H. Marshall Ward

“ Nor is it pertinent to remark on the apparent hopelessness of physiology being at present able to explain the seemingly infinite complexity of the living machinery of protoplasm and its activities. The modern locomotive is also a complex affair in its way, but it is profitable to investigate it and to know all one can of its working and possibilities, for obvious reasons: a little reflection will convince us that it is also worth while to investigate that complex machine, the plant—the working organism which alone can really enrich a country. ”

“ Even the distinction into physiological diseases versus parasitic diseases cannot be maintained from the standpoint of the nature of the disease itself. All disease is physiological in so far as it consists in disturbance of normal physiological function, for pathology is merely abnormal physiology, no matter how it is brought about. ”

“ We are now in a position to see how another fundamental cause of disease or predisposition to disease exists in the deprivation of the protoplasm of any of the elements needed to supply—in the food-materials—the place of those which have been permanently put aside in the form of cell-walls, or burnt off in respiration, passed out as excretions, or in other ways lost. ”

“ It is customary to regard the soil, between the particles of which the root-hairs of plants are distributed, as if it were merely a dead matrix of smaller or larger pieces of rock, such as sand, gravel, stones, etc., and organic remains, such as bits of wood, leaves, bones, etc., with water and air in their interstices. As matter of fact, however, soil is a much more complex body than was suspected until comparatively recent times. ”

“ Fundamentally the whole of life and work on our planet is to be referred to the one great source of energy which renders possible the establishment of differences of potential.
This machine, then, doing work in various ways, adapts itself—or goes to the wall—to the conditions of its work among competing organisms or opposing circumstances. Curiously enough, while in some cases it suffers from the competition, in others it is benefited by its life-actions fitting in between those of other organisms, which in their turn supplement it. ”

“ I confess it seems to me as reasonable to look upon scientific agriculture as a branch chiefly of chemistry as it would be to look upon horse-breeding or pigeon-rearing from the same point of view; and why the professed chemist's advice is regarded as so comforting and final in the one case and not in the other is one of those mysteries which seem inherent in human nature. ”

“ Again, probably no one factor of the non-living environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the accumulation of foul products, etc., is the principal factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases. ”

“ In other words, the more far-seeing pioneers of scientific agriculture and forestry, etc., are recognising that agricultural chemistry is not the be-all and end-all of agricultural science; but that, in place of the study of the chemical analyses of dead soil, water, air, and plant-remains, which has so long held sway, largely owing, I think, to the influence of Liebig, the student should have his attention more concentrated on the living plant itself and on the physiological actions which make up its life. ”

“ But, when all due honour is paid to the scientific chemist, it must still be allowed that his problems are different from the real problems of agriculture. To take one set of instances alone. The chemist can analyse a given soil or a given manure, and can even go a long way towards making them, but his analyses do not tell us what conditions are necessary in order that their ingredients may be presented to the roots so as to be absorbed and become built up into the plant. ”

“ Having now obtained some insight into the factors concerned in disease, let us enquire further into the bearing of variation on these. It is evident that pathological conditions may vary; indeed they are themselves symptoms of variation, as we have seen. The history of all our cultivated plants shows abundantly that many of the variations obtained by breeding in our gardens, orchards, fields, etc., involve differences of response on the part of the plant to the very agencies which induce disease. ”

“ If we agree that a living plant in a state of health is not a fixed and unaltering thing, but is ever varying and undergoing adaptive changes as its life works out its labyrinthine course through the vicissitudes of the also ever-varying environment, then we cannot [92] escape the conviction that a diseased plant, so long as it lives, is also varying in response to the environment. ”

“ We have seen that all the essential phenomena of disease concern only the living substance—the protoplasm—of the plant, and that however complex the symptoms of disease may be, the occurrence of discolorations, lesions, hypertrophies, and so forth are all secondary matters subsidiary to the fundamental alterations of structure and function constituting the disease. ”

“ As regards any particular weed, the cultivator should learn all he can respecting its duration, seeding capacity, method of dissemination, the depth and spread of its root-system, and any other particulars which enable him to judge when and how to attack it. It is only necessary to see the victory of such drought-resisting weeds as Hieracium pilosella, Plantains, Hypochaeris, on lawns to realise how weeds may win in the struggle for existence with the finer grasses. ”

“ Now it turns out to be by no means a foregone conclusion that all or any of the substances, even though freely soluble in the water, can pass through the protoplasm into the interior of the cell. Some may be allowed easy access, others may only be permitted to pass in small quantities, and others, again, may be absolutely refused access by the delicate living filter, so long as it is vigorously alive. Nor, as proved by numerous experimental cultures since De Saussure's time, is the entrance of a salt, etc., ruled by its indispensability or otherwise in the economy of the plant. ”

