The three text-books just mentioned contained little or nothing new either in facts or ideas on the subject of the nutrition of plants; all three were rather compilations of what was already known, and differed from each other only in their selection of material and in the form which each sought to give to the general theory; but this is a reason why we should take a nearer look at them, that we may learn how the spirit and tendencies of the time were reflected in vegetable physiology, and made themselves felt particularly in the theory of nutrition.
De Candolle’s work appeared in French in 1832 in two volumes, the first only being devoted to the subject of the nutrition of plants, and in German in 1833 with many valuable annotations by the translator Roeper, under the title, ‘Pflanzen-physiologie oder Darstellung der Lebenskräfte und Lebensverrichtungen der Gewächsc.’ It suffers, in common with the other two books we have mentioned on the same subject, and with the earlier works of Du Hamel, Mustel, and other writers, from a too discursive mode of treatment, which has the effect of burying the points of fundamental importance under a huge mass of facts and statements from other writers. It contains much that might have been omitted as obsolete, and much empirical material of a purely chemical nature, which could not at that time be applied to the purposes of physiology. Nevertheless, it deserved the great consideration which it enjoyed for a long time, especially in Germany, for its author had undertaken to treat vegetable physiology as a separate and peculiar branch of knowledge, not ignoring at the same time its connection with and dependence on physics, chemistry, phytotomy, and biology proper, and thus to give a full and complete delineation of vegetable life; whereas the best works that had been written since Du Hamel’s time, especially on the nutrition of plants, had proceeded from chemists and physicists or from plant-growers like Knight and Cotta, who treated the subject in a one-sided manner, each from his own point of view, and made no attempt to give a connected account of all the phenomena of vegetation. For this reason De Candolle’s ‘Physiologie végétale’ is the most important performance that appeared after Du Hamel’s ‘Physique des Arbres’; and if we wish to know what progress was made in vegetable physiology generrally, and in the doctrine of nutrition particularly, in the period from 1758 to 1832, we have only to compare the contents of these two books. That this progress was a considerable one, appears plainly from a short summary at the end of the first volume of the general theory of nutrition, as De Candolle himself conceived it; this summary will show us at the same time that he aimed rather at giving a clear account of the whole of the internal economy of the plant, than at searching into the moving forces, the causes and effects. From this he was necessarily withheld by his assumption of a vital force. He distinguished four kinds of forces; the force of attraction which produces the physical, and that of elective affinity which causes the chemical phenomena; then the vital force, the original source of all physiological, and the soul-force, the cause of all psychical phenomena. Only the first three of these forces operate in the plant, and though it is necessary to find out what phenomena in vegetation are due to physical or chemical causes, yet the main task of the vegetable physiologist is to discern those which proceed from the vital force, and the chief mark of such phenomena is that they cease with the death of the plant (p. 6). Of course therefore all the peculiar phenomena of nutrition, which are manifested only in the living plant, come within the domain of the vital force. It must be allowed, however, that De Candolle has made a very moderate use of the vital force, and confines himself wherever he can to physical and chemical explanations; and when he has recourse to the vital force, it is owing less to the influence of his philosophical point of view than to the fact that his account is based rather on tradition and information at second hand than on actual research. It is true that De Candolle was perhaps better acquainted than any contemporary botanist with the physics and chemistry of his day, and it is part of his great merit that he should have acquired so much knowledge on these subjects while engrossed in his splendid labours as a systematist and morphologist; but he betrays, at least in his later years, a want of practice in the study of physics and a want also of the habit of mind which this imparts, and which is more important to the physiologist than a knowledge merely of many facts. But this defect is still more apparent in Treviranus and Meyen, whose works on physiology were published soon after that of the great systematist.
De Candolle first brings together all the facts in physiology which have been discovered from the beginning, not omitting the chemical researches of more modern times into the substance of plants, and then gives a general delineation of the processes of nutrition in the plant: ‘The spongioles (an unfortunate invention of his own which has not yet disappeared from French books, and plays a great part in Liebig’s latest work)—the spongioles of the roots, being actively contractile and aided by the capillarity and hygroscopic qualities of their tissue, suck in the water that surrounds them together with the saline organic or gaseous substances with which it is laden. By the operation of an activity which is manifested principally in the contractility of the cells and perhaps also of the vessels, and is maintained by the hygroscopic character and capillarity of the tissue of the plant and also by the interspaces produced by exspiration of the air and by other causes, the water sucked in by the roots is conducted through the wood and especially in the intercellular passages to the leaf-like parts, being attracted in a vertical direction by the leaves and in a lateral direction by the cellular envelope (cortical parenchyma) at every period of the year, but chiefly in the spring; a considerable part of it is exhaled all day long through the stomata into the outer air in the form of pure water, leaving in the organs in which the evaporation takes place all the saline, and especially all the mineral particles which it contained. The crude sap which reaches the leaf-like parts of the plant there encounters the sun-light, and by it the carbonic acid gas held in solution by the sap, whether derived from the water sucked in by the roots or from the atmospheric air, or being part of that which the oxygen of the air produced with the surplus carbon of the plant, is decomposed in the day-time; the carbon is fixed in the plant and the oxygen discharged as gas into the air. The immediate result of this operation appears to be the formation of a substance which in its simplest and most ordinary state