If we examine what both these writers have said on the subject of the nutrition of plants, we shall find that the difference in their general views in physiology as described above appears at once in their treatment of the work of suction in the roots, and of the means by which the sap ascends; here in Treviranus the vital force is everything; it makes the vessels of the wood conduct the juices from the roots into the leaves, with other antiquated notions of the kind; Meyen on the contrary adopts Dutrochet’s position, and even rejects De Candolle’s spongioles. Treviranus knows not what to make of respiration; Meyen explains it without hesitation as a function that answers to respiration in animals, and finds in it the main cause of the natural heat which Treviranus derives in the old mystical fashion from the vital force. In one point however they agree, namely, in a complete misconception of the connection between the decomposition of carbon dioxide in the leaves and the general nutrition of the plant. It is necessary to the understanding of the confusion of ideas which had crept at this time into the doctrine of nutrition, and to a right estimate of the services of Liebig and Boussingault on this point, that we should look a little more closely into the chemical part of the theory of nutrition in Treviranus and Meyen.
Treviranus in the introduction to his book repudiated the idea of a vital force separable from matter, but he was in fact a prisoner within that circle of ideas, and he made a much freer use of the vital force than De Candolle; he went even farther than this, and in his want of chemical experience he hit upon the grossly materialistic notion of a vital matter [I. p. 6]. This vital matter is a half-fluid substance, which may be obtained from all bodies that were once alive by boiling and by decay; it is formed from other elements, but it is itself the true elementary matter with which alone physiology has to do; it is common to the animal and vegetable kingdom, and is purest when in the form of mucilage, albumen, and gelatine; that animals and plants alike consist of this vital matter explains the circumstance, that plants serve as food for animals and animals as food for plants. He goes on to show that a similar unctuous substance, called by chemists extract of the soil, and considered by many of them to be an important ingredient in the nutrition of plants, is their true and proper food. This extract of the soil was therefore the vital matter which plants take up; it was natural that Treviranus should no longer attribute any importance to the decomposition of carbon dioxide in the leaves, especially as he was unable to understand the chemical connection of all that Ingen-Houss, Senebier, and de Saussure had written. He explained the co-operation of light in the nutrition of plants to be a merely ‘formal condition,’ and the salts in solution in the water of the soil were in his opinion stimulants for the use of the extremities of the roots, which were thus put into a condition of ‘vital turgescence’; and as the functions of the leaves, such as Malpighi and Hales had conjectured, and Ingen-Houss, Senebier, and de Saussure had proved it to be, had no existence for Treviranus, he made the assimilation of the soil-sap take place on its way, as it flowed upwards and downwards through the plant. We see that nothing can be conceived more deplorable than this theory of nutrition; it would have been bad at the end of the 17th century, it is difficult to believe that it could have been published thirty years after de Saussure’s work.
There is much in the details of Meyen’s views on the chemical processes in the nutrition of plants that is better than what we find in Treviranus; it is a great point that he concluded from earlier experiments, that the salts which find their way with the water into the roots are not merely ‘stimulants’ but food-material, and, as was before said, he explained the respiration of oxygen by plants correctly in accordance with de Saussure’s observation. But he too stumbled over the assimilation of carbon; he, like so many before and after him, was confused by the simple fact, that gaseous matter takes part both in the nutrition and the respiration of the plant; and taking the processes in both cases for processes of respiration, he considered the absorption of oxygen to be the only important and intelligible function, and the decomposition of carbon dioxide in light to be a matter of indifference as regards the internal economy of the plant. Instead of ascertaining by a simple calculation, whether the apparently small quantity of carbon dioxide in the atmosphere was perhaps sufficient to supply vegetation with carbon, he simply declared it to be insufficient, and because plants will not flourish in barren soil merely by being supplied with water containing carbon dioxide, he gave up the importance of that gas altogether. He too found the humus-theory, which had been constructed by the chemists, more convenient for his purpose, and like Treviranus derived the whole of the carbon in plants from ‘extract’ of the soil, without any close attention to the facts of the case; he refused to believe that the soil is rendered not poorer but richer in humus by the plants that grow on it. It is obvious then that the account given by Treviranus and Meyen of the chemical processes that take place in the nutrition of plants, though correct in some of the details, could afford no true general view of the processes of nutrition, because it entirely misconceived the cardinal points in the whole theory, namely the source of the carbon, and the co-operation of light and of the atmosphere; and thus the best results of the observations of Ingen-Houss, Senebier, and de Saussure were lost upon the German vegetable physiologists.