“ This is sufficiently exemplified by the disastrous spread of such diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera, Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other maladies which have now become historic in agricultural, planting, and forest annals. Providing the favourite food-supply in large quantities is not the only factor of an epidemic, but it is a most important one in that it not only facilitates the growth and reproduction of a pest, but affords it every opportunity of spreading rapidly and widely. ”

“ The proper study of plant pathology presupposes and involves a knowledge of the physiology of plants, of the normal relations of the latter to their environment, and of the biology of those animals and plants (principally insects and fungi) which are parasitic on them. It is of the first importance to understand that a disease is a condition of abnormal physiology, and that the boundary lines between health and ill-health are vague and difficult to define. ”

“ He must regard the living plant as a sort of working machine—infinitely more complex than any machine made by man, but a machine nevertheless—the purpose of which is to store up energy from the sun, and so to add to our wealth on this planet, at the expense of the extra-terrestrial universe.
It is not, be it noted, that the new study proposes to ignore or abandon the old studies: modern physiology owes too much to the physics and chemistry on which it is partly based, and to the labours of De Saussure, Ingenhousz, Priestley, and others, for that. ”

“ Indeed we might compare the cases of fungi which attack the root and collar, and so strike at and cut off the water supply, to a compact army which at once cuts off the enemy from his narrow base; whereas the innumerable units which bring about an epidemic attack on the leaves, and so surround the enemy and cut off his food supplies all round, is rather like a much larger army which cannot get in [116] beyond the natural barriers of the tissues, and so puts a cordon all round the territory and seizes the multitudes of food-stuffs at the frontiers. ”

“ Moreover, the cultivator knows a good deal about his plants which I do not know, and although I should much like to know it, his plea of want of time rings in my ears and the conviction strikes home that one ought to try and meet his views, and tell him something about disease as manifested in plants without insisting on his becoming a professional mycologist, entomologist, agricultural chemist, and philosopher. ”

“ As matter of experience also it is found that rotation of crops, planting forests of mixed species, and breaking up large areas of cultivation into plantations, fields, etc., of different species afford natural and often efficient checks to the ravages of fungus and insect pests. Over and over again it has been found that a fungus or an insect which is merely endemic so long as it is isolated in the forest, where its host is separated from other plants of the same species by other plants which it cannot attack, becomes epidemic when let loose on the continuous acres so beloved of the planter. ”

“ Man's position in regard to the struggle is that of an intelligent being who steps in at certain stages and protects, fosters, and in every way favours the agricultural plant—the living machine—and sees that every opportunity is given it to do its best work in the best way—from his points of view! ”

“ But the gravest misapprehensions current among us are due to the crude ideas as to what a plant really is: this, I take it, is owing to the difficulty of grasping what physiologists mean by organised structure, and leads to regarding the living being either as a mere aggregation of chemical compounds, built up by the ordinary play of chemical forces, as we know them, acting on dead matter, or, as in the days before organic chemistry, as a mysterious entity endowed with "vital force," and with properties not amenable to scientific investigation. ”

“ Unsuitable temperature is one of the commonest of all sources of disease, for every plant is adapted to certain ranges of temperature, and best adapted to a given optimum somewhere between the maximum and minimum temperature for each function. Consequently any serious departure from the mean may bring about physiological disturbances of the nature of disease, and this in very various ways, as exemplified by the results of frost, sun-scorching, drought, hail-storms, forest fires, and so forth. ”

“ Moreover, the essentials are found to be the same in the animal kingdom also, and the bearing of all these discoveries on the phenomena of reproduction, variation, and heredity in living organisms has been and is of the highest importance, for they support, control, explain and correct so many of the splendid results of Knight, Kölreuter, Sprengel, Hildebrand and Hermann Müller, and in every direction throw side-lights into the crevices of that magnificent structure, the theory of Natural Selection, erected for all time by our countryman, Charles Darwin. ”

“ Finally, the invested or invaded cell may so adapt itself to the demands of the invader that a sort of arrangement is arrived at by which life in common—Symbiosis—is established, each organism doing something for the other and each taking something from the other. In this latter case, which is often realised—e.g. lichens, leguminous plants and the organisms in their root-nodules, mycorrhiza, etc.—we leave the domain of disease, which supervenes indeed if the other symbiont is lacking. ”

“ If these ideas are correct, we may define Disease as dangerous disturbances in the regularity, or interference with the completeness or range of the molecular activities constituting normal Life—i.e. Health—and it is evident that every degree of transition may be realised between the two extremes. Now, if we further assume, as I think we must do, that a considerable range or "play" must exist in the molecular activities of the protoplasm constituting life, we obtain a sort of expression of what we mean by limits of variation. ”

“ For we must recognise at the outset that disease is but variation in directions dangerous to the life of the plant. Health consists in variation also, but not in such dangerous grooves. That the passage from health to disease is gradual and ill-defined in many cases will readily be seen. In fact we cannot completely define disease. ”