We see what good use Hales could make of the small stock of ideas in physics and chemistry at his disposal, and that he succeeded with their help in rising to a point of view, from which he was able to form some idea of the phenomena of vegetation in their most important relations to the rest of nature, and in their inner course and connection. But his successors did not comprehend the fundamental importance of these considerations, and made no use of the pregnant idea, that a much larger part of the substance of plants comes from the air and not from the water or the soil; they were for ever wondering that so little is furnished by the soil to the plant, as Van Helmont had shown, though they did not confess to supposing that the water was changed into the substance of the plant, as he had imagined. Thus physiologists lost sight of the principle, which might long before the time of Ingen-Houss have sufficiently explained the most important of all the relations of the plant to the outer world, namely that it derives its food from the constituents of the atmosphere, and so neglected further experimental enquiry into the matter; they quoted and repeated Hales’ experiments and observations again and again, but forgot that which in his mind bound all the separate facts together.
Hales is the last of the great naturalists who laid the foundations of vegetable physiology. Strange as some of their ideas may seem to us, yet these observers were the first who gained any deep insight into the hidden machinery of vegetable life, and handed down to us a knowledge both of individual facts and of their most important relations. If we compare what was known before Malpighi’s time with the contents of Hales’ book, we shall be astonished at the rapid advance made in less than sixty years, while scarcely anything had been contributed to the subject in the period between Aristotle and Malpighi.
3. Fruitless attempts to explain the movement of the sap in plants.
1730-1780.
If those, who studied the nutrition of plants and especially the movement of their sap in the period between Hales and Ingen-Houss, had kept a firm hold on Malpighi’s view, that the nutritive substances are elaborated in the leaves, and had combined it with Hales’ idea, that plants derive a large portion of their substance from the air, they would have had a principle to guide them in their investigations into the movement of the sap; and by experimenting on living plants they might have succeeded in giving a more definite expression to these ideas, even though chemistry and physics supplied during that time no new aids. We have said already that such was not the course of events; physiologists confined their attention to the obvious phenomena of vegetation, and trusted in so doing to gain a firmer footing, but in this they never got beyond a commonplace and unreflecting empiricism, because their observation was without an object, and their conclusions without a principle. They wandered from the right direction, as always happens when observation is not guided by a well-considered hypothesis; and their conceptions were rendered more obscure by their imperfect acquaintance with one of the most important aids to understanding the movement of the sap, namely the structure of the more delicate parts of the plant, the knowledge of which had not advanced since the days of Malpighi and Grew. Since most of them made no phytotomical investigations of their own, and only partially understood the descriptions of those writers, they had to be content with misty and often quite inaccurate ideas of the inner structure of wood and bark, and yet expected to obtain an insight into the movement of the sap in them. In reading the writings of Malpighi, Grew, Mariotte, Hales and even Wolff, notwithstanding many mistakes in details we find a pleasure in the connected reasoning, and in the sagacity which knew how to distinguish between what was important and what was not; whereas the observers, whom we have now to mention, give us only isolated statements, nor have we the satisfaction of feeling that we are conversing with men of superior understanding.
We may pass over the unimportant writings of Friedrich Walther (1740), Anton Wilhelm Platz (1751) and Rudolph Böhmer (1753), as merely barren exercises; but some notice should be taken of those of De la Baisse and Reichel, since these authors at least endeavoured to bring to light something new. But the method which they employed of making living plants suck up coloured fluids was calculated to give rise to serious errors both at the time and afterwards. Magnol had mentioned experiments of the kind in 1709, and the Jesuit father Sarrabat, known by the name of De La Baisse, occupied himself with them and described them in a treatise, ‘Sur la circulation de la sêve des plantes,’ 1733, which received a prize from the academy of Bordeaux[125]. He set the roots of different plants in the red juice of the fruit of Phytolacca, and found that in two or three days the whole of the bark of the roots and especially the tips of the root-fibres were coloured red inside. It was a natural conclusion at that time, that it was these parts which chiefly absorbed the red colouring matter, and in fact this opinion was maintained till quite recent times, and it was on such results that Pyrame de Candolle founded his theory of the spongioles of the root, which is still accepted in France. At present it is known, that the bark and especially the youngest tips of the fibres of the root are not coloured under these circumstances, until they have been first poisoned and killed by the colouring matter; these experiments therefore, which have been frequently repeated since De la Baisse’s time, prove nothing respecting the action of living roots, but they were from the first the cause of a pernicious error in vegetable physiology, which as we shall see gave rise to others also. One result however of De la Baisse’s experiments was less misleading; he placed the cut ends of branches of woody plants in the coloured fluid, and found that not only the general body of the wood, but the woody bundles which pass from it into the leaves and parts of the flowers, were coloured red, while the succulent