We have noticed in the previous section the rise of views during the period between 1830 and 1840 which were calculated to make the hypothesis of a vital force appear superfluous, at least as an explanation of certain important phenomena in vegetation; such were the referring the natural heat of plants to chemical processes, and the movement of the sap to osmose; in the domain of chemistry also, in which Berzelius had in the year 1827 made the distinction between organic and inorganic matter to consist in the fact, that the former is produced under the influence of the vital force, the opinion was openly expressed that such an intrusion of the vital principle could not be allowed, since organic compounds had been repeatedly produced from inorganic substances by artificial means, and therefore without its aid. The general tendency of scientific thought was now in fact unfavourable to the nature-philosophy of former days; it inclined to free itself from the obscurity that attended the idea of a vital force, and to assert the belief, that chemical and physical laws prevail alike outside and inside all organisms; this idea became an axiom with the more eminent representatives of natural science after 1840, and if not always expressed in words, was made the basis of all their attempts to explain physiological phenomena.
Thus a freer course began to open for the intellectual movement of the time even before the year 1840, and strict inductive research, and above all the establishment of facts and closer reasoning were now demanded in the question of the nutrition of plants, as they were also in the domain of morphology and phytotomy. But in dealing with the theory of nutrition, the first thing required was not the discovery of new facts so much as the forming a correct appreciation of the discoveries of Ingen-Houss, Senebier, and de Saussure, and clearing away the misconceptions that had gathered round them. The chief modern representatives of vegetable physiology, De Candolle, Treviranus, and Meyen, had increased the difficulty of the task by neglecting to keep the several questions of their science, the chemical especially and the mechanical, sufficiently distinct from one another. The question, what are the materials which as a rule compose the food of plants, though one of the first and most immediate importance, had been very imperfectly investigated, while attention had been diverted to a confused mass of comparatively unimportant matters, and the solution of that question had been rendered impossible for the time by the humus-theory, an invention of chemists and agriculturists, which Treviranus and others had fitted so readily into the doctrine of a vital force. To Liebig belongs the merit of removing these difficulties and all the superfluous matter which had gradually gathered round the subject, and of setting forth distinctly the points which had to be considered; this was all that was required to ensure a satisfactory solution of the problem, for former observations had supplied an abundance of empirical material. But some points of minuter detail were brought out in the course of his investigations which required new and comprehensive experiments, and for these a most capable and successful observer was found between 1840 and 1850 in the person of Boussingault.
But before we go on to give a fuller account of the work of Liebig and Boussingault, we may mention a circumstance which serves to indicate the character of the revolution in scientific opinion before and after 1840. An anonymous ‘Friend of science’ had put a prize at the disposal of the Academy of Göttingen for an answer to the questions, ‘whether the inorganic elements, which are found in the ashes of plants, are found in the plants themselves, in cases where they are not supplied to them from without; and whether these elements are such essential constituents of the vegetable organism, as to be required for its full development.’ The first question appears in the present day absurd, since it implies the possibility of elementary matter coming into being, and of certain special elements coming into being in the plants themselves, an idea however not unfamiliar to the nature-philosophy and the vital force school. It was easy for Wiegman and Polstorff, the authors of the essay that gained the prize (1842), men of the new school, to answer the first question in the negative, and indeed their answer to the second question involved a negative answer to the first. The investigations made by Wiegman and Polstorff in connection with the subject of the second question were conducted in a thoroughly intelligent manner, though they set out from the hypothesis that a certain quantity of compounds of humic acid, as it was called, must be present in the food-mixtures. Their experiments, better adapted to the purpose than any previous ones, showed convincingly that it is necessary to the normal nutrition of the plant that it should take up the constituents of the ash; the observers also took into consideration a number of other questions connected with nutrition, in which however we may already sec the influence of Liebig’s book which had come out during their investigations.
