that there must be contrivances in them, which allow of the ingress of the water but not of its egress. The existence of the pressure is well demonstrated by the outflow from plants which contain milky juice when they are wounded, and is compared with the pressure on the blood in the veins. Equally striking is his further conclusion, that the pressure of the sap expands the roots, branches, and leaves, and so contributes to their growth. The sap, he adds, would not be able to remain at this pressure, if it did not enter by pores, which forbid its return. In these remarks lay the first germs of speculation on the growth of plants, such as we shall meet with in Hales also in a somewhat different form, but in the backward state in which phytotomy then was they could not at present be further developed; we shall recur to them further on, though in a different connection.

Mariotte concluded that the primary sap finds its way into the plant through the leaves as well as through the roots from the fact, that if a branch is taken from a tree, and one of its smaller branches kept in water, another will remain fresh for some days; the conclusion was not quite justified, as the future showed. His remarks on the necessity of sun-light to nutrition, on the ripening of fruit, and other matters, rests on very imperfect experience and need not be noticed.

The characteristic and the important point in Mariotte’s theory of nutrition is the marked contrast between his point of view in natural science and the Aristotelian and scholastic doctrines still widely diffused, and thus he is led to declare war also against Aristotle’s vegetable soul. He connects his remarks on this point with a fact which excites his astonishment, namely that every species of plant reproduces its properties so exactly; no explanation of this fact, he says, is gained by the assumption of a vegetable soul, of which no one knows what it is. He declares as decidedly against the theory of evolution, also much in vogue in his day. In opposition to the notion that all future generations are shut up one inside another in the seeds of a plant, he thinks it much more probable that the seeds only contain the essential substances, and that their influence on the crude sap brings about the successive formation of the rest of the constituents of the plant, a view which we may still allow to be correct. He regards the whole process of nutrition and life in plants as a play of physical forces, as the combination and separation of simple substances, but he believes at the same time that he can prove the commonly received doctrine of spontaneous generation to be a necessary conclusion from this view. On this point he went wrong from want of sufficient and well-sifted experience, for he regarded it as a proof of generatio spontanea that numerous plants spring up from the soil thrown out from ditches and swamps that have been laid dry. ‘We may therefore suppose,’ he says, ‘that there are in the air, in the water, and in the earth an infinite number of minute bodies so fashioned that two or three uniting together may make the beginning of a plant, and represent the seed of such a plant, if they find a soil favourable to their growth. But it is not probable that this little complex body contains already all the branches, leaves, fruits, and seeds of this plant, and still less that this seed contains all the branches, leaves, flowers, etc., which proceed ad infinitum from the first germination.’ The contrary he thinks is proved by the fact, that a rose-bush which has lost its leaves in the winter may produce in the next year nothing but leafy shoots from its flower buds, which shows that the blossoms were not previously formed in those buds, and that a similar conclusion is to be drawn from another fact, that the seeds of one and the same fruit-tree or of a melon produce descendants that differ from one another by variation; here we have an argument against the theory of evolution much more to the purpose than the greater part of those which were alleged against it before Koelreuter obtained his hybrids.

Other prejudices also of his day were opposed by Mariotte, and on good grounds; the medicinal effects, commonly known as the ‘virtutes’ of plants, played an important part in the botany, and still more in the medicine and chemistry of that time. He rejects the old theory of heat and cold, moisture and dryness, things supposed to be essentially immanent qualities of the substance of plants and used to explain their medicinal effects, and pointing to the fact, that poisonous plants grow in the same soil as harmless ones and side by side with them, he concludes, as he had before concluded, that different plants do not derive their peculiar constituents immediately from the soil, but that they form them themselves by separation and combination of the common principles. Finally he declared against one of the grossest errors which had come down from the previous century, the ‘signatura plantarum,’ which supposed that the medicinal properties of plants could be deduced from their external features, and especially from resemblances between their organs and the organs of the human body. Mariotte insists that the medicinal properties of plants are to be ascertained by trying them on sick people.

Mariotte’s letter, the most important parts of which have here been given, presents us with a lively picture of the views which prevailed in the second half of the 17th century respecting the life of plants; it shows at the same time how an eminent investigator of nature, adopting the principles of a more modern philosophy and knowing how to make a skilful use of the facts that were known to him, was led to oppose antiquated error, the result of prepossessions and want of reflection. If we combine the views of Malpighi on the internal economy of the plant, derived chiefly from its anatomy, with the chemical and physical disquisitions of Mariotte, we have an entirely new theory of the nutrition of plants, not only antagonistic to the Aristotelian doctrine, but distinguished from it by a much greater wealth of ideas and by more sagacious combinations.

