we do not look on their depredations as diseases, though the gradual deteriorat1on of hay under the exhausting effects of root-parasites like Rhinanthus, and the onslaught of Cuscuta when unduly abundant, should teach us how unimportant to the definition the question of size may be.
It is, however, among the Fungi that we find the most disastrous and elusive agents of disease. Parasitic Fungi may be, as regards their direct action, purely local—e.g. Schinzia, which forms gall-like swellings on the roots of rushes; Gymnosporangium, causing excrescences on juniper stems; numerous leaf Fungi such as Puccinia, Aecidium, Septoria, &c., causing yellow, brown or black spots on leaves; or Ustilago in the anthers of certain flowers. In such cases the immediate damage done may be slight; but the effects of prolonged action and the summation of numerous attacks at numerous points are often enormous, certain of these leaf diseases costing millions sterling annually to some planting and agricultural communities. In other cases the Fungus is virulent and rampant, and, instead of a local effect, exerts a general destructive action throughout the plant—e.g. Pythium, which causes the “damping off” of seedlings, reducing them to a putrid mass in a few hours, and Phytophthora, the agent of the potato disease. Many Fungi, in themselves not very aggressive, slowly bring about important and far-reaching secondary effects. Thus, many Hymenomycetes (Agarics, Polyporei, &c.) live on the wood of trees. This wood is in great part already dead substance, but the mycelium gradually invades the vessels occupied with the transmission of water up the trunk, cuts off the current, and so kills the tree; in other cases such Fungi attack the roots, and so induce rot and starvation of oxygen, resulting in “fouling.” Numerous Fungi, though conspicuous as parasites, cannot be said to do much individual injury to the host. The extraordinary malformations known as “Witches' Brooms” caused by the repeated branching and tufting of twigs in which the mycelium of Exoascus (on birch) or Aecidium (on silver fir) are living, may be borne in considerable numbers for years without any very extensive apparent injury to the tree. Again, the curious distortions on the stems of nettles attacked by the Aecidium form of the heteroecious Puccina Caricis (see Fungi for Heteroecism), or on maize stems and leaves attacked by Ustilago Maydis, or on the inflorescence of crucifers infested with Cystopus, &c., are not individually very destructive; it is the cumulative effects of numerous attacks or of extensive epidemics which eventually tell. Some very curious details are observable in these cases of malformation. For instance, the Aecidium elatinum first referred to causes the new shoots to differ in direction, duration and arrangement, and even shape of foliage leaves from the normal; and the shoots of Euphorbia infected with the aecidia of Uromyces Pisi depart so much from the normal in appearance that the attacked plants have been taken for a different species. Similarly with Anemone infested with Puccinia and Vaccinium with Calyptospora, and many other cases of deformations due to hypertrophy or atrophy. Instances of what we may term tolerated parasitism, where the host plant seems to accommodate itself very well to the presence of the Fungus, paying the tax it extorts and nevertheless not succumbing but managing to provide itself with sufficient material to go on with, are not rare; and these seem to lead to those cases where the mutual accommodation between host and guest has been carried so far that each derives some benefit from the association—symbiosis (see Fungi).
II. The kinds of disease due to these various agencies are very different. A plant may be diseased as a whole, because nearly all its tissues are in a morbid or pathological condition, owing to some Fungus pervading the whole—e.g. Pythium in seedlings—or to a poison diffusing from cell to cell; in the case of unicellular plants—e.g. an alga infested with a Chytridium—indeed, matters can hardly bc otherwise. But the case is obviously different where a plant dies because some essential organ or tissue tract has been destroyed, and other parts have suffered because supplies are cut off—e.g. when the upper parts of a tree die off owing to destruction of the roots, or to the ringing of the stem lower down, and consequent interference with the transpiration current. In a large number of cases, however, the disease is purely local, and does not itself extend far into the organ or tissue affected.
