Disease in Plants

Protoplasm—Hypothesis as to its structure and behaviour—Assimilation—Growth—Respiration—Metabolism—Action of the environment—Nuclear protoplasm—Pollination—Grafting—Parasitism—Graft-hybrids—Life—Death—Variation—Disease.

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. It remains to see if we can adopt any hypothesis as to the nature of this physical basis of life—the protoplasm—which shall help us to understand still more clearly in what must reside those processes which, so long as they proceed harmoniously and uninterruptedly, constitute life and health, and which when interfered with result in disease and death. The protoplasm of the living plant-cell looks like a slimy translucent mass which has been superficially compared in appearance to well-boiled sago or clear gum. Fifty years of observations and experiments with it have convinced physiologists that it is not a mere solution or emulsion, however, or even a chemical compound in the ordinary sense of the term, although chemical analysis gets little out of it beyond water, proteids, carbohydrates and fats, and traces of certain mineral salts; for living protoplasm does not respond to the laws of physics and mechanics in obeying them, simply as do ordinary solutions and liquids. On the other hand, the most delicate chemical manipulation fails us, because when killed it is no longer protoplasm. Nor does the microscope advance matters far, beyond convincing us that this marvellous material must have a structure far more intimate than anything visible to the highest magnifying powers at our disposal.

Nevertheless, some information is forthcoming from the comparative examination of the protoplasm of numerous different kinds of organisms, for we have learnt that certain ingredients and no others are necessary for its composition—namely, carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, calcium[Note: See note at end of chapter.], magnesium, potassium—and it is as a rule of no use trying to foist on to it any substitute for any one of these. Moreover, these chemical elements must be given in certain definite proportions and forms: for instance it is of no use to offer the carbon and sulphur in such a form as carbon disulphide, or the nitrogen and hydrogen in that of hydrocyanic acid, but the carbon must be given to the protoplasm in the form of a carbohydrate or in some similar form, the nitrogen as an ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so forth, and thus water, air, carbohydrates, and the nitrates, sulphates, and phosphates of potassium, calcium, and magnesium become the chief natural sources of the essential ingredients. Again, we have learnt that while there are different forms of protoplasm in the cell, and that these react on each other, and go through cycles of arrangement and rearrangements, the intimate structure must be of that kind termed molecular—beyond the region of vision, just as is the microscopic structure of a crystal; but, while like the latter affording evidence of order and sequence when properly examined, the structural arrangements and changes must be infinitely more complex.

All these, and numerous other results of enquiry, have led to the conclusions that we must regard living protoplasm as a complex made up of very large molecular units, each containing atom-groupings of the elements named; and, partly on account of the large number of atoms they contain, and partly due to the vibrations of absorbed heat, these units must be extremely labile. Moreover, they are linked up into an invisible and intricate meshwork, bathed in a watery liquid held in the interstices somewhat as water is held in a sponge. In this imbibed liquid are dissolved the substances, consisting of the same elements, which are to serve as food, and which are to be taken up into the molecular framework and built up into the structure of new molecular units—or, as they may be shortly termed, molecules of protoplasm: in the bathing liquid are also dispersed the fragments—again containing the elements named—which have resulted from the breaking asunder of some of the complex protoplasm molecules, and which are partly destined to be used up again, partly to be burnt off in respiration, and partly to be put aside as metabolic products such as reserves, secretions, permanent structure, etc. Among the elements carried into this liquid and dissolved in it the free oxygen of the air also plays an important part.

As new molecules are formed, by mutual combinations of the food-materials selected by molecular attractions, they are taken up into the protoplasmic framework, and built in between those already in existence, thus distending the whole, and we say that the protoplasm Assimilates food-materials and Grows. When distended beyond a given degree, or disturbed in various other ways, the molecular framework breaks, and some of the molecules are shattered, and as they fall to pieces certain of their constituent parts containing carbon and hydrogen forcibly combine at the moment of liberation with the oxygen in the fluid around and are burnt off in the form of carbon-dioxide and water, heat being of course evolved. This is the fundamental process of Respiration.