tissue of the bark and leaves remained uncoloured. It appeared therefore that the red juice passed only through the wood, and a somewhat bold analogy might lead to the further conclusion that this is true also of the nutrient substances dissolved in the watery sap; but the view so stated is not at present considered to be correct, and that the sap which ascends from the roots to the leaves, the water especially, is conveyed through the wood only, and not through the rind, had been already sufficiently proved by the experiments of Hales and others. The uncritical treatment of experiments of this kind by Georg Christian Reichel[126] afterwards led to new errors, though his dissertation, ‘De vasis plantarum spiralibus,’ shows to advantage by the side of similar productions of the day owing to its careful notices of the literature, and the author’s original researches in phytotomy. Reichel was not satisfied with the arguments of Malpighi, Nieuwentyt, Wolff, Thümmig and Hales for the view that the vessels of the wood contain air. He observed quite correctly, that if branches are cut off from woody and herbaceous plants and the cut surfaces are placed in red decoction of brazil-wood, the red colouring matter spreads through all the vascular bundles, even those of the flowers and fruit; but on examination with the microscope he found the red fluid to some extent in the cavities of the vessels, and hastily concluded that they too in the natural condition convey sap and not air. His description and his drawing show however, that only some vessels had received any of the red fluid and that none of these were filled with it. Reichel and the many who repeated his statements forgot to ask whether the vessels had contained air or fluid before the experiment, or whether the result would have been the same, if plants with uninjured and living roots had absorbed the coloured fluid, and no divided vessels had therefore come in contact with it. There was no reason why observers of that day should not have been alive to the simple consideration, that the vessels of a branch parted from the stem and placed in a fluid must necessarily show the capillary action of narrow glass tubes if they are filled with air in their natural condition, and that in the experiment the transpiration of the leaves must favour the ascent of the red juice in the cavities of the vessels, as was to be gathered from other and better experiments made by Hales. But these obvious reflections were not made; the supposed results of the experiment were heedlessly accepted, and the unfounded notion, that vessels are natural sap-conducting organs, was set up in opposition to the trustworthy decision of Malpighi and Grew, that they convey air. Thus on the strength of badly interpreted experiments one of the most important of physiological discoveries was called in question, and a hundred years later there were persons, who, relying on the same experiments as Reichel, supposed that the vessels of the wood convey the ascending sap, a view which made it impossible from the first to arrive at any real understanding of the movement of the sap in plants provided with organs of transpiration. But even the other great discovery which we owe to Malpighi, that leaves are organs for elaborating the food, was denied by Bonnet, who substituted for it the utterly false view, that they chiefly serve to absorb rain-water and dew. Bonnet[127], who had previously done good service to insect-biology, and had discovered the asexual propagation of aphides, having injured his eyes in these studies, found an agreeable pastime in a variety of experiments on plants. Much that he did was unimportant, yet he obtained some results, which could afterwards be turned to account by more competent persons, for the weakness of his own judgment is shown even in his more serviceable observations, such as those on the curvature of growing plants. We notice the same defect in his observations on the part played by leaves in the nutrition of the plant. It shows the character of the time that a book like Bonnet’s ‘Recherches sur l’usage des feuilles des plantes,’ a mere accumulation of undigested facts, should have been generally considered an important production. He tells us, that his attention was called by Calandrini to the fact, that the structure of the under side of leaves seems to show that they were intended to absorb ‘the dew that rises from the ground’ and introduce it into the plant. Starting from this sensible suggestion, as he calls it, he proceeded to make a variety of senseless experiments with leaves, which were cut off from their plants, and having been smeared over with oil or other hurtful substances were laid on water, some on their upper some on their under side, the object being to note the time which they took to perish. It is impossible to imagine worse-devised experiments on vegetation; for if Bonnet wished to test Calandrini’s ‘sensible’ conjecture, he ought certainly to have left the leaves on the living plants and have observed the effect of the supposed absorption of dew on the vegetation. It is to be observed, that by rising dew he evidently meant aqueous vapour, for the real dew descends chiefly on the upper side of the leaf; and what could he have expected to learn by laying cut leaves on water? how could this prove that leaves absorb dew? Nevertheless Bonnet came to the conclusion that the most important function of leaves was to absorb dew, and in order to make this result agree with Hales’ investigations on transpiration, he propounded the theory[128], that the sap which rises by day from the roots into the stem is carried by the woody fibres assisted by the air-tubes into the under side of the leaves, where there are many stomata to facilitate its exit (evaporation). At the approach of night, when the leaves and the air in the air-tubes are no longer under the influence of heat, the sap returns to the roots; then the under side of the leaves commences its other function; the dew slowly rising from the earth strikes against it, condenses upon it, and is detained there by the fine hairs and by other contrivances (this really takes place to a much greater extent on the upper side). The fine tubes