CHAPTER XIII.
NATURE OF DISEASE.
General and local disease—General death owing to cutting-off supplies, etc.—Disease of organs—Tissue-diseases, e.g. timber—Root-diseases—Leaf-diseases, etc.—Diseases of Respiratory, Assimilatory, and other organs—Physiological and Parasitic diseases—Pathology of the cell—Cuts—Cork—Callus—Irritation—Stimulation by protoplasm—Hypertrophy.
On going more deeply into the nature of those changes in plants which we term pathological or diseased, it seems evident that we must at the outset distinguish between various cases. A plant may be diseased as a whole because all or practically all its tissues are in a morbid or pathological condition, such as occurs when some fungus invades all the parts or organs—e.g. seedlings when completely infested by Pythium, or a unicellular Alga when invaded by a minute parasite; or it may die throughout, because some organ with functions essential to its life is seriously affected—e.g. the roots are rotten and cannot absorb water with dissolved minerals and pass it up to the shoot, or all the leaves are infested with a parasite and cannot supply the rest of the plant with organic food materials, in consequence of which parts not directly affected by any malady become starved, dried-up, or poisoned or otherwise injured by the results or products of disease elsewhere.
In a large number of cases, however, the disease is purely local, and never extends into the rest of the organs or tissues—e.g. when an insect pierces a leaf at some minute point with its proboscis or its ovipositor, killing a few cells and irritating those around so that they grow and divide more rapidly than the rest of the leaf tissues and produce a swollen hump of tissue, or gall; or when a knife-cut wounds the cambium, which forthwith begins to cover up the dead cells with a similarly rapid growth of cells, the callus. Numerous minute spots due to fungi on leaves, cortex, etc., are further cases in point, the mycelium never extending far from the centre of infection.
Many attempts have been made to classify diseases on a basis which assumes the essential distinction of the above cases, and we read of diseases of the various organs—root-diseases, stem-diseases, leaf-diseases, and so forth; or of the various tissues—timber-diseases, diseases of the cambium, of the bark, of the parenchyma, and so on. Furthermore, attempts have been made to speak of general functional disease, of diseases of the respiratory organs, of the absorptive organs, and so forth, as opposed to local lesions.
Critical examination, however, shows that no such distinctions can be consistently maintained, partly because the organs and functions of plants are not so sharply marked off as they are in animals, the diseases of which have suggested the above classification, and partly because all disease originates in the cells and tissues, and it is a matter of detail only that in some cases—e.g. severe freezing or drought of seedlings, or when some ingredient is wanting in the soil—the diseased condition affects practically every cell alike from the first, while in others it spreads more or less rapidly from some one spot.
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. This is not saying that no importance is to be attached to the mode in which disease is incurred or induced: it is merely insisting on the truth that the disease itself consists in the living cell-substance—the protoplasm—not working normally as it does in health, and this, whether want of water, minerals, or organic food be the cause, or whether the presence of some poison or mechanical irritant be the disturbing agent, as also whether such want or irritation be due to some defect in soil or air, or to the ravages of a fungus or an insect.
This being understood I need not dwell on the common fallacy of confounding the fungus, insect, soil or other agent with the disease itself, or of making the same blunder in confusing symptoms with maladies. In this sense, wheat rust is not a disease: it is a symptom which betrays the presence of a disease-inducing fungus, the Rust fungus. Similarly, chlorosis is not a disease: it is a symptom of imperfect chlorophyll action, and the best proof of the truth of both statements is that in both cases the fundamental disease-action is the starvation of the cell-protoplasm of carbohydrates and other essential food matters—in the one case because the fungus steals the carbohydrates as fast as the leaves can make them, in the second because the leaf is unable to make them.
The foundation of a knowledge of disease in plants therefore centres in the understanding of the pathology of living cells.