It is probably the alternation of these processes of Assimilation—the building up into the protoplasmic structure of new complex labile molecules—and Destruction—the shattering of such molecules with redistribution, oxidation, etc., of their fragments—which constitute the fundamental process of life. Different authorities attempt to explain the details of these processes in various ways, but there is practical agreement on the one point, that life consists in the alternate building up of new protoplasm from the food-materials—Assimilation—and the breaking down of the molecular complexes to simpler ones—Disintegration, or Dis-assimilation, as we may call it. During the periods when assimilation prevails, and the protoplasm increases in mass, we recognise Growth, and since this is usually associated with the vigorous imbibition of water, owing to the powerful osmotic attractions for that liquid exhibited by some of the products, and with consequent further stretching of the invisible molecular plexus, the growth may be so evident in increased size, that we are accustomed to look upon the visible increase in volume alone as growth; but it is essential to understand that growth of the protoplasm is always proceeding during life, even when as many older molecules are being shattered and dispersed as new ones are being formed by assimilation, and when, therefore, no visible permanent enlargement occurs. Similarly, during periods when disintegration of the molecules prevails, we must not assume that the assimilation of new molecules is not occurring and that growth is not proceeding. The two processes are always going on during the active life of the protoplasm: in fact life consists in the play of these processes, as already said.

That numerous chemical rearrangements of the atom-complexes take place outside the protoplasmic molecules—both of those left unemployed in assimilation and of those rejected during the destructive processes—will be readily understood: many of the bye-products found in plants, such as vegetable acids, alkaloids, colouring matters, crystalline bodies, etc., etc., are due to these, so to speak, fortuitous combinations and re-combinations.

The part played by respiration has often been misunderstood. It consists in the burning off of some of the carbon and hydrogen of the shattered protoplasm molecules, by means of the oxygen of the air, which finds its way into the fluids around the protoplasm, and when it is active every act of combustion—which is here an explosion—leads to the shattering of more protoplasm molecules, and consequently to more respiratory combustion of the products. If the supply of oxygen is limited the breaking down of the molecules of protoplasm does not cease, but the carbon and hydrogen which would otherwise have been oxidised are now in part left to form other compounds in the surrounding liquid, and thus incompletely oxidised bodies, such as vegetable acids, alcohols, etc., accumulate. Even in the complete absence of atmospheric oxygen the protoplasm may go on breaking down and accumulating various compounds containing relatively much carbon and hydrogen—so-called intramolecular respiration; but in ordinary plants this process soon comes to an end, because the blocking up of the molecular plexus leads to obstruction and interferes with the normal assimilation and dis-assimilation, and, if prolonged, leads to pathological conditions, and eventually death.

Here, then, we meet with a cause of disease, or of predisposition to disease. The deprivation of oxygen interferes with the normal processes of building up and breaking down of the protoplasmic molecules, and bodies we term poisonous accumulate and may lower the vitality or even bring life to an end.

During normal life other products of the disruption of the protoplasm molecules are nitrogenous bodies, such as proteids, and these we have reason to believe are used up again, acting as the nuclei, so to speak, of the new molecules, and so being built up again with fresh food-materials into the plexus, to be again set free, and again used up, and so on. Others are the carbohydrates, such as cellulose, which pass out of the molecule into an insoluble form, and are accumulated outside the protoplasm in the form of cellulose membranes, and so forth. It is these formed products of metabolism (Metabolites), especially cellulose and bodies which result from its subsequent transformation, which constitute the main permanent mass of the ordinary plant.

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 clear that the indispensability of an element must mean that the protoplasmic molecule cannot be completed without it: the same conclusion is supported by the experimental proof that these elements cannot be replaced by chemically similar elements.