of the leaves absorb it at once, (this is evidently not so, since the dew increases in quantity till sunrise), and conduct it to the branches, whence it passes into the stem. He thought so highly of this strange theory, that he believed he found in it a teleological explanation of the heliotropic and geotropic curvature of leaves and stems, two things which he did not distinguish, and of the position of leaves on the stem. Bonnet’s view of the functions of leaves, foolish as it is, is historically important and therefore required to be noticed, because it was really accepted during many years in preference to the older and better ideas, and because it shows how the power of judging of such matters had fallen off since Malpighi’s time. It appears to have been the praise lavished on Bonnet by his contemporaries that made later physiologists, who might have known better, take him for an authority on the nutrition of plants. His experiments on the growth of plants in another material than earth are if possible more worthless than those with cut leaves. Here too the idea was not his own; for hearing that land-plants had been grown in Berlin in moss instead of earth, he made numerous experiments of the kind, and found that many plants grow vigorously in this way, and bloom and bear seed. But the theory of nutrition gained nothing by these experiments, which were only a childish amusement. The few pages which Malpighi wrote on the nutrition of plants are worth more than all Bonnet’s book on the use of leaves; the former by the help of some simple considerations and conclusions from analogy really discovered the use of leaves; Bonnet on the faith of many unmeaning experiments ascribed to them another function than the true one.
We are unable to pass a much more favourable judgment on the views respecting the nutrition of plants of another writer, who otherwise did good service to vegetable physiology, and to whom we shall return in our last chapter. It is true that Du Hamel[129], of whom we speak, was not an investigator of nature, as were Malpighi, Mariotte or Hales; compared with those great thinkers he was only a compiler, and a somewhat uncritical one. But he was not a dilettante in science, like Bonnet; he made the vegetable world the subject of serious and diligent study, and he endeavoured to turn the results of that study to practical account. Long familiarity with plants gave him a kind of instinct for the truth in dealing with them, as is shown in his observations and experiments, many of which are still instructive; but he had neither that faculty of combination which can alone bring a meaning out of experiments and observations in physiological investigations, nor the power to distinguish between matters of fundamental and secondary importance. So thinks also his biographer Du Petit-Thonars.
The merits and the faults here mentioned are combined in an especial degree in Du Hamel’s most famous work, ‘Physique des arbres,’ which appeared in two volumes in 1758 and is a text-book of vegetable anatomy and physiology with numerous plates. His remarks on the nutrition of plants and the movement of the sap are a lengthy compilation chiefly from Malpighi, Mariotte and Hales, though he has not succeeded in appropriating exactly that which is theoretically important or adopting the most commanding points of view. He introduces the results of his own experiments into his account, and these are often instructive in themselves, but are never made use of to establish a definite view with respect to the connection between the processes of nutrition. He hits upon the right view only when he is dealing with plain and obvious matters; for instance, he restores the vessels of the wood to their old rights, and concludes from experiments, as had been already done in the 17th century, that an elaborated sap moves in the reverse direction in the rind; so too he perceives that if bulbs, tubers, and roots, with or without the help of water which they have absorbed, produce shoots and even flowers, this must be done at the expense of material laid up in reserve, but he does not turn this fact to any further account. But he utterly spoilt the best part of his subject; he made the leaves nothing but pumps that suck up the sap from the roots; he quotes Malpighi’s better view as a curiosity, and never mentions it again; but he accepts Bonnet’s unfortunate theory, though he himself adduces many facts, which make for Malpighi’s interpretation of the leaves. He is almost more unsuccessful with chemical points in nutrition; he repeats Mariotte’s statements with regard to the necessity of a chemical change in the nutrient substances in the plant, and even supplies further proof of it; but he cannot shake off the Aristotelian dogma, that the earth like an animal stomach elaborates the food of plants, and that the roots absorb the elaborated matter like chyle-vessels (II. pp. 189, 230). He concludes from his own attempts to grow land-plants without earth and in ordinary water that the latter supplies the plant with very little matter in solution, but he makes no use of Hales’ statements with regard to the co-operation of the air in the building up of the plant, and ends by saying (II. p. 204) that he only wished to prove that the purest and simplest water can supply plants with their food, which his experiments do not prove. Thus almost all that Du Hamel says on the nutrition of plants is a mixture of right observations in detail with wrong conclusions, and reflections which never rise above the individual facts and give no account of the connection of the whole. These faults appear in a still higher degree in a later and almost more comprehensive work, the ‘Traité théorique et pratique de la végétation’ of Mustel (1781). The further the distance from the founders of vegetable physiology, the larger were the books that were written on the subject; but the thread that held the single facts together became thinner and thinner, till at last it broke. The theory of nutrition, like a forced plant, needed light that it might recover strength. This light came with the discoveries of Ingen-Houss, and with the mighty strides made by chemistry after 1760 in the hands of Lavoisier.
