That plants take up certain substances from their environment for the purpose of building up their own structures could not be a matter of doubt even in the earliest times; it was also obvious, that movements of the nutrient material must be connected with this proceeding. But it was not so easy to say, what was the nature of this food of plants, in what manner it finds its way into and is distributed in them, and what are the forces employed; it was even for a long time undecided, whether the food taken up from without suffers any change inside the plant, before it is applied to purposes of growth. Such were the questions which had engaged the attention of Aristotle, and which formed the chief subject of Cesalpino’s physiological meditations.
But the questions respecting the nutrition of plants acquired a much more definite shape in the latter half of the 17th century, when the various phenomena of vegetation began to be more closely observed, and some attempt was made to understand their relations to the outer world. Malpighi, the founder of phytotomy, was the first who undertook to explain the share which belongs to the different organs of the plant in the whole work of nutrition; guided by analogy, he perceived that the green leaves are the organs which prepare the food, and that the material so prepared by them passes into all parts of the plant, there to be stored up or employed for purposes of growth. But this gave no insight into the nature of the substances from which plants prepare their food. On this point Mariotte endeavoured to give such information as could be obtained from the chemistry of his day; and he has the merit of having shown, in opposition to the old Aristotelian notion, that plants convert the food-material which they derive from the ground into new chemical combinations, while the earth and the water supply the same elements of nutrition to the most different kinds of plants. It could not escape the notice of physiologists even of that time, that the water which plants take up from the ground introduces into them but very small quantities of matter in solution. Van Helmont in the first half of the 17th century had shown this by an experiment, the results of which, however, led him to think that plants were able to produce both the combustible and incombustible parts of their substance from water. Hales at the beginning of the 18th century formed a different opinion, being led by the evolution of the gases in the dry distillation of plants to conclude, that a considerable part of their substance was absorbed in a gaseous form from the atmosphere.
The views propounded by Malpighi, Mariotte, and Hales contained the most important elements of a theory of the nutrition of plants; fully understood they would have taught that one part of the food of plants comes from the earth and the water, and another part from the air; that the leaves change the materials thus obtained in such a manner as to produce from them the substance of plants and to apply this to the purposes of growth; but the ideas were not combined in this way, for during some years after their time botanists were chiefly engaged in observations on the movement of the sap in plants, and they arrived even on this point at very obscure and even contradictory results, because they overlooked the function of the leaves which had already been recognised by Malpighi. All insight not only into the chemical processes in the nutrition of plants, but also into the mechanical laws of the movement of the sap, and generally into the whole internal economy of plants, depends on a knowledge of the fact, that it is only the cells which contain chlorophyll, and therefore in the higher plants the leaves chiefly as consisting largely of such cells, which have the power of converting the gaseous food supplied by the atmosphere into the substance of the plant with the aid of the materials taken up from the soil. This fact is of fundamental importance to the whole theory of the nutrition of plants; it is only by a knowledge of it that we can explain the movement of material connected with nutrition and growth, the dependence of vegetation on light, and to a great extent also the function of the roots.
But this principle could not be discovered till the new chemical system founded by Lavoisier took the place of the old phlogistic chemistry, and it is remarkable that the discoveries, which laid the foundation of modern chemistry in the period between 1760 and 1780, contributed essentially to the establishment at the same time of the modern doctrine of the nutrition of plants. Ingen-Houss, in reliance on Lavoisier’s antiphlogistic views on the composition of air, water, and the mineral acids, succeeded in proving that all parts of plants are continually absorbing oxygen and forming carbon dioxide, but that the green organs at the same time under the influence of light absorb carbon dioxide and exhale oxygen; and as early as 1796 he considered it probable that plants obtain the whole mass of their carbon from the carbon dioxide of the atmosphere. Soon after (1804) de Saussure proved, that plants, while they decompose carbon dioxide, increase in weight by a greater amount than that of the carbon which they retain, and that this is to be explained by the fact that they at the same time fix the elements of water. He likewise showed that the small quantities of saline compounds, which plants take up from the soil, are a necessary part of their food, and that it was at least probable, that the nitrogen of the atmosphere does not contribute to the formation of nitrogenous substances in plants. Senebier had before insisted on the fact, that the decomposition of carbon dioxide under the influence of light only takes place in green organs.
Thus the most important points in the nutrition of plants were discovered by Ingen-Houss, Senebier and de Saussure. But, as often happens in the case of discoveries of such magnitude, their ideas were for a long time exposed to great misunderstanding. They were better appreciated in France than in any other country; Dutrochet and De Candolle were able to see the importance of the interchange of gases in the green organs to the general nutrition and respiration; but others, and especially German botanists, were not content with these simple chemical processes as the foundation of the whole system of nutrition and consequently of the whole life of the plant; the theory of the vital force, which was elaborated in connection with the nature-philosophy during the first years of the 19th century, and was generally accepted by philosophers and physiologists, chemists and physicists, preferred to supply the plant with a mysterious substance for its food, which had its source in the life itself and which it called humus. The most obvious considerations, which must at once have shown that this humus-theory was absurd, were entirely overlooked; and thus in the face of de Saussure’s results the food of plants was once more referred entirely to the soil and the roots, as it was in the earliest times; one of the consequences of this humus-theory in combination with the vital force was that the ash-constituents of plants were supposed to be merely accidental admixtures or stimulants, or to be directly produced in the plant by the vital force.
