Disease in plants

by H. Marshall Ward

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.