This work was the one entitled ‘Die organische chemie in ihrer Anwendung auf Agricultur und Physiologie,’ which appeared first in 1840 and was afterwards repeatedly reprinted and enlarged. The name of the author, the first chemist of Germany, raised an expectation that the questions respecting nutrition would be dealt with otherwise than they had hitherto been, and this expectation was more than fulfilled by the novelty and boldness with which Liebig cleared up the most important points of the theory, seized upon all that was essential and fundamental, and disregarded the unimportant matter which had before only served to confuse the question. Moreover, he was able to rest on long-accepted facts in just those points which were the most important, and on these he had only to throw the light of his chemical knowledge to dispel the previous darkness. In accordance with his main purpose, which was to apply organic chemistry and vegetable physiology to the service of agriculture, Liebig directed the severity of his criticism first of all against the humus-theory constructed by chemists and agriculturists and thoughtlessly adopted by various physiologists; this was the first thing that must be got rid of, if the question was to be answered, of what substances does the food of plants consist, for the humus-theory was at once incorrect, and the product of a want of reflection which overlooked facts which lay before men’s eyes. Liebig showed that what was known as humus is not diminished but constantly increased by vegetation, that the quantity in existence would not suffice for any length of time for the support of a vigorous vegetation, and that it is not taken up by plants. This once established, and Liebig’s calculations left no doubt on the point, there remained one source only for the carbon of the plant, namely, the carbon dioxide of the atmosphere, with regard to which it was shown by a very simple calculation resting on eudiometric results that its quantity is sufficient to supply the vegetation of the whole earth for countless generations. It is true that Liebig in his zeal went much too far, when he found something contradictory in the true respiration of plants, because it is connected with the elimination of carbon dioxide, and simply denied its reality. On the other hand the theoretical significance of the fact established by de Saussure, that the elements of water are assimilated at the same time as the carbon, was now for the first time clearly explained. Liebig was better able to realise the importance of this fact for the theory of nutrition than de Saussure had been. But these weighty points were not the ones which attracted most attention with the adherents and opponents of Liebig; the practical tendency of his book made the discussion, to which it gave rise especially among chemists and agriculturists, turn rather on the question of the source of the nitrogen in the substance of plants. The humus-theory had made the nitrogen like the carbon enter the plant in the form of organic compounds. De Saussure in his great work of 1804 had named ammonia as a compound of nitrogen which might be taken into consideration with others, but he arrived at no definite conclusion. Liebig, from different points of view and in reliance on his own investigations into the nature of nitrogen and its compounds, arrived at the result, that ammonia must ultimately be the sole source of the nitrogen in the plant, and that the ammonia in the atmosphere and in the soil is quite sufficient to supply vegetation with the requisite amount of nitrogen just as the carbon dioxide of the atmosphere is the sole source of the carbon of the plant; and so he concluded that ‘carbon dioxide, ammonia, and water contain in their elements the requisites for the production of all the substances that are in animals and plants during their life-time. Carbon dioxide, ammonia, and water are the ultimate products of the chemical process of their putrefaction and decay.’
Liebig was less happy, at least as regards his mode of treating the subject, in his remarks on the necessity and specific importance of the constituents of the ash to the nutrition of plants. Instead of insisting on an experimental answer to the question, what constituents of the ash are absolutely indispensable to the health of one or all plants, he lost himself in ingenious chemical theories, intended to show the operation of inorganic bases in fixing vegetable acids, the extent to which different bases can replace each other, and similar matters.
It is not requisite for our purpose to follow Liebig in his applications of his theoretical remarks to agriculture, still less to occupy ourselves with the sensation and the discussions which his work excited among practical and theoretical farmers and agricultural chemists. The scientific value of Liebig’s considerations on the nutrition of plants stood out in a purer and more definite form for the vegetable physiologists, who turned their attention chiefly to the points mentioned above. It is true that Liebig’s work encountered lively opposition from these men also, and the two foremost representatives of vegetable physiology at that time, Schleiden and von Mohl, criticised it unsparingly; this was due partly to the deductive method adopted by Liebig, to which botanists were unaccustomed in physiological questions, and partly to the derogatory expressions in which he indulged against the vegetable physiologists, whom he held responsible with the botanists generally for all the absurdities connected with the humus-theory. Von Mohl asked, and justly, whether de Saussure, Davy, Carl Sprengel, Berzelius and Mulder, the real founders of the theory, were botanists. But it was unnecessary for von Mohl, Schleiden and others to feel touched by Liebig’s reproach, at least so far as it was addressed to professed physiologists, for they were no more physiologists than Davy, Berzelius or Mulder. Professed vegetable physiologists, official public representatives of vegetable physiology there were none, and then as now every one who occupied himself occasionally with questions of the kind was called a vegetable physiologist. In this way the contest became a dispute about words, and Liebig, von Mohl and Schleiden lost an excellent opportunity for influencing public opinion in favour of the idea, that it was high time to establish public official representatives of so important a branch of science, who should devote themselves entirely to it; how could it be expected that Professors of botany, who were required by the government and the public to work for the advancement of systematic botany, phytotomy, and medical botany, to give instruction in these subjects, and to devote a large portion of their time to the management of botanic gardens, should do much to promote the study of vegetable physiology, which demands very considerable acquaintance also with physics and chemistry? and where were the laboratories and the instruments for the serious prosecution of this branch of science? But these questions were not raised, and the old state of things remained for the time unchanged.