In the period between 1820 and 1840 the reaction set in from different quarters against the theory of vital force; chemists succeeded in producing by artificial means certain organic compounds, which had hitherto been regarded as products of that force; Dutrochet discovered in endosmose a process, which served to refer various vital phenomena in plants to physico-mechanical principles; de Saussure and others showed that the heat of plants is a product of respiration, and by 1840 the earlier theory of a vital force might be looked upon as antiquated and obsolete. It remained to restore to their rights the observations of Ingen-Houss and de Saussure, which under the influence of that theory and of the notions respecting the humus had been so utterly misconstrued. Liebig set aside the humus-theory in 1840, and referred the carbon of plants entirely to the carbon dioxide of the atmosphere, and their nitrogenous contents to ammonia and its derivatives; he claimed the components of the ash as essential factors in the nutrition, and taking his stand on the general laws of chemistry endeavoured to obtain chiefly by the method of deduction an insight into the chemical processes of assimilation and metabolism. The whole theoretical value of the facts discovered by Ingen-Houss, Senebier and de Saussure was first made apparent by the connection which Liebig succeeded in establishing between the phenomena of nutrition. The doctrine of nutrition burst suddenly into new life; firm ground was gained, and the botanist, no longer distracted by the difficulties raised by the vital force but resting on physical and chemical principles, might now resume the task of investigation. Oxygen-respiration denied by Liebig was first of all re-established by von Mohl and others. Liebig’s views on the source of nitrogen in plants and on the importance of the ash-constituents rested chiefly on general considerations and observations and on calculation, and had now to be tested by systematic investigation and especially by experiments on vegetation in individual plants. And here the place of honour must be assigned to Boussingault, who pursued the path of pure induction as contrasted with Liebig’s deductive mode of proceeding, gradually improved the methods for experimenting on vegetation, and soon succeeded in so producing plants in a purely mineral soil free from all humus, that he finally settled the question of the derivation of the carbon from the atmosphere and of the source of the nitrogen also. He showed from the plants thus artificially nourished, and with due consideration of the many sources of error which beset the question, that the uncombined nitrogen of the atmosphere does not contribute to the nutrition of plants, but that a normal increase in the nitrogenous substances in a plant takes place when the roots take up nitrates as well as the necessary constituents of the ash.
With the exception of some doubts which still remained respecting the necessity of certain constituents of the ash, such as sodium, chlorine and silicic acid, the source of the materials which take a part in the chemistry of the nutrition of plants was known before 1860; but the knowledge obtained with regard to processes in the interior of the plant, the origination of organic substances in the processes of assimilation, and the further changes which they undergo was still fragmentary and uncertain, and led to no general and conclusive results.
Aristotle had sought to determine the nature of the materials which plants take up as food, and had laid down the proposition, that the food of all organisms is not simple but composed of various substances. This view was correct, but he united with it the erroneous notion, that the food of plants is elaborated beforehand in the earth, as in a stomach, and is made applicable to purposes of growth, so as to exclude the necessity of any separation of excrements in the plant; this error was refuted by Jung, as we shall see, but nevertheless it continued to live as late as into the 18th century, and ultimately quite spoilt Du Hamel’s theory of nutrition.
Cesalpino, whom we have learnt to regard as a faithful and gifted disciple of Aristotle, directed his speculations to the mechanical rather than to the chemical side of the question, and chiefly tried to explain the movement of the nutrient sap in plants. He had a larger stock of material drawn from experience at his disposition than his master, and it is instructive therefore to make a nearer acquaintance with his views, because they show how far the old philosophy was in a condition to turn better empirical knowledge than Aristotle possessed to a satisfactory use; they will also show that Cesalpino’s first essays led him to views which can no longer be said to be strictly Aristotelian.
In the second chapter of the first book of the work from which we have already quoted, ‘De plantis libri XVI,’ 1583, he raises the question, in what way the food of plants is taken in and their nutrition accomplished. In animals we see the food conveyed from the veins to the heart, which is the laboratory of the warmth of the body, and after it has been finally perfected there, spread abroad through the arteries into all parts of the body; and this is effected by the operation of the force (spiritus) which is generated in the heart from the food. In plants on the contrary we see no veins, or other channels, nor do we feel any warmth in them, so that it is difficult to understand how trees grow to so great a size, since they seem to have much less natural heat than animals. Cesalpino explains this enigma by saying, that animals require much food for maintaining the activity of the senses and the movements of their organs. The larger quantity of animal food also requires larger receptacles, namely the veins. Plants on the other hand need less food, because this is only used for purposes of nutrition, or to a very small extent for the production of internal heat as well, and therefore they grow more vigorously and bear more fruit than animals. At the same time plants are not without internal heat, though it cannot be perceived by the touch because all objects seem cold to us, which are less warm than our organ of feeling. That plants moreover have veins, though only narrow ones in accordance with the small mass of their food, is shown by those which yield a milky juice, such as Euphorbia and Ficus, which when cut bleed like the flesh of animals; Cesalpino adds ‘and this is very frequent also in the vine,’ which shows that he made no distinction between milky juice and the exuding water of the weeping vine-stock. These narrow veins cannot be seen on account of their fineness; but in every stem and in every root things may be discerned which like nerves in animals can be split longitudinally and are called the nerves of the plant, or also certain thicker things, such as those which branch in most leaves and are there called veins. These should be considered as food-passages and as answering to the veins in animals; but plants have no main vein like the vena cava in animals, but many fine veins pass from the root to the heart of the plant (cor, root-neck, see above, Book I. chap. 2), and ascend from it into the stem; for it was not necessary that the food should be collected in a common receptacle in plants, as it is in the heart in animals, where this is necessary for the production of the spiritus, but it was sufficient that the fluid in plants should be changed by contact with the medulla cordis (in the root-neck), as it is changed in animals in the marrow of the brain or in the liver; and in these organs the veins are very narrow, as they are in plants.