If a suitable mass of living cells is neatly cut with a sharp razor the first perceptible change is one of colour: the white "flesh" of a potato or an apple, for instance, turns brown as the air enters the cut cells, and the microscope shows that this browning affects cell-walls and contents alike. The cut cells also die forthwith; and the oxygen of the air combining with some of their constituents forms the brown colouring matter which soaks into the cell-walls. The uninjured cells below them grow longer, pushing up the dead débris, and divide across by walls parallel to the plane of the wound, and so form series of tabular cells with thin walls, which also soon turn brown and die, the cell-walls meanwhile undergoing changes which convert them into cork. The living cells deeper down are now shut off from the outer world by a skin, of several layers, of cork-cells, which prevent the further free access of air or moisture. During the period of active cell-division which initiates the cork, the temperature of the growing cells rises: a sort of fever (wound-fever) is induced, evidently owing to the active respiration of the growing cells.
This healing by cork occurs in any tissue of living cells exposed by a cut—leaf-tissue, young stem or root, fruit, cambium, etc.; and the same applies to any other kind of cutting or tearing injury—such as a prick with a needle or the proboscis of an insect, a stripping, or even a bruise.
Such healing is prepared for and carried out very thoroughly in the case of falling leaves and cast branches, the plane of separation being covered by a cicatrix of cork.
If the cell-tissue under the wound is actually growing at the time, however, a further process is observed when the wound-cork has been formed. The uninjured cells below go on growing outwards more vigorously than ever, the pressure of the overlying tissues taken off by the cut having been removed, and, lifting up the cork-layer as they do so, they rapidly divide into a juicy mass of thin-walled cells which is of a cushion-like nature and is termed a Callus. This callus is at first a homogeneous tissue of cells which are all alike capable of growing and dividing, but in course of time it undergoes changes in different parts which result in the formation of tracheids, vessels, fibres and other tissue-elements, and even organs, just as the embryonic tissues of the growing points, cambium, etc., of the healthy plant give origin to new growths. Such wound-wood, however, is apt to differ considerably in the arrangement, constitution and hardness of its parts as compared with normal wood, and its peculiar density and cross-graining are often conspicuous.
If instead of a simple tissue, the cut or other wound lays bare a complex mass such as wood, the resultant changes are essentially the same to start with. The living cells bordering the wound form cork, and then those deeper down grow out and form a callus. The exposure of the wood however, entails alterations in its non-living elements also. The lignified walls of tracheids, fibres, etc., turn brown to a considerable depth, and this browning seems to be—like all such discolorations in wounds—due to oxidation changes in the tannins and other bodies present: the process is probably similar to what occurs in humification and in the conversion of sap-wood into heart-wood in trees. Such wood is not merely dead, but it is also incapable of conveying water in the lumina of its elements, which slowly fill with similarly dark-coloured, impervious masses of materials termed "wound-gum," the nature of which is obscure, but which slowly undergoes further changes into resin-like substances.
The exposure of wood by a wound results also in another mode of stopping up the vessels and so hindering the access of air, loss of water, etc., for the living cells of the medullary rays and wood-parenchyma grow into the lumina of the larger vessels through the pits, forming thyloses, again a phenomenon met with in heart-wood. In Conifers the stoppage of the lumina is increased by deposition of resin, which also soaks into the cell-walls and the wounded wood becomes semi-translucent owing to the infiltration.
Every living cell in an active condition is irritable, and one of the commonest physiological reactions of growing tissues is that of responding to the touch of a resistant body, as is vividly shown by the movements of the Sensitive plant, Dionaea, etc., and by those of tendrils, growing root tips, etc., on careful observation. We have reason for stating that if a minute insect, too feeble to pierce the cuticle, cling on to one side of the dome-shaped growing point of any shoot, the irritation of contact of its claws, hairs, etc., would at once cause the protoplasm of the delicate cells to respond by some abnormal behaviour; and, as matter of experiment, Darwin showed long ago that if a minute piece of glass or other hard body is kept in contact with one side of the tip of a root, the growth on the side in contact is interfered with. Moreover we know from experiments on heliotropism, thermotropism, etc., that even intangible stimuli such as rays of light, etc., impinging unsymmetrically on these delicate cells cause alterations in their behaviour—e.g. arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due to the presence of the entire protoplasmic body of one organism in the cell of another, each living its own life for the time being, but the protoplasm of the host cell showing clearly, by its abnormal behaviour, that the presence of the foreign protoplasm is affecting its physiology. A simple example is afforded by Zopfs' Pleotrachelus, the amoeboid protoplasmic body of which lives in the hypha of Pilobolus, causing it to swell up like an inflated bladder, in which the parasite then forms its sporangia. The Pleotrachelus does not kill the Pilobolus, but that its protoplasm alters the metabolic physiology of the latter is shown by the hypertrophy of the cells, and by the curious fact that it stimulates the Pilobolus to form its sexual conjugating cells, otherwise rare, an indication of very far-reaching interference with the life-actions of the host.