It does not follow, however, that the protoplasm molecule must always have the same number of atoms of these elements, and grouped always in the same atom-complexes before being assimilated; nor that the protoplasm molecule, when once built up, always breaks down in exactly the same way. On the contrary, while the protoplasm of corresponding parts of a daisy and of a rose must contain all the elements named, we must believe that the atom groupings are different in the protoplasm molecule in each case; and though the molecules of the cell-protoplasm, of the nucleus, of the chlorophyll-corpuscles, etc., of one and the same plant must have all these elements, the atom groupings and modes of building up and breaking down may be very different in each case.

Again, the cell-protoplasm, bathed by the sap taken in by roots from the soil or fed directly by that derived from the leaves, must be exposed to very different stimuli and modes of nourishment, etc., from those incurred by the protoplasm of the nucleus which it encloses: and similar conclusions must apply in turn to the protoplasm of the root in the dark moist soil and of the leaf in the light dry air, or to that of the superficial epidermis cells as contrasted with that of the deeply immersed pith, and so on.

It is no doubt in these directions that we must seek for the explanation of many life-phenomena at present quite beyond explanation. Thus, it is tolerably easy to modify the action of the cell-protoplasm of a plant, by exposing it to differences of illumination, temperature, moisture, and so forth, within certain limits; at least, since the changes in stature, tissue differentiation, cell-secretions, flowering capacity, etc., of plants affected by such factors of the environment—e.g. alpine plants brought into the plains—must be due to changes in the mode of activity of the protoplasm, we must assume that the above factors affect the latter. But it is extremely difficult to reach the nuclear-protoplasm directly by such stimuli, as proved by the experience that even where we allow the factors to act for a long time, no permanent change can be detected in the behaviour of the nuclear-protoplasm—the essential material in the reproductive organs and reproductive process. At least we must infer that no change has been permanently stamped on this nucleo-plasm from such facts as the characters of the seedlings of the progeny of the plain-raised plants: if they are again sown in an alpine situation they forthwith behave again as alpines.

Must we not conclude, then, that this difficulty of reaching the nuclear-protoplasm is owing to the fact that it is nourished and influenced directly only by the cell-protoplasm? That the cell-protoplasm is its environment, and not so directly the outer world? We may influence the cell-protoplasm—we may make it work harder or less actively, respire vigorously or slowly, build up and break down in various different ways, or at different rates, and so forth, within limits; but it is nevertheless cell-protoplasm of its specific kind, with its own range of molecular variations and activities within these limits, and it supplies the nuclear-protoplasm with what it wants so long as these limits are not exceeded. Consequently, while it is very easy to make the cell-protoplasm vary within the limits of its range, it is not easy to induce it to vary its effects on the nuclear-protoplasm to such an extent or in such a way that the latter is permanently or materially altered in constitution.

Nevertheless it would appear that cases do occur where the nuclear-protoplasm is reached and affected by external stimuli, as evinced by some of the phenomena of hybridisation and of cross- and self-fertilisation, because we find the results expressed in the mingling of the characters of parents, in strengthened or enfeebled progeny, and even in the appearance of unexpected properties, which, from the facts of Reproduction, we know must have taken their origin in some alteration of the nuclear substance of the embryo.

Here, however, we know in most cases that the principal agent which has reached the nuclear-protoplasm, is another portion of nuclear-protoplasm. In hybridisation, one which has been fed and influenced by cell-protoplasm of a very different plant; in cross-fertilisation, one fed and influenced by the cell-protoplasm of a different plant of the same species, and in self-fertilisation, one fed and influenced by the same cell-protoplasm.