Since plants have no sense-perception, they cannot seek their food like animals, but they draw up the moisture from the ground into themselves in a way of their own; but it is not easy to see how this takes place. Cesalpino, in trying to explain this, gives us a glimpse into the physics of the day, and we observe also to our surprise an attempt made to explain phenomena in living creatures by physical laws, a step beyond the limits of Aristotelian modes of thought and in the right direction. It is not the ratio similitudinis, which draws iron to the magnet, that can cause the attraction of the juice by the roots, for then the smaller would be drawn to the larger; and if the attraction of the fluid of the earth by the roots were the same thing as the attraction of the iron by the magnet, the moisture of the earth would draw out the juice from the plant, which is just what does not happen. Nor can it be the ratio vacui; for since not moisture only but air also is contained in the earth, the plant would be filled not with juice but with air. But Cesalpino hits upon a third kind of cause by which juices may be drawn into the plant. Do not many dry things, he says, in accordance with their nature attract moisture, as linen, sponge and powder, while others repel it, as the feathers of many birds and the herb Adiantum, which are not wetted even when dipped in water; but the former absorb much water, because they have more in common with it than with air; of this kind Cesalpino thinks those parts of plants must be, which the nourishing soul employs to take in food. Therefore these organs are not traversed by a continuous canal such as the veins in animals, but formed like the nerves of a fibrous substance; and thus the power of suction (bibula natura) conveys the moisture continually to the place, where the principle of internal heat is placed, just as may be seen in the flame of a lantern, to which the wick continually conducts the oil. The absorption of the moisture is also increased by the outer warmth, for which reason plants grow more vigorously in spring and summer.
That Cesalpino had no suspicion of the use of the leaves in the nutrition of plants appears incontestably from his repeating the Aristotelian idea, that the leaves are only for the protection of young shoots and fruits from air and sun-light; this idea is no result of speculation, but came simply from observing a vineyard in a hot country.
All that Aristotle and his school, Cesalpino not excepted, are able to tell us about the phenomena of vegetable life, was the result of the most every-day observations, none of which were critically and exactly tested to ascertain their actual correctness, while the larger part of their physiological axioms were not derived from observations on plants at all, but from philosophical principles, and especially from analogies taken from the animal world.
The first step towards a scientific treatment of the doctrine of nutrition was an enlargement and critical examination of the materials to be gained from experience; nor were any difficult observations or experiments needed to discover contradictions between the truths of nature and the old philosophy; all that was necessary was to look into things more closely and to judge of them with less prejudice.
In this way Jung was led to oppose one important point of the Aristotelian account of nutrition. In the second fragment of his work ‘De plantis doxoscopiae physicae minores’ is to be found a remark, which is evidently directed against the notion that plants receive their food already elaborated from the earth, and therefore give off no excrements[116]. Plants, says Jung in accord with Aristotle, appear not to need a thinking soul (anima intelligente), which would be able to distinguish wholesome from unwholesome food, and Aristotle therefore provided them with food which had already been perfectly prepared in the earth. But Jung takes another view founded on actual observation. It is very possible, he says, that the openings in the roots which take in liquid matter are so organised, that they do not allow every kind of juice to enter, and who can say that plants have the peculiarity of only absorbing what is useful to them, for like all other living creatures they have their excreta, which are exhaled through the leaves, flowers, and fruits. But among these he reckons the resins and other exuding liquids, and says that it is possible after all that a large part of the juices of plants escapes by imperceptible evaporation, as happens in animals.
According to Aristotle’s view the plant itself was quite passive in the work of nutrition; since food was offered to it which had been already prepared for it in the earth, growth was to some extent merely a process of crystallisation unaccompanied by chemical change. In pointing to the formation of excreta Jung on the contrary ascribed a chemical activity to the plant, and by supposing that the organisation of the root was such as to prevent the entrance of certain matters and to favour that of others, he made the plant co-operate in its own nourishment, though he did not assume that it needed a thinking soul for this purpose.
Johann Baptist van Helmont[117], physician and chemist, and a contemporary of Jung, took up a position still more decidedly opposed to Aristotelian doctrines. He rejected the four elements of that philosophy, and regarding water as a chief constituent of all things he considered that the whole substance of plants, the mineral parts (the ash) as well as the combustible, was formed from water. Thus while Aristotle made the component parts of plants be introduced into them by water in a state ready for use, Van Helmont, on the contrary, ascribed to the plant the power of producing all kinds of material from water. It would scarcely have been necessary to mention this resistance to old dogmas, originating as it did in the notions of the alchemists, if Van Helmont had not made an attempt to establish his views by experiment; this was the first experiment in vegetation undertaken for a scientific purpose of which we have any information, and it was repeatedly quoted by many later physiologists, and employed in support of their theories. He placed in a pot a certain quantity of earth, which when highly dried weighed two hundred pounds; a willow-branch weighing five pounds was set in this pot, which was protected by a cover from dust, and daily watered with rain-water. In five years’ time the willow had grown to be large and strong, and had increased in weight by a hundred and sixty-four pounds, though the earth in the pot, when once more dried, only showed a loss of two ounces. Van Helmont concluded from this experiment that the considerable increase of weight in the plant had been gained entirely at the cost of the water, and consequently that all the materials in the plant, though distinct from water, nevertheless come from it.