An equally remarkable example is that of Plasmodiophora, the amoeboid naked protoplasm of which lives and creeps about in the protoplasm of a cell of the root of a turnip, to which it gains access through the root-hairs. It does not kill the cell, but stimulates its protoplasm to increased activity and growth and division, itself dividing also and passing new amoebae into each new daughter-cell of the host. Here the processes of stimulation, hypertrophy and further division are repeated, until hundreds or thousands of the turnip root-cells are infected. The externally visible result is the formation of distorted swellings on the root (Finger and Toe), most of the cells of which are abnormally large and filled with amoeboid Plasmodiophora protoplasm, which finally devours the turnip-protoplasm and itself passes over into spores. Here we have most convincing proof of the stimulation of protoplasm by other protoplasm in direct contact with it; and that the metabolism of the host-cells is profoundly altered is shown not only by the abnormal growth of the cells, but also by the starvation of the rest of the turnip plant as the Plasmodiophora gets the upper hand. We have here, in fact, a local intracellular parasitic disease, gradually invading large tracts of tissue and eventually inducing general disease resulting in death—a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by parasites which never bring their protoplasm into direct contact with that of the host. Many Chytridiaceae penetrate the cells of plants, and grow inside them as short tubes, vesicles, etc., the protoplasm of which is separated by their own cell-walls from that of the host-cell; nevertheless hypertrophy and abnormal cell-divisions and secretions are induced, and the effect even extends to neighbouring cells—e.g. Synchytrium—showing that some influence is exerted through cells themselves not directly affected. This latter point need not surprise us now we know that the cells of plant-tissues are connected by fine protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still stimulate them through both its own and their own cell-walls to abnormal growth. This is well shown by the intercellular mycelium of Exoacus and Exobasidium, and the latter affords an excellent illustration of the far-reaching effects of hyphae on the cells (of Vaccinium) into which they do not penetrate. Not only are the cells stimulated to grow larger and divide oftener than normally, thus producing large gall-like swellings, but the chlorophyll disappears, the cell sap changes colour to red, the numerous compound crystals normally found in the tissues diminish in number and are different in shape, large quantities of starch are stored up, and even the vascular bundles are altered in character. All these changes indicate very profound alterations in the physiological working of the protoplasm of the cells of the host, and yet the fungus has done its work through both its own cell-walls and those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may stimulate the cells in their immediate neighbourhood to increased growth, e.g. Anabaena in the roots of Cycads.
Notes to Chapter XIII.
With reference to cork-healing and wound-fever the student may consult Shattock "On the Reparative processes which occur in Vegetable Tissues," Journal of the Linnean Society, 1882, Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in the Aspen," Journal of Botany, 1883, Vol. XXI., p. 306. Also Richards, "The Respiration of Wounded Plants," Annals of Botany, Vol. X., 1896, p. 531; and "The Evolution of Heat by Wounded Plants," Ann. of Bot., Vol. XI., 1897, p. 29.
For details and figures respecting callus, see Sorauer, Physiol. of Plants, p. 175.
In respect to the irritable movements referred to see Darwin, The Power of Movements in Plants, 1880, chapter III. The recent work of Nawaschin, Beobachtungen ueber den feineren Bau u. Umwandlungen von Plasmodiophora, Flora, Vol. LXXXVI., 1899, p. 404, should be read for details and literature concerning "Finger and Toe."
