Disease In Plants Part 20

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 ma.s.ses 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 a.s.sume 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 a.s.similatory, 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 a.s.sume 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.

A step further, and we come to instances of _Symbiosis_, where the commingled ma.s.ses of protoplasm of host and invader continue this harmonious action during life. Clearly there are resemblances between these latter cases and successful grafts, and between both and successful s.e.xual unions where the resulting embryo-cell gives rises to a vigorous and healthy plant; and the more these resemblances are examined in the light of what we know of symbiosis the more they support our contention.

Such considerations as the foregoing suggest, then, that life consists in the regular and progressive building up and breaking down of the complex protoplasm molecules, and is necessarily accompanied by the influx of the indispensable food-elements in certain combinations and atom-complexes for a.s.similation, and by the combustion of some of the debris of the shattered molecules, which combine with the oxygen in respiration and so afford explosions which raise the temperature and enhance the lability of existing molecules, and act as stimuli to the shattering of further molecules. The results of these rhythmical buildings up (a.s.similation) and shatterings (dis-a.s.similation) of the protoplasm molecules are the growth of the protoplasm, with further intercalations of water and new food-supplies, etc., on the one hand, and the formation of metabolic products (proteids, cellulose, sugars, fats, etc.), some of which are again used up, others respired, others deposited as stores, cell-walls, etc., on the other.

That the building-up process depends on the action of molecular forces comparable to those by which a growing crystal goes on selecting atom-complexes of its particular kind from the solution around seems highly probable, and this being the case we can understand how under certain circ.u.mstances _subst.i.tutive_ selections may occur. That is to say, just as a crystal will sometimes build up into its structure atom-complexes of a kind different from its normal molecules, so, given the proper conditions, a protoplasmic molecular unit will build up into its structure atom-complexes somewhat different from those it had hitherto taken up--_i.e._ a.s.similated--with consequent modifications of its behaviour. If this occurs, the modes of further building up and breaking down will be affected by the subsequent action of these slightly modified protoplasm units, _and it may well be that the whole significance of variation turns on this_. Whether the resulting variation makes for the welfare or otherwise of the organism will then be decided by the struggle for existence, and the natural selection which ensues. Such a view also implies that the energy concerned is primarily what is usually termed chemical energy, and that every compound entering into the protoplasm carries in a supply of this, available in various ways.

_Death_, on the contrary, is the cessation of these rhythmical processes of building up and breaking down of the protoplasm molecules. It does not imply the cessation of chemical changes of other kinds, but that these rhythmical constructions of the complex and labile protoplasm molecules breaking down on stimulation to bodies partly re-a.s.similable, partly combustible in respiration, and partly excretory, etc., have ceased, and that further chemical changes in the material are thenceforth simpler and different in kind and degree, eventually leading to total disintegration so that no units are left capable of restoring the rhythm.

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 const.i.tuting 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 a.s.sume, as I think we must do, that a considerable range or "play" must exist in the molecular activities of the protoplasm const.i.tuting life, we obtain a sort of expression of what we mean by limits of variation. The fact that life can go on in a given plant at temperatures between from 1-5 and 35-40 C., or in lights of different intensity, or within considerable ranges of water supply, concentration of salts, partial pressure of oxygen, etc., implies that the molecular activities of the protoplasm are of the normal _kind_ all the time, though they may differ in rapidity, and even in _quant.i.tative_ and _qualitative_ respects within certain limits; and the meaning of the _optimum_ temperature, illumination, oxygen pressure, etc., is, from this point of view, not that the molecular activities differ in kind from those nearer the minima and maxima, so much as that they are running at the best rates for the welfare of the plant--_i.e._ for permanent health.

If we transcend the cardinal points limiting the range of this play, however, and we get variations in the _kind_ as well as _rates_ of molecular constructions and disruptions, then we pa.s.s by imperceptible gradations into ill-health--_i.e._ _Disease_.

And similarly in relation to other protoplasm. That of the right kind of pollen grain from another plant of its own species, stimulates the contents of the ovule to produce a vigorous embryo and healthy seedling: that of a similar pollen grain in its own flower either does no positive harm, but has a feebler effect, or it may act like a poison. That of another pollen grain again may refuse to unite at all; while that of a fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the style as does the pollen tube and produce death and destruction throughout the ovule.

Or again, in Clover, we may have the hypha of a _Botrytis_ with its protoplasm unable to do more than penetrate into the cellulose walls and diffuse a poison into the adjacent cells, being utterly incapable of directly facing, or mingling with the living protoplasm of such cells, whereas the protoplasm of another organism--_e.g._ _Rhizobium_--will penetrate directly into the cells, live in them for weeks or months without injury--nay even with advantage to their life. And hundreds of similar cases can be selected.

We may, therefore, conclude that _Variation_ depends fundamentally on alterations in the structure or mode of building up and disintegration of the protoplasmic molecular unit, brought about either by direct modifying action of the inorganic environment--nutrition, temperature, oxygen supply, light, etc., etc.--or by the mingling with it of other protoplasm, the molecules of which since they have already a slightly different composition, configuration, mode of breaking down and building up, etc., affect its molecules by supplying them with altered nutritive atom-complexes, by competing with them for oxygen, etc., etc. Once these molecules are affected, we must a.s.sume that long sequences of other chemical and molecular changes will be also modified; and although we have no conception of _how_ these changes bring about changes in form, that they do so is only a conclusion of the same order as that which we hold regarding the much simpler changes concerned in the formation of crystals.

That such variations may be of every degree as regards profundity, permanence, kind, etc., may well be imagined; and there is nothing surprising in our being able to induce them more easily by the action of external factors _in the readily accessible cell-protoplasm_ than in the _less exposed nuclear-protoplasm_; because the latter is only accessible through the former, or through the agency of _other nuclear protoplasm already modified_. On these and similar phenomena depend the relative permanency and transmissibility of the variations. Our measure of the latter only begins when the effects referred to have become manifest in large ma.s.ses of cells, because only then do they become appreciable to our senses.

Further, variations thus induced may be of advantage to the continued life of the plant, or in all degrees disadvantageous or threatening to its existence. These latter variations are _Disease_, and if their interference with the normal rhythmical play of the building up and breaking down of the protoplasm molecules proceeds beyond certain limits, life ceases, and we have death supervening on disease.

NOTES TO CHAPTER x.x.x.

It appears probable that calcium is not always needed by living cells, and may not enter into the composition of protoplasm; on the other hand traces of iron are perhaps necessary.

The criticisms and summary of facts on which the hypothesis regarding protoplasm here adopted is based are developed at length in Ka.s.sowitz, _Allgemeine Biologie_, Wien, 1899, B. I.

and II., where the collected literature may be found, and the reader introduced to the huge ma.s.s of controversial writings put forward since Darwin and a.s.sociated with the names of Weismann and others.

It will probably be noticed that I have employed the term molecular unit of protoplasm, and have not discussed the question of organised structure in the latter: this is because it seems clear to me that living protoplasm as such does not possess "organised structure" in the true sense of that term--it is, rather, busy preparing and making "organised structure," and a molecular const.i.tution would have to be ascribed to all "physiological units" of the nature of micellae, pangens, ids, etc., as truly as to the structural units of a starch-grain or cell-wall, or even of a crystal. In this connection, the student will find the necessary points of view put forward in Pfeffer, _Physiology_, pp. 37-83.