That somewhere, and somehow, such nuclear-protoplasm as induces the changes in the characters of hybrids, etc., has been influenced by its immediate environment—the cell-protoplasm of the plant—appears to be a conclusion from which there is no escape. We may obtain similar evidence from the experience of grafting. It is relatively easy to influence the cell-protoplasm of a scion by a suitable stock, obviously because the latter, while handing on to the former all necessary materials from the soil, presents the indispensable elements and compounds in somewhat different proportions, dilutions, etc., from those which its own roots would have done, and probably mingles with them a certain amount of its own peculiar products, as well as affects the modes of working and interaction of both by the molecular impetus impressed on them. Consequently the cell-protoplasm of the scion, while obtaining from the stock all it needs within the limits of its own variations of structure and activity, nevertheless builds up and breaks down in ways or at rates slightly different from those hitherto normal to it, and perceptible variations result when the sequences and correlations of these material and mechanical changes have affected a sufficiently large mass for the accumulation of visible effects. The limits to grafting suggest not that an inappropriate stock does not offer to the protoplasm of the scion the right materials, but that it presents them in proportions and in forms which are unsuitable for the assimilable powers of the latter, or, possibly, mingled with substances poisonous in themselves or capable of becoming so in conjunction with bodies in the scion.

What has been said of the action of stock on scion, will also be true, mutatis mutandis, of the reciprocal action of scion on stock. Here again we may have causes for disease, or predisposition to disease.

It occasionally happens, however, that the nuclear protoplasm of the stock or scion is affected in grafting, and we infer from the difficulty of modifying it in any other way in ordinary reproduction than by means of other nuclear protoplasm—e.g. in hybridisation—that in such cases a fusion of the nuclei of stock and scion has occurred during the grafting, and a graft-hybrid has resulted—e.g. Cytisus Adami.

It is not impossible however that the nuclear protoplasm has in such graft-hybrids been subsequently modified by the differences in nutrition to which it has been subjected, in the modified cell-protoplasm affected by the mingling of the juices, etc., of scion and stock; for it is quite conceivable that such materials may affect the protoplasm far more profoundly than anything derived directly from the environment.

If Daniel's researches are confirmed, however, it appears that in some cases, at any rate, the nuclear-protoplasm is so altered by the grafting that when the new embryo is developed, after fusion with nuclear substance from another plant of the same species, the results are apparent only in the progeny, and the effects of alteration in the cell-protoplasm have been transmitted to the nuclear protoplasm of the germ-cells—i.e. acquired characters have been transmitted and fixed by heredity. Should this prove true the importance of the results can hardly be over-estimated. The matter is too problematical for further discussion here, but we see that any such action may profoundly affect the "constitution" of the resulting plant.

Turning now to the case of fungi or other organisms which obtain access to the cell-protoplasm. At the one extreme we have cases where the protoplasm of the diseased plant is rapidly and directly poisoned and destroyed, as in the killing off of seedlings in "Damping Off": near the other extreme we have cases where the foreign protoplasm of the parasite, although it gains complete access to that of the host, merely stimulates the latter to greater activity and itself works for its own ends in conjunction with it—e.g. Plasmodiophora. In such instances we must figure to ourselves the cells of the root of the Crucifer handing on food-materials to both masses of protoplasm—that of the Plasmodiophora and that of the cell into which it penetrates; and it is immaterial whether both obtain the food-materials directly, or, what seems more likely, the fungus only at second hand and by the medium of the host's protoplasm. In any case, the latter is for a long time at least not poisoned or maimed, or in any perceptible way injured by excreta from the fungus-protoplasm, although it is evident that each must excrete various metabolites which may soak into and be taken up by the other: on the contrary the host-protoplasm grows larger, attracts more food supplies, makes larger cells, and is evidently stimulated to greater activity for the time being, its behaviour reminding us of the stimulation of cells by means of slight doses of poison referred to previously. We must therefore assume that the general course of building up and breaking down of its protoplasm-molecules go on as usual—or nearly so—in both the host cell and the invader; and that the assimilatory, respiratory, excretory and other functions are carried on in the former as in the normal cell, or are but slightly modified to an extent which does no immediate injury to its life. But we must further assume that the same is also true of the invading protoplasm, and that the Plasmodiophora is also supplied with suitable atom-complexes to build up its protoplasm molecules, as fast as they are shattered and the rejecta burnt off in respiration.