4. The modern theory of nutrition founded by Ingen-Houss and Theodore de Saussure.
1779-1804.
The two cardinal points in the doctrine of the nutrition of plants, namely that the leaves are the organs which elaborate the food, and that a large part of the substance of the plant is derived from the atmosphere, were established, as we have seen, by Malpighi and Hales, and employed by them in framing their theory; it remained to supply a direct and tangible proof of the fact that the green leaves take up a constituent of the atmosphere and apply it to purposes of nutrition. It was evidently the want of such direct proof which caused the successors of the first great physiologists to overlook the importance of the propositions thus obtained by deduction, and so to grope their way in the dark with no principle to guide them.
The discoveries of Priestley, Ingen-Houss and Senebier, and the quantitative determinations of de Saussure in the years between 1774 and 1804, supplied the proof that the green parts of plants, and the leaves therefore especially, take up and decompose a constituent of the air, while they at the same time assimilate the constituents of water and increase in weight in a corresponding degree; but that this process only goes on copiously and in the normal way, when small quantities of mineral matter are introduced at the same time into the plant through the roots. The discoveries and facts, from which this doctrine proceeded, were those which overthrew the theory of the phlogiston, and from which Lavoisier deduced the principles of modern chemistry; the new theory of the nutrition of plants was indeed directly due to Lavoisier’s doctrines, and it is necessary therefore to take at least a hasty glance at the revolution which was effected in chemistry between 1770 and 1790. It is a well-known fact[130] that this revolution dates from the discovery of oxygen-gas by Priestley in 1774. Priestley himself was and continued to be a stubborn adherent of the phlogiston; but his discovery was made by Lavoisier the basis of an entirely new view of chemical processes. By the combustion of charcoal and the diamond, Lavoisier proved as early as 1776 that ‘fixed air’ was a compound of carbon and ‘vital air.’ In like manner phosphoric acid, sulphuric acid and, after a preliminary discovery by Cavendish, nitric acid also were found to be compounds of phosphorus, sulphur and nitrogen with vital air; in 1777 Lavoisier showed that fixed air and water are produced by the combustion of organic substances, and after establishing within certain limits the quantitative composition of fixed air, he named it carbonic acid, and the gas which had up to that time been known as vital air he called oxygen. Cavendish in 1783 obtained water by the combustion of hydrogen-gas, and then Lavoisier proved that water is a compound of hydrogen and oxygen. These discoveries not only did away step by step with the old theory of the phlogiston, and supplied the principles of modern chemistry, but they also affected exactly those substances which play the most important part in the nutrition of plants; every one of these discoveries in chemistry could at once be turned to account in physiology. In 1779 Priestley discovered that the green parts of plants occasionally exhale oxygen, and in the same year Ingen-Houss described some fuller investigations, which showed that this only takes place under the influence of light, and that the green parts of plants give off carbon dioxide in the dark, as those parts which are not green do both in the light and the dark. A correct interpretation of these facts was not however possible in 1779; it was not till 1785 that Lavoisier succeeded in setting himself quite free from the old notions, and developed his antiphlogistic system into a connected whole. It should be mentioned that he had discovered in 1777 that the respiration of animals is a process of oxidation which produces their internal heat, heat being the product of every form of combustion. This fact was equally important for vegetable physiology, but it was some time before it was used to explain the life of plants.