As regards the scientific questions at issue between Liebig and von Mohl, Schleiden, and various agricultural chemists, the contest was chiefly about matters of secondary importance, and among these might be included the objection that Liebig knew scarcely anything of the anatomy of the plant. The main point was, that he had corrected mistaken views as to the way in which plants are fed, had refuted gross errors, had shown what was fundamental and essential and what was unimportant. Everything that was written on the subject after 1840 shows that he did all this completely; the publications called forth by the controversy on his book occupied in the main the ground which Liebig had cleared. Now every body knew all at once what was meant by the decompositon of carbon dioxide in the green parts of plants, that the constituents of the ash are not mere seasoning to the vegetation, and the like; firm ground had been won for all, a number of scientific truths had become common property for ever; this did not of course make it less meritorious in others, to test the rest of Liebig’s theories, or even to correct his great mistake about the respiration of plants, as was done emphatically by von Mohl.
It would not be consistent with the design of this work to go into all the details of the discussion excited by the appearance of Liebig’s book, into questions for instance respecting the first products of assimilation in plants, and their further transformations by metabolism. Whether the primary use of the basic mineral constituents is merely to fix the vegetable acids, whether these acids are the first products of assimilation, or whether carbo-hydrates are the immediate result of that process, and similar questions, were for some time only matter of conjecture, deduction and combination, unsupported by certain observation obtained by suitable methods; it was not till after 1860 that new paths were struck out on these subjects, and important results achieved. More important at the time for the advance of the science was the further examination of the question respecting the source of the nitrogen which plants assimilate; it was the more necessary that this point should be finally settled, because Liebig’s deductions still gave room for many doubts, and the first of vegetable physiologists, de Saussure, in his later days made the mistake of coming forward in opposition to Liebig as a defender of the humus-theory, maintaining (1842) that ammonia or the nitrates are not themselves the food-material of plants, but only serve to dissolve the humus. Others also found it difficult to give up entirely the old and favourite doctrine of the humus; though von Mohl and others acknowledged that the carbon of plants is mainly derived from the atmosphere, yet they thought themselves obliged to assign to the humus, on account of the nitrogen which it contains, a very important share in promoting vegetation. Under these circumstances it was extremely fortunate for physiology that Boussingault took up the question. He had occupied himself before the appearance of Liebig’s book with experimental and analytical investigations into germination and vegetation, and specially into the source of nitrogen in plants. His experiments in vegetation in 1837 and 1838 produced no very decisive results; but he continued them for some time longer, improving his methods of observation from year to year; and between the years 1851 and 1855 he succeeded in establishing with all certainty as the result of many repeated trials, that plants are not capable of assimilating the free nitrogen of the atmosphere, but that a normal and vigorous vegetation is produced, when they are supplied with nitrogen from the nitrates in the soil. It appeared also that plants will flourish in a soil from which all trace of organic substance has been removed by heat, if a nitrate is added to the constituents of the ash; this proves at the same time that the whole of the carbon in such plants is derived from the carbon dioxide of the atmosphere without the co-operation of the humus, and that consequently the favourable effect of a soil rich in humus on vegetation must be due to other causes than those which were assumed by the humus-theory. We cannot describe the further services rendered by Boussingault to the theory of nutrition, for this would take us too much into technical details, and the best and most important of his results were first given to the world after 1860, and do not fall therefore within the limits of this history. But it should be mentioned that Boussingault must be considered the founder of modern methods of conducting experiments in vegetation. Liebig had before spoken in terms of sufficient severity of the miserable way in which experiments on the subject of the nutrition of plants were managed after de Saussure’s time till later than 1830, but he did not himself introduce better methods; this was reserved for Boussingault. One instance may be given; those who desired to decide the question of the humus by experiment, such as Hartig in conjunction with Liebig and others, generally adopted the plan of supplying plants with compounds of humus-acid, and seeing what would be the result. Boussingault did as Columbus with the egg; he simply made plants supply themselves with food in a soil artificially deprived of all trace of humus and containing a mixture of food-material, in order to prove beyond question that they do not need humus.