These objections to Aristotelian teaching on the part of Jung and Van Helmont remained isolated and unproductive, but an incentive to new investigations in vegetable physiology was supplied from a different quarter, and its influence lasted till far into the 18th century. This was the suggestion, that not only does a nutrient sap taken up by the roots ascend to the leaves and fruits of plants, but that there is also a movement of the same sap in the opposite direction in the rind. But this idea assumed from the first two different forms. Some botanists, evidently resting on the analogy of the circulation of the blood in animals, supposed that there was also an actual circulation of the sap in plants; others on the contrary were content with supposing that while the watery sap absorbed by the roots rises in the wood, an elaborated sap capable of ministering to growth moves in the rind, the laticiferous vessels, and the resin-ducts. The two views were at a later time repeatedly confounded together, and those who refuted the first believed that they had refuted the other also. It appears that a physician from Breslau, Johann Daniel Major[118], Professor in Kiel, first gave expression to the opinion, that there is a circulation of the nourishing substance in plants as in animals; and from this time to the end of the 18th century the circulation of the juices of plants was a favourite subject of discussion, but more often chosen by the impugners of the doctrine than by its defenders.
The better form of the idea, namely, that there is a return-movement of material towards the root, combined with the view, that the leaves are the organs which produce the substances required for growth from the crude material supplied to them, was expressed by Malpighi as early as 1771 in the shape of a well-considered theory. In his ‘Anatomes plantarum idea’ of that year he devotes the last pages to a short account of the theory of nutrition, as he understood it. He regarded the fibrous constituents of the wood as the organs for conducting the sap taken up by the roots, and the vessels as air-passages, which he named tracheae on account of their resemblance to the tracheae of insects. He was in doubt whether the air came from the earth through the roots, or from the atmosphere through the leaves, for he had never succeeded in finding openings for the entrance of air in the roots or the leaves; but he thought it more probable that the air is absorbed by the roots, because they are well supplied with tracheae, and air has besides a tendency to ascend. Beside these fluid-conducting fibres and air-conducting tracheae in the wood he called attention to the existence of special vessels, which conduct peculiar juices in many plants, as the laticiferous vessels, gum-passages, and turpentine-canals.
Respecting the movement of the juices, he notices that the direction may be reversed, because shoots planted upside down send out roots into the earth from what is organically their upper end, and grow into trees; and though they do not grow vigorously, yet the experiment proves that the movement of the sap in them is in the reverse direction.
After these preliminary remarks he proceeds to prove, that it is in the leaves that the crude juices of nutrition undergo the change which fits them for the maintenance of growth. The way in which Malpighi arrives at this view is as simple as it is original. He considers the cotyledons of young plants to be genuine leaves (in leguminibus seminalis caro, quae folium est conglobatum), as is shown in the gourd, where the cotyledons grow into large green leaves. Liquid is conveyed to them through the radicle, and a portion of the substances which they contain passes from them into the plumule to make it grow, which it will not do if the cotyledons are removed; hence he concludes that all other leaves also are intended to elaborate (excoquere) the nutritive juice contained in their cells, which the woody fibres have conveyed to them. The liquids mingled together in their long passage through the network of fibres are changed in the leaves by the power of the sun’s rays, and blended with the sap before contained in their cells, and thus a new combination of the constituent parts is effected, transpiration proceeding at the same time; he compares the whole process with that which goes on in the blood of animals.
We see that Malpighi’s view of the function of the leaves in nutrition approaches very closely to the truth, as closely indeed as was at all possible in the existing condition of chemical knowledge. He was induced by the results of anatomical investigation to carry this view farther and indeed correctly; he supposed that the parenchymatous tissue of the rind acts in the same way as the leaves; but he went a step too far in assigning the function of the leaves to the colourless parenchyma also, which only serves for the storing up of assimilated matter. He says we must ascribe a character similar to that of the leaf-cells to the corresponding cells in the rind and to those also which lie transversely in the wood (the medullary and cortical rays), and that it is not unreasonable to conclude that the food of the plant is elaborated and stored up in these cells. As he makes no sharp distinction between elaboration and mere storing up, he ascribes the function of the leaves to the parenchyma of fleshy fruits also and to the scales of bulbs; he concludes from the exudations from stumps of trees and from the cut surfaces of other parts of plants, that they are filled with reserve-matter (asservato humore turgent).
Thus the essential points in Malpighi’s theory of nutrition in the year 1671 were, that the vessels of the wood are primarily air-conducting organs, that the leaves elaborate the crude sap for purposes of growth, that the sap so elaborated is stored up in different parts of the plant, and that the fibrous elements of the wood convey upwards to the leaves the crude materials of nutrition which are absorbed by the roots. No mention is made of a circulation of juices, comparable to the circulation of the blood, though this idea was in later times often imputed to him; and we find by his later remarks, that while he was in no doubt as to the elementary organs which convey the ascending sap, he confined himself to conjecture with respect to the way by which the sap elaborated in the cell-tissue of the leaves, rind and parenchyma generally is carried on its further course. But he was in no doubt about the direction of that course; he believed that this sap forces itself downwards through the stem into the roots, and upwards in the branches above the leaves and so into the fruit. Thus Malpighi had formed a more correct idea of the movement of assimilated matter than the majority of his successors who introduced the very unsuitable expression, ‘descending sap.’ He further thought it probable that the elaborated sap passes through the bast-bundles[119], but without a continuous flux and reflux (absque perenni et considerabili fluxu et refluxu); that it rests to some extent in the laticiferous vessels, but that it is also driven sometimes, when occasion requires, by transpiration and external causes into the higher parts of the plant, where it is the means of maintaining growth and nutrition. These later remarks also are better than much that was said about the movement of the sap in the 18th and even in the 19th century, and at all events they prove that to speak of Malpighi as a defender of the circulation of the sap in Major’s sense, as was often done in later times, was an entire misunderstanding of his views.