is a kind of gum consisting of one atom of water and one of carbon, and which may be changed with very little alteration into starch, sugar, and lignine, the composition of which is almost the same. The nutrient sap thus produced descends during the night from the leaves to the roots, by way of the rind and the alburnum in Exogens, by way of the wood in Endogens. On its way it falls in with glands or glandular cells, especially in the rind and near the place where it was first formed; these fill themselves with the sap and generate special substances in their interior, most of which are of no use in the nutrition of the plant, but are destined either to be discharged into the outer air or to be conducted to other parts of the tissue. The sap deposits in its course the food-material, which becoming more or less mixed up with the ascending crude sap in the wood, or sucked in with the water which the parenchyma of the rind draws to itself through the medullary rays, is absorbed by the cells and chiefly by the roundish or only slightly elongated cells, and is there further elaborated. This storing up of food-material, which consists chiefly of gum, starch, sugar, perhaps also lignine, and sometimes fatty oil, takes place copiously in organs appointed for the purpose, from which this material is again removed to serve for the nourishment of other organs. The water, which rises from the roots to the leaf-like parts of the plant, reaches them in an almost pure state, if it passes quickly through the woody parts, the molecules of which are but slightly soluble. If, on the other hand, the water flows through parts in which there is much roundish cell-tissue filled with food-material, it moves more slowly and mixes with this material and dissolves it; when it is drawn away from these places by the vital activity of the growing parts, it reaches them not as pure water but charged with nutrient substances. The juices of plants appear to be conveyed chiefly through the intercellular passages. The vessels probably share in certain cases in these functions, but serve generally as air-canals. The cells appear to be the really active organs in nutrition, since decomposition and assimilation of the juices take place in them. Cyclosis (of Schultze’s vital sap[136]) is a phenomenon which appears to be closely connected only with the preparation of the milky juices, and to be caused by the actively contractile nature of the cell-walls or of the tubes. Woody and other substances are deposited in every cell in different quantities according to their kinds and the accompanying circumstances, and clothe their walls; the unequal thickness of the layer so deposited appears according to Hugo von Mohl to have given rise to the supposition of perforated cells; that is, the parts of the cell-wall that remain transparent appear under the microscope as pores. Every cell may be regarded as a body which prepares juices in its interior; but in vascular plants their activity stands in such a connection with a complex of organs, that a single cell does not represent the whole organism, as may be said of the cells of certain cellular plants, which are all like one another. There is no circulation in plants like the circulation in animals, but there is an alternating ascent and descent of the crude sap and of the formative sap which is often mixed with it. Both these phenomena depend perhaps on the contractile power in cells that are still young, and if so, this power would be the true vital energy in plants.‘
What is strange to us in De Candolle’s theory of nutrition is due chiefly to the predominance of the vital force; yet at the same time it gives the facts in their general connection, and its best feature is, that the true function of the leaves, the decomposition of carbon dioxide in light and the production of organisable substance, is made the central point of the whole circle of the processes of nutrition. Very different in this respect were the views of the two most eminent German vegetable physiologists at the close of the period before us, Treviranus and Meyen, though they are not in accord with one another in their general conception of the subject. It may be said that all the prejudices and errors, built up on the foundation of the hypothesis of a vital force during the first thirty years of the 19th century, culminated in Treviranus; while others were already setting up the mechanical explanation of the phenomena of vegetation as the one object to be attained, Treviranus produced once more the whole machinery of the obsolete doctrine of the vital force, and with such effect, that his ‘Physiologie der Gewächse’ was already obsolete when it appeared in 1835. The second volume of Meyen’s ‘Neues System der Pflanzenphysiologie’ was a striking contrast to the work of Treviranus; Meyen endeavours as far as possible to trace back the phenomena of vegetation to mechanical and chemical causes, though he does not often succeed in bringing anything to light that is new or of lasting service. He, like Treviranus, was deficient in sound training in chemistry and physics; they did not stand in this respect, as Hales and Malpighi once did, at the highest point of knowledge of their time. At the same time there was a great difference in the way in which each dealt with the writings of his predecessors; Treviranus, who had done good service in former years in phytotomy, was not equal to the task which he had now undertaken; his physiological expositions are marked by feebleness of thought and by an inability to survey as from a higher ground the connection between the facts; he distrusts all that had been done during the previous thirty years, and almost everywhere appeals to the publications of the 18th century; he lives indeed in the ideas of the past, without gaining vigour from the forcible reasoning and freshness of thought of a Malpighi, a Mariotte, or a Hales. Meyen’s treatment of his subject is on the contrary fresh and vigorous; he does not disregard the old, but he holds chiefly to the modern conquests of science; while Treviranus with singular ill-luck constantly overlooks what is valuable in itself and important in its results, Meyen generally picks out the best things from the books before him; Treviranus timidly avoids expressing any view decidedly and maintaining it; Meyen, amid the multiplicity of the labours which we have already described, finds no time to arrange his thoughts, is hasty in judgment and often contradicts himself. But with all these defects, he is still the champion of the new tendencies that were being developed, while Treviranus lives entirely in the past, and shows no trace of the actively creative spirit which was soon to burst forth so mightily in every branch of natural science.
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.
6. Settlement of the question of the food-material of plants.
1840-1860.
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.