These two men had in truth discovered all the principles of vegetable life and nutrition, which could have been discovered in the existing condition of phytotomy and chemistry; Mariotte especially had succeeded in applying the very best that was to be obtained from the uncertain chemical knowledge of his day to the explanation of the phenomena of vegetation. Chemistry was at that time beginning to set herself free from the notions of the medical science, the iatro-chemistry of a former age, only to throw herself into the arms of the theory of the phlogiston; and how little she could contribute to the explanation of the processes of nutrition in plants, how little the methods then in use were adapted to the examination of organised bodies, may be learnt from a little book published in 1676 and again in 1679, ‘Mémoires pour servir à l’histoire des plantes,’ which appeared indeed in Dodart’s name, but which was compiled and approved by the body of members of the Academy of Paris. It contains no results of investigation, but a detailed scheme for researches into botanical science, and more particularly into the chemical part of it. There we read, that plants must be burnt slowly, in order that the destroying and transmuting power of the fire may have less effect; the ‘virtutes plantarum’ play an important part in the chemical examination of plants, and blood was mixed with their juices, in order to discover their properties. A writer named Dedu in a treatise, ‘De l’âme des plantes’ (1685) derived the generation and growth of plants from the fermentation and effervescence of the acids in combination with the alkalies, as Kurt Sprengel informs us. It is by comparison with these and similar notions that we recognise the full superiority of the utterances of Malpighi and Mariotte respecting the nutrition of plants, and their sagacity is still further shown by the fact, that there are some things which they forebore to say, evidently because they thought that they were not clearly proved.

The views of Malpighi and Mariotte on the nutrition of plants were respected and often quoted by their contemporaries and immediate successors; but as has happened in other cases unfortunately up to recent times, much that was fundamentally important and significant in them was neglected from the first for comparatively unimportant matters, and the views of these clear thinkers were so mixed up with indistinct ideas and actual misconceptions, that no real advance was made, though a variety of new facts were from time to time brought to light. It has been already noticed that Malpighi’s correct idea of the connection of the leaves with the nutrition of the plant was at a later time commonly supposed to be equivalent to Major’s theory of circulation, and since the latter was for various reasons considered to be incorrect, it was thought that Malpighi’s view was dismissed with it. Yet even Major’s theory deserved the preference over the views of those who assumed only an ascent of the sap in the wood, because it at least attempted to account for certain phenomena of growth. It found a new supporter in 1680 in the person of Claude Perrault, who does not however appear[122] to have added anything essentially new to Malpighi’s conclusive arguments for a returning sap. Nor did his opponent Magnol in his very weak treatise published in 1709 succeed in saying anything that will bear examination against the theory of circulation, which he too ascribed to Malpighi.

Among the phenomena of vegetation in woody plants, there is scarcely one so striking as the outflow of watery sap from wounded vines and from some tree-stems in the spring. This phenomenon, like the outflow of milky juice, gum, resin and the like, could not fail to be regarded with lively interest by those who occupied themselves with vegetable physiology in the 17th century. Even supposing the movements of water in the wood and of the milky and other juices in their passages not to be necessary accompaniments of the nutrition of plants, yet it was natural that the physiologists of the 17th century should see in them striking proofs of that movement of the sap which is connected with nutrition, and should therefore make them a subject of study. It might also seem to them that the problem in question was easy to solve, for it was not till long after that it came to be understood that these movements are in reality one of the most difficult questions of vegetable physiology. We discover the interest taken in these matters from a series of communications in the form of letters from Dr. Tonge, Francis Willoughby, and especially from Dr. Martin Lister, to be found in the Philosophical Transactions for 1670[123]. The phenomenon to which these men chiefly directed their attention was just the one best calculated to lead to misconceptions respecting the movements of water in woody plants, namely that which is known as the bleeding of the wood in winter, and which depends on entirely different causes from those which produce the weeping of the vine and other woody plants in spring; but the two things were supposed to be identical, and hence arose an unfortunate confusion of ideas. Lister indeed showed that it is possible to force water out of the wood of a portion of a branch cut from a tree in winter time by warming it artificially, and then to cause the water to be sucked in again by cooling it; but it was reserved for a modern physiologist to prove that this phenomenon has nothing to do with the bleeding of cut stems from root-pressure, and cannot be used to explain it.

John Ray, who gave a clear and intelligent summary of all that was known respecting the nutrition of plants in the first volume of his ‘Historia plantarum’ (1693), also communicated some experiments made by himself on the movements of water in the wood. He follows Grew’s nomenclature, who called the ascending sap in the wood lymph and the woody fibres therefore lymph-vessels, and notices particularly that the lymph especially in spring cannot be distinguished in taste or in consistence from common water. He agrees with Grew that in spring the lymph fills the true vascular tubes of the wood and oozes from them in cross sections, while in summer these are filled with air, and the lymph at that time, when there is strong transpiration in woody plants, ascends only in the lymph-vessels, that is in the fibrous elements of the wood and the bast. By suitable incisions Ray proved that the lymph can also move laterally in the wood; and by causing water to filter in opposite directions through pieces of a branch cut off at both ends, he refuted those who thought that the cavities of the wood and especially the vessels were furnished with valves to hinder the return of the lymph. But his knowledge of the mechanical causes of the movement of water in the wood was not very great.