In Germany also Prince Salm-Horstmar made similar experiments to those of Boussingault; he occupied himself chiefly in determining the relative importance of the acids and bases of the ash in the nutrition of plants, whether any and which of them are indispensable; these are questions which approached their solution only after 1860, and some are not yet decided.
The establishment of the facts, that plants containing chlorophyll derive the whole of their carbon from the carbon dioxide of the atmosphere, and that the latter is also the original source of the carbon in plants and animals which do not contain chlorophyll; further that the nitrogen which plants assimilate is derived from ammoniacal salts or nitrates, and that the alkalies, alkaline earths in the form of sulphates and phosphates, are indispensable ingredients in the food of plants, must be considered to be the great results of the labour bestowed on the theory of nutrition in the period from 1840 to 1860, while the way was also prepared for many points, which were afterwards of the first importance in the enquiry.
On the other hand the advance made in the theory of the movement of the sap from the time of Dutrochet till nearly 1860 was so small as to be scarcely worth mentioning; yet it was an advance, that the physiological value of the doctrine of endosmose was more and more highly estimated, and that more solid proofs of the theory itself and a more exact acquaintance with osmotic processes were making it possible to explain more of the details of the movement of material in the plant, though the whole question was far from being finally settled. One discovery must be specially mentioned, the establishment by Hofmeister in 1857 of the fact, that the phenomenon observed for centuries in the grape-vine and other trees, and more recently in Agave and in many tropical climbing plants, known by the name of bleeding or weeping and supposed to be confined to certain periods of vegetation, not only occurs in all plants with true woody cells, but may be produced in them at all times by suitable means. The knowledge of this fact was an aid to the investigation of the cause of the weeping.
The theory of the descending sap was in the least advanced condition during this period; appeal was still made to experiments of the kind which Malpighi, Du Hamel, and Cotta had made, and which in reality show nothing more than that in dicotyledonous woody plants a food elaborated in the leaves is carried downwards through the cortex. As soon as it was understood, that all organic substance originates in the leaves, a fact which no one could doubt after 1840, no experiment was required to prove that the formative matter necessary for the growth of the roots, buds, and fruit, must be conducted to those parts from the leaves. It could no longer be a question whether such a movement of assimilated material takes place; it remained only to consider what are the conducting tissues, and what is the nature of the substances which are produced in the leaves and conducted to the rest of the organs. Both questions in accordance with the organisation of the plant could be properly answered only by microchemical methods, and these were not adopted and further developed till after 1857. We have already said that nothing certain was known even as late as 1860 about the chemical combinations formed by assimilation in the leaves; De Candolle supposed that the primary formative sap was a gum-like substance, from which the rest of the various vegetable substances were secreted in the different tissues. Theodor Hartig, who had done good service between 1850 and 1860 by his investigations into the starch in the wood of trees and into proteid in seeds, by the discovery of sieve-tubes, by observations on the amount of water in woods at different times of the year, and by other contributions to botanical science, also occupied himself with the subject of the descending sap, which he conceived of as a formless primary mucilage, from which, as from De Candolle’s gum, the various substances in the plant were deposited as it travelled through the plant. He says (‘Botanische Zeitung’ for 1858, p. 341), ‘The crude sap is changed in the leaves into primitive formative sap,’ and ‘the formation of solid reserve-material (from this) involves the elimination of large quantities of watery fluid.’ The occasional remarks of vegetable physiologists of all sorts between 1840 and 1860 prove, that similar ideas respecting the formation of a primary mucilage of this kind in the leaves were generally entertained.