Malpighi published his theory in a brief and connected form in 1671; it appeared again further worked out in detail in the fuller edition of the Phytotomy in 1674; he attributed a special value to his discovery, that plants require air to breathe as much as animals, and that the vessels of the wood answer in function to the tracheae in insects and to the lungs in other animals; he recurs also several times to the importance of leaves as organs for the elaboration of the food.
If we compare Malpighi’s theory of the nutrition of plants with the views of his predecessors, we cannot help seeing, that it was an entirely new creation, in which Aristotelian doctrines had no share. If his successors had apprehended the important and essential points in his doctrine and had striven by experimenting on living plants to support and illustrate them by new facts, we should have been spared many erroneous notions which established themselves in the theory, and made it a perfect chaos of misconceptions. That particular misconception, which we have already mentioned more than once, namely, that Malpighi, like Major and Perrault after him, assumed a continuous circulation of the juices of the plant, necessarily involved an incorrect idea of the function of the leaves; that function was by many later writers either quite neglected, or sought for chiefly in transpiration, the chemical activity of the leaves being quite overlooked.
Malpighi’s theory can hardly be said to take into consideration the chemical nature of the food of plants; it is chiefly occupied with the relation of the organs to the main points in the nutritive process; its foundations are for the most part laid in the anatomy of the plant. Grew, who in all essential points adopted Malpighi’s views, but without doing much to advance them by his lengthy discussions on particular questions, made some attempt to extend the knowledge of the chemistry of the subject; but his notions were entirely borrowed from the corpuscular theory of Descartes, and he may be said to have constructed his own chemical processes; the consequence was that he usually overlooked the points that were of fundamental importance, and brought nothing to light that could assist the further development of the theory of nutrition. But there is another writer, whose name is in the present day known to few in the history of vegetable physiology, but whose ideas on the chemistry of plants are of great interest. This writer is Mariotte[120], the discoverer of the well-known law of gases, one of the greatest physicists of the latter half of the 17th century, who also enriched the physiology of the human body with some valuable discoveries. We have a tolerably copious treatise of Mariotte’s in the form of a letter to a M. Lantin in the year 1679, to be found in the ‘Œuvres de Mariotte,’ Leyden, 1717, under the title, ‘Sur le sujet des plantes.’ It is highly instructive to gather from this letter the ideas of one of the most famous and ablest of the natural philosophers of that day on chemical processes and conditions in the nutrition of plants, a few years after the appearance of Malpighi’s great work and about the time that Grew’s Phytotomy was being published. It is to be expected that Mariotte should give but an incidental and superficial attention to the more delicate structure of plants; but we are compensated for this by his making us acquainted with everything fundamentally important and new which could at that time be said on the chemistry of the food of plants. Speaking of the ‘elements’ or ‘principles’ of plants, Mariotte propounds three hypotheses. The first is, that there are many immediate principles (principes grossiers et visibles, evidently what we should call proximate constituents) in plants, such as water, sulphur or oil, common salt, nitre, volatile salt or ammonia, certain earths, etc.; and that each of these immediate constituents is a compound of three or four more simple principles, which have united together into one body; nitre for instance has its ‘phlegma’ or tasteless water, its ‘spiritus,’ its fixed salt, and other things; common salt in the same way has the like constituents, and it may be assumed with much probability, that these more simple principles also are compounds of parts that differ among themselves, but are too small to be distinguished by any artificial means as to figure or any other characters. Having shown how certain principles unite together, he goes on to say, that he is unwilling to ascribe to them any sort of consciousness (connaissance) by which they seek to unite together; but he thinks that they are endowed with a natural disposition to move towards one another, and to unite closely as soon as they touch one another; though it is very difficult to define the nature of this disposition, it is enough to know that there are many instances of such movements to be found in nature; thus heavy bodies move towards the centre of the earth, and iron to the magnet; nor are these movements more difficult to conceive, than that of the planets in their courses or of the sun round its axis, or that of the heart in a living animal. With this first hypothesis Mariotte places himself, in opposition to the Aristotelian doctrine with its entelechies and final causes which prevailed at that time among botanists and physiologists, upon the firm ground of modern science with its atoms, and its assumption of necessarily active forces of attraction.
Mariotte’s second hypothesis more specially concerns the chemical nature of plants; he supposes that several of his principes grossiers are contained in every plant, and he endeavours first to explain their source; the motes in the air, he says, which when burnt by lightning smell of sulphur, are carried by rain into the earth, and parts of them are taken up into the plant. Moreover distillation in all plants produces a water, which the chemists call phlegma, and also acids and ammonia, and if the residuum is burnt there remains an ash, from which we obtain an earth which is without taste and insoluble in water, and fixed salts; these salts differ from one another according as they are mixed with more or less acid and ammoniacal spirit or other unknown principles, which the fire could not volatilise. It is not to be wondered at that these principles are found in plants, since they derive their food from the earth which contains them. We see how great has been the advance since the time when Van Helmont believed that he had proved by his experiment, that all the materials in plants come from pure water.
