Disease in Plants. Ward Harry Marshall
Ingenhousz, Priestley, and others, for that. But it is that the new study recognises that the central point, to which all views must be focussed, is not the one that it was formerly supposed to be. The student is still taught that the chemistry of soils yields valuable information, and that lessons of importance are derived from comparisons of the analyses of the ashes, etc., of plants; but he is no longer able to cherish the hope, however remotely, that such studies solve his most important problems.
The scene—or rather the point to which attention is now directed—is the living, working, energy-accumulating plant itself, and not the dead store of materials in the soil. The reason for the change is not far to seek: it is due to the enormous strides made in the study of the physiology of plants during the last quarter of a century, and the subject abounds in examples illustrating the marvellous advances that have been made, and at the same time showing how, in the progress of researches, made for their own sake—i.e. in pursuit of satisfaction for the intense curiosity of the scientific man—all kinds of side issues turn up which prove to be of value in practice, and suggestive of further thinking.
At the beginning of the nineteenth century—i.e. about 1820—the best thinkers were giving up the old ideas that the environment supplied food, as such, to plants, and had recognised that the plant takes up substances from without and rearranges these in its own body.
The next twenty years or so form a very dark interval in plant physiology, chiefly owing to the influence of the assumption of a special "vital force," an assumption which was not allowed merely to serve as a hypothesis put forward to stimulate research and suggest better ideas, but which gained a hold over men's powers of reasoning to an extent which now appears monstrous and phenomenal.
Many errors crept in during this reign of terror, one of the most fatal of which was De Candolle's revival of the idea of "spongioles"; and another, equally disastrous in many of its effects, was the conception of a sort of vegetable food-extract, humus, existing in the soil in a form peculiarly suitable for direct use by plants. It was during this period that the confusion between the processes of respiration and carbon-dioxide assimilation arose, and exerted its effects for evil into our own day.
The now astounding statement that oxygen-respiration in plants did not occur, laid the foundation of many subsequent difficulties, and so did the positive and authoritative views on the uses of minerals to the plant. Liebig, in fact, stood in the invidious position of being a high authority on purely chemical questions, who was impelled to give opinions on matters which can only be solved by physiological experiments: his great service was to clear up mistakes as regards the chemistry of soils and of plants—his great mistakes were due to his pronouncing on physiological matters; and it may be doubted whether his great services to the purely chemical side of subjects connected with agricultural matters are the more to be admired, or the disastrous influence of his statements on subjects which do not belong to the domain of chemistry should be the more deplored. Be that as it may, he handed on to succeeding generations some weighty errors as regards plant-life, and taught the agriculturist to regard chemical analyses of soils and plant ashes with a reverence which obstructed progress for some time. As a set-off to this we must place his contributions to the destruction of the bugbear vitalism, which was simply preventing enquiry, and his services in bringing together and sifting with power and originality all that had been then acquired as regards the chemistry of the plant, the soil, and the atmosphere.
That Liebig was indispensable in 1840-1850 is one thing; but that his influence should extend to the present day is quite another, and his inevitable mistakes were almost as powerful for future evil, as his clear exposition of the chemistry of his day was productive of immediate good.
Boussingault, working at the same time, 1837-1855, but experimentally with the living plant, taught us more about these matters than any investigator of the time, though it is very probable that the stimulus of Liebig's speculations, good and bad, had its effect in impelling Boussingault to devote his splendid methods to problems of plant-nutrition. Boussingault's contributions to our knowledge of the composition of the dead plant cannot be over-estimated; but he did more than this, because he so clearly apprehended the necessity for asking his questions directly of the living plant, instead of deducing from chemical principles what might be supposed to occur in it; and although future researches showed that even so careful an investigator solved a problem of first importance—viz. the question of the fixation of free nitrogen—the wrong way, it will be found that so far as he did go his conclusions were sound, and well calculated to inspire the confidence with which the world received them. As we are here concerned more especially with the botany of agriculture, however, it is unnecessary to dwell longer on these matters, or on the similar and even more extensive experiments, of world-wide reputation, carried on for so many years, and still being carried on under the liberal auspices of Sir John Lawes, at Rothamsted. Moreover it may be necessary to return to some of these points later on.
Notes to Chapter I
The reader will find a further general account of these matters in Sachs' Lectures on the Physiology of Plants, especially Lectures I. and XII., Engl. ed., Oxford, 1887. He may then proceed to Pfeffer's Physiology of Plants, Engl. ed., 1899, chapter I., and to the account of the history of the subject in Sachs' History of Botany, Oxford, 1890, especially pp. 359-375 and 445-524. References to more special literature will be found in Pfeffer.
CHAPTER II.
THE PLANT AND ITS FOOD
The food of plants—"Vital force"—Other errors—Liebig and Boussingault—The botany of agriculture. The synthesis of carbohydrates—The physiology of plant-nutrition. The persistence of misconceptions.
The year 1860 may be regarded as a landmark of importance in the history of plant physiology, for it was in that year that Sachs discovered that the bringing together of water and carbon-dioxide, in the green chlorophyll-corpuscles of the plant exposed to sunlight, results in the formation of the grains of starch found in these corpuscles.
Previous to this date Dutrochet (1826-37) had introduced the then crude idea of osmosis into physiology; vegetable anatomy had improved, and the modern conceptions of the living cell, protoplasm, nucleus, etc., were slowly looming; sieve-tubes had been discovered, and the proteids and starch in various parts of the plant examined; and the suggestion was abroad, replacing Liebig's idea that plant acids were the first products of carbon-assimilation, that some substance, of a slimy nature, was manufactured in the cells of the leaves and thence distributed as the formative material from which the plant constructed its parts. Davy and Boussingault had even surmised that a carbohydrate might be the first-formed product in assimilation.
There can be little doubt that Sachs' classical proof, by direct physiological observation and experiment, first brought forward the truth of organic synthesis in the plant in a concrete and convincing form.
But it did more than that. It laid the foundation of the modern physiology of plant-nutrition on ground already prepared by De Saussure and the earlier workers; for, in addition to emphasising the truth of organic synthesis—a truth which had been gradually impressing itself on the world for some years—Sachs' discovery showed clearly the real meaning of carbon-assimilation as a process for obtaining combustible food, which the plant then proceeds to make use of.
Many points were rapidly cleared up at once, or if not explained were at least put into a strong light for further enquiry, and plant-nutrition soon ceased to be the mysterious subject for all kinds of wild conjectures that it had hitherto been.
The meaning of thin leaves, with numerous stomata and finely ramified or divided vascular bundles, became more apparent, as also did the significance of the ascending transpiration current; the storage of starch-grains in tubers, medullary rays, roots, seeds, etc., obtained meanings not understood before; the spread of roots in the soil, and the gradually discovered properties of the finer rootlets and of the root-hairs, fitted naturally into their places; and, in short, a thousand facts, otherwise isolated, became collated into an intelligible system, full of suggestions for new work, such as has since gone on and is now being pursued with an activity and success never before realised in the history of science.
As time went on, while the general truth of Sachs' views was confirmed, a number of detailed discoveries