It remained to confront one view of the source of the substances in plants, which was also drawn from the treasure-house of Aristotelian conceptions, and was still in vogue. It was supposed that the very materials of which the plant is composed were contained in their own form in the earth, and had only to be taken up by the roots. Aristotle had himself said: ‘Everything feeds on that of which it consists, and everything feeds on more than one thing; whatever appears to feed only on one thing, as the plant on water, feeds on more than one thing, for earth in the case of the plant is mixed with the water; therefore the country-people water plants with mixtures of things.’ This passage might leave some doubt about Aristotle’s view, if we did not find the following: ‘As many savours as there are in the rinds of fruits, so many it is plain prevail also in the earth. Therefore also many of the old philosophers said, that the water is of as many kinds as the ground through which it runs[121].’ These passages taken with those quoted above show that Aristotle made the substances required for the growth of plants reach them from the earth ready elaborated, as has been before observed; and this view, still maintained in Mariotte’s time, may yet be met with among those who are ignorant of physiology. It is interesting then to see, how vigorously Mariotte exposes the incorrectness and absurdity of this idea, though he has no new discovery to help him. In his third hypothesis he maintains, that the salts, earths, oils, and other things, which different species of plants yield by distillation, are always the same, and that the differences are due entirely to the way in which these principes grossiers and their simplest parts are united together or separated, and he proves it thus: If a bonchretien pear is grafted on a wild one, the same sap, which in the wild plant produces indifferent pears, produces good and well-flavoured pears on the graft; and if this graft has a scion from the wild pear again grafted on it, the latter will bear indifferent fruit. This shows that the same sap in the stem assumes different qualities in each graft. But still more forcible is his proof of the fact, that plants do not take their substance direct from the earth, but produce it themselves by chemical processes. Take a pot, he says, with seven to eight pounds of earth and grow in it any plant you like; the plant will find in this earth and in the rain-water which has fallen on it all the principles of which it is composed in its mature state. You may put three or four thousand different kinds of plants in this earth; if the salts, oils, earths were different in each species of plant, all these principles must be contained in the small quantity of earth and rain-water which falls upon it in the course of three or four months, which is impossible; for each of these plants would yield in the mature state a dram of fixed salt at least and two drams of earth, and all these principles together with those which are mixed with the water would weigh at least from two to three ounces, and this multiplied by four thousand, the number of the species of plants, would give a weight of five hundred pounds.
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.
Some years elapsed before Hales’ labours added materially to the progress which had been already made in the study of these processes in vegetation. His important services to vegetable physiology close our present period, but before we pass on to them, we must first notice a few less important writers. The pages of Woodward and Beale on transpiration and the absorption of water are not very valuable contributions to the theory of nutrition. The fact stated by Woodward, that a Mentha growing in water took up and discharged by evaporation through the leaves forty-six times as much water as it retained in itself, was perhaps the most important of all that he discovered, but his own conclusions from it were of no value.
None of Malpighi’s doctrines had from the first excited so much attention as the one which makes the air which is necessary for the respiration of the plant circulate in the spiral vessels of the wood, as it does in the tracheae in insects; while Grew and Ray after him agreed with Malpighi in the main, his countryman Sbaraglia in 1704 ventured even to deny the existence of such vessels, and before long phytotomy was fallen into such a state of decadence that the question, whether there were any vessels, or as they were then called spiral vessels, at all, was repeatedly affirmed and as often denied again, and ultimately it was thought better in the interest of physiological questions to take counsel of experiment rather than of the microscope. Thus in 1715 Nieuwentyt endeavoured with the help of the air-pump to make the air contained in the vessels issue in a visible form under a fluid. Here we again encounter the philosopher Christian Wolff as a zealous representative of vegetable physiology in Germany; in the third part of his work, ‘Allerhand nützliche Versuche,’ 1721, among other experiments he mentions some which confirmed the presence of air in plants; the question was more interesting, in the state in which physics and chemistry then were, than that of the anatomical character of the air-conducting organs. Wolff submitted leaves lying in water containing no air to the vacuum of the air-pump, and saw air-bubbles issue, especially on the under side; but when he allowed the atmospheric pressure to come into play again the leaves became filled with water, and a piece of fir-wood treated in a similar manner sank after the infiltration. In similar experiments with apricots air issued from the rind and especially from the stalk. Wolff’s pupil Thümmig described similar experiments in his ‘Gründliche Erläuterung der merkwürdigsten Begebenheiten in der Natur,’ 1723, and both continued in this question, as in all their physiological and phytotomical views, faithful adherents of Malpighi, as it was wisest then to be. We must linger a moment longer over Christian Wolff, because he published a few years later a general view of the nutrition of plants in a popular form. Wolff’s services in the dissemination of natural science in Germany seem not to have been as highly appreciated up to the present time as they deserve to be; his various works on natural science, some of which took a wide range and were partly founded on his own observations, were full of matter and for his time very instructive; they contributed moreover to introduce more liberal habits of thought at a time when gross superstitions, such as that of palingenesia, reigned even among men who published scientific treatises in the German Academy of Sciences (the ‘Acta of the Leopoldina).’ If Wolff’s own scientific researches show more good will than skill, yet he had an advantage over many others in a really philosophical training, a habit of abstract thought which enabled him to fix with certainty on what was fundamentally important in the observations of others, and thus to expound the scientific knowledge of his day from higher points of view. For this reason his work which appeared in 1723, ‘Vernünftige Gedanken von den Wirkungen der Natur,’ deserves recognition. It is a work of the kind which would now be called a ‘Kosmos,’ and treats of the physical qualities of bodies generally, of the heavenly bodies and specially of our own planet, of meteorology, physical geography, and lastly of minerals, plants, animals and men. In accordance with his chief object, general instruction, it is written in German and in a good homely style, and contains the best information that was at that time to be obtained on scientific subjects; among these he gives an account of the processes of nutrition in plants, in which he made careful and intelligent use of all that had been written on the subject, bringing together all the serviceable material which he could gather from Malpighi, Grew, Leeuwenhoek, Van Helmont, Mariotte and others into a connected system, and occasionally introducing pertinent critical remarks. If we consider the state of scientific literature in Germany in the first years of the 18th century, we shall be inclined to assign as great merit to comprehensive text-books of this popular character as to new investigations and minor discoveries. Wolff’s chapter on nutrition has however a special interest for us, because it contains several observations of value which were lost sight of after his time. These refer chiefly to the chemistry of nutrition and touch many problems which were not solved before our time; for instance, the statement that it is a well-known fact that the earth loses its fruitfulness, if much is grown on it; that it requires much to feed it, and must be manured with dung or ashes; in these few words we have the questions of the exhaustion of the soil, and the restitution of the substances taken from it by the crop, brought into notice by Wolff at this early period. ‘It should be particularly noted,’ continues Wolff, ‘how fruitful nitre makes the soil; Vallemont has praised the usefulness of nitre, and has mentioned other things which have a like operation by reason of their saline and oily particles, such as horn from the horns and hoofs of animals; dung likewise contains saline and oily particles, which are present in the ash also, and we see therefore that such particles should not be wanting, if a plant is to be fed from water. The seed also, which supplies the first food of the plant, shows the same thing, for there are none which do not contain oil and salt, and there are many from which the oil may be squeezed out; and oil and salt are found in all plants if they are examined chemically.’ He insists on the correctness of the view taken by Malpighi and Mariotte, that the constituents of the food must be chemically altered in the plant. Since every plant, he says, has its own particular salt and its own particular oil, we must readily allow that these are produced in the plant and not introduced into it. But at the same time since plants cannot grow where the soil does not supply them with saline and especially with nitrous particles, it is from these that the salts and oils in the plant must be produced, and the water also changed into a nutritious juice. Further on he alludes to the saline, nitrous and oily particles which float in the air, and says that daily experience shows that most of the substance of putrefying bodies passes into the air, and that if we admit light through a narrow opening into a dark place, we can see a great number of little particles of dust floating about; water also readily takes up salt and earth, and mineral springs show that metallic particles are mixed with it. Therefore there is no reason to doubt that rain-water also contains a variety of matters which it conveys to the plant. Alluding once more to the chemical changes in the constituents of the food which must be supposed to take place in the plant, he connects the subject with some remarks on the organs of plants, in which he closely follows Malpighi; he says that these changes cannot take place in tubes, because the sap merely rises or falls in them; we can only therefore suppose that it is in the spongy substance (the cellular tissue) that the nutrient sap is elaborated, and accordingly the vesicles or utriculi are a kind of stomach; but the change in the water can only be this, that the particles of various substances which are in rain-water are separated from it and united together in some special manner, and this cannot be effected without special movements. But his ideas on these movements in the sap are somewhat obscure. He employs the expansion of the air and the capillarity of the woody tubes as his moving forces. He agrees decidedly with those who postulated a returning sap as well as an ascending crude sap, but he appeals in this matter to Major, Perrault, and Mariotte, and not to Malpighi; yet like Malpighi he notices the growth of trees set upside down as a proof that the juices can move in opposite directions in the conducting organs, and with Mariotte he ascribes the enlargement of growing organs to the expanding power of the juices which force their way into them.
But these well-meant efforts on the part of Christian Wolff, and indeed all that was done from Malpighi and Mariotte to Ingen-Houss to advance the knowledge of the nutrition of plants, was thrown into the shade by the brilliant investigations of Stephen Hales[124] in whom we see once more the genius of discovery and the sound original reasoning powers of the great explorers of nature in Newton’s age. His ‘Statical Essays,’ first published in 1727, reappeared in two new editions in English, and afterwards in French, Italian and German translations; in the last with a preface by Christian Wolff. This was the first work devoted to a more complete account of the nutrition of plants and of the movements of the sap in them, and while it noticed what had been already written on the subject, it was chiefly composed of the author’s own investigations. An abundance of new experiments and observations, measurements and calculations combine to form a living picture of the whole subject. Malpighi endeavoured to discover the physiological functions of organs by the aid of analogies and a reference to their structure; Mariotte discerned the main features of the connection between plants and their environment by combining together physical and chemical facts; Hales may be said to have made his plants themselves speak; by means of cleverly contrived and skilfully managed experiments he compelled them to disclose the forces that were at work in them by effects made apparent to the eye, and thus to show that forces of a very peculiar kind are in constant activity in the quiet and apparently passive organs of vegetation. Penetrated with the spirit of Newton’s age, which notwithstanding its strictly teleological and even theological conception of nature did endeavour to explain all the phenomena of life mechanically by the attraction and repulsion of material particles, Hales was not content with giving a clear idea of the phenomena of vegetation, but sought to trace them back to mechanico-physical laws as then understood. He infused life into the empirical materials which he collected by means of ingenious reflections, which brought individual facts into connection with more general considerations. Such a book necessarily attracted great attention, and for us it is a source of much valuable instruction on matters of detail, though we now gather up the phenomena of vegetation into a somewhat differently connected whole.
His investigations into transpiration and the movement of water in the wood were greeted with the warmest approbation. He measured the quantity of water sucked in by the roots and given off by the leaves, compared this with the supply of moisture contained in the earth, and endeavoured to calculate the rapidity with which the water rises in the stem, and to compare it with the rapidity of its entrance into the roots and its exit by the leaves. The experiments, by which he showed the force of suction in wood and roots, and that of the root-pressure in the case of the bleeding vine, were particularly striking and instructive. His measurements and the figures, on which he founded his calculations, were not so exact as they were often at a later time supposed to be, but he was himself satisfied with obtaining round, approximative numbers; these under given circumstances supplied a sufficient basis for propositions which were new and afforded a certain amount of insight into the economy of the plant. This mode of proceeding showed his understanding; for the case of living bodies is different from that of metals and gases; in these we seek for constants which can then be inserted in general formulae, and to which therefore the nicest accuracy is applied; but in plants we have to deal with individual cases, and it is from a right interpretation of the measurements taken from them that we can arrive at general laws of vegetation.
To show that the forces of suction and pressure which operate in plants are not something sui generis, but prevail also in dead matter, in other words that they are an example of the general attraction of matter, a subject of particular interest at that time, Hales observed the absorption of water by substances with fine pores; and measured the force employed. These processes he compared with the force which swelling peas exert on the obstacles which they encounter, and thus obtained a more correct idea of the forces concerned in the movement of water in the plant than that given by the capillarity of glass-tubes, which Mariotte and Ray had employed to illustrate them.
Hales failed to appreciate the value of Malpighi’s observations on the function of leaves, and was induced by the copiousness of the evaporation of water from their surfaces to overrate the physiological importance of that process; hence he saw in leaves chiefly organs of transpiration, which raise the sap by suction from the roots through the stem. In accordance with this view he denied the existence of a descending sap in the bark, and only admitted that the ascending sap in the wood might possibly sink in the night in consequence of the lowering of the temperature, like the quicksilver in a thermometer, and that so far there might be a return-movement. This was the weak point in Hales’ system.
One of his most important discoveries has generally been overlooked even in modern times, probably because it was entirely neglected by his successors in the 18th century; he was the first who proved, that air co-operates in the building up the body of the plant, in the formation of its solid substance, and that gaseous constituents contribute largely to the nourishment of the plant; consequently that neither water, nor the substances which it carries with it from the earth, alone supply the material of which plants are composed, as had been generally imagined. He showed also with the aid of the air-pump, and better than Nieuwentyt and Wolff, that air enters the plant not only through the leaves but also through apertures in the rind, and circulates in the cavities of the wood. He then connected this with the fact which he had confirmed by numerous experiments, that large quantities of ‘air’ are obtained from vegetable substance by fermentation and dry distillation; the air thus set free by fermentation and heat must in his opinion be condensed and changed to a solid condition during the period of vegetation. He says in chap. 7, that we find by chemical analysis (dry distillation) of vegetables, that their substance is composed of sulphur, volatile salt, water and earth; these principles are all endowed with mutual power of attraction (of their parts). But air also enters into the composition of the plant, and this in its solid state is powerfully attractive, but in an elastic condition has the highest powers of repulsion. It is on infinitely various combinations, actions, and reactions of these principles that all activity in animal and vegetable bodies depends. In nutrition the sum of the forces of attraction is greater than that of the forces of repulsion, and thus the viscid ductile parts are first produced, and then by evaporation of the water the harder parts. But if the latter again absorb water, and the forces of repulsion consequently gain the preponderance, then the consistence of the vegetable parts is dissolved, and this decomposition restores to them the power of forming new vegetable products; therefore the stock of nutritive substance in nature can never be exhausted; this stock is the same in animals and plants, and is fitted by a small change of texture to feed the one or the other.
He goes on to say, that it results from his experiments, that leaves are very useful for the nourishing of the plant, inasmuch as they draw up the food from the earth; but they seem also to be adapted to other noble and important services; they remove the superfluous water by evaporation, retaining the parts of it that are nutritious, while they also absorb salt, nitre, and the like substances, and dew, and rain; and since, like Newton, he regarded light as a substance, he concludes by asking: ‘may not light, which makes its way into the outer surfaces of leaves and flowers, contribute much to the refining of the substances in the plant?’
It might be gathered from these expressions that Hales attributed importance for purposes of nutrition only to the substances suspended in the air; but this was not the case; for we read in the 6th chapter, that he had proved by experiment that a quantity of true permanently elastic air is obtained from vegetable and animal bodies by fermentation and dissolution (dry distillation); the air is to a great extent immediately and firmly incorporated with the substance of these bodies, and it follows therefore that a large quantity of elastic air must be constantly used in forming them.
But Hales not only regards the air as a nourishing substance, but he sees also in its elasticity, which counteracts the attraction of other substances, the origin of the force which maintains the internal movements in the plant. He says that if all matter were endowed only with forces of attraction, all nature would at once contract into an inactive mass; it was therefore absolutely necessary in order to set in movement and animate this huge mass of attracting matter, that a sufficient quantity of strongly repellent and elastic matter should be mixed with it; and since a large portion of these elastic particles are constantly changing to a solid condition through the attraction of the other parts, they must be endowed with the power of again assuming their elastic condition, when they are set free from the attracting mass. Thus the formation and dissolution of animal and vegetable bodies go on in constant succession. Air is therefore very important to the production and growth of animals and plants in two ways; it invigorates their juices while it is in the elastic state, and contributes much to the firm union of the constituent parts, when it has become fixed.
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.