If a mass of living plant-tissue is cut, the first change observed is one of colour: the white “flesh” of a potato or an apple turns brown as the air enters, and closer examination shows that cell walls and contents are alike affected. The cut cells die, and oxidized products are concerned in the change of colour, the brown juices exuding and soaking into the cell-walls. The next change observable after some hours is that the untouched cells below the cut grow larger, push up the dead surface, and divide by walls tangential to it, with the formation of tabloid cork-cells. The layer of cork thus formed cuts out the dead débris and serves to protect the uninjured cells below. Such healing by cork formation is accompanied by a rise of temperature: the active growth of the dividing cells is accompanied by vigorous metabolism and respiration, and a state of “wound fever” supervenes until the healing is completed. The phenomena described occur in all cases of cicatrization of wounds in nature—e.g. leaf tissue, young stems, roots, &c., when cut or pierced by insects, thorns and so forth They are concerned in the occlusion of broken twigs and of falling leaves, and it is from the actively growing “callus” developed at the surface of the wounded tissues of cuttings, buddings, prunings, &c., that the healing and renewal of tissues occur of which advantage is taken in the practice of what might well be termed plant surgery. A third phenomenon observable in such healing tissues is the increased flow and accumulation of plastic materials at the seat of injury. The enhanced metabolism creates a current of draught on the supplies of available food-stuffs around. The phenomenon of irritability here concerned is well shown in certain cases where a parasitic organism gains access to a cell—e.g. Pleotrachelus causes the invaded Pilobolus to swell up, and changes the whole course of its cell metabolism, and similarly with Plasmodiophora in the roots of turnips, and many other cases.
Irritation and hypertrophy of cells are common signs of the presence of parasites, as evinced by the numerous malformations, galls, witches'-brooms, &c., on diseased plants. The now well-known fact that small doses of poisonous substances may act as stimuli to living protoplasm, and that respiratory activity and growth may be accelerated by chloroform, ether and even powerful mineral poisons, such as mercuric chloride, in minimal doses, offers some explanation of these phenomena of hypertrophy, “wound fever,” and other responses to the presence of irritating agents. Still further insight is afforded by our increasing knowledge of the enzymes, and it is to be remarked that both poisons and enzymes are very common in just such parasitic Fungi as induce discolorations, hypertrophies and the death of cells—e.g. Botrytis, Ergot, &c. Now it is clear that if an organism gains access to all parts of a plant, and stimulates all or most of its cells to hypertrophy, we may have the latter behaving abnormally—i.e. it may be diseased throughout; and such actually occurs in the case of Euphorbia pervaded with Uromyces Pisi, the presence of which alters the whole aspect of the host-plant. If such a general parasite carries its activities farther, every cell may be killed and the plant forthwith destroyed—e.g. Phytophthora in potatoes. If, on the other hand, the irritating agent is local in its action, causing only a few cells to react, we have the various pimples, excrescences, outgrowths, &c., exhibited in such cases as Ustilago Maydis on the maize, various galls, witches'-brooms, &c.
It must not be overlooked that the living cells of the plant react upon the parasite as well as to all external agencies, and the nature of disease becomes intelligible only if we bear in mind that it consists in such altered metabolism—deflected physiology—as is here implied. The reaction of the cells may be in two directions, moreover. For instance, suppose the effect of a falling temperature is to so modify the metabolism of the cells that they fill up more and more with watery sap; as the freezing-point is reached this may result in destructive changes, and death from cold may result. If, on the contrary, the gradual cooling is met by a corresponding depletion of the cells of water, even intense cold may be sustained without injury.
Or, take another case. If the attack of a parasite is met by the formation of some substance in the protoplasm which is chemotactically repulsive to the invader, it may be totally incapable of penetrating the cell, even though equipped with a whole armoury of cytases, diastatic and other enzymes, and poisons which would easily overcome the more passive resistances offered by mere cell-walls and cell-contents of other plants, the protoplasm of which forms bodies chemotactically attractive to the Fungus.
The various degrees of parasitism are to a certain extent explained by the foregoing. In order that a Fungus may enter a plant, it must be able to overcome not merely the resistance of cell-walls, but that of the living protoplasm; if it cannot do this, it must remain outside as a mere epiphyte, e.g. Fumago, Herpotrichia, &c., or, at most, vegetate in the intercellular spaces and anchor itself to the cell-walls, e.g. Trichosphaeria. The inability to enter the cells may be due to the lack of chemotactic bodies, to incapacity to form cellulose-dissolving enzymes, to the existence in the host-cells of antagonistic bodies which neutralize or destroy the acids, enzymes or poisons formed by the hyphae, or even to the formation and excretion of bodies which poison the Fungus. But even when inside it does not follow that the Fungus can kill the cell, and many cases are known where the Fungus can break through the cell's first lines of defence (cell wall and protoplasmic lining); but the struggle goes on at close quarters, and various degrees of hypertrophy, accumulation of plastic bodies or secretions, discolorations, &c., indicate the suffering of the still living cell. Finally, cases occur where the invaded cell so adapts itself to the presence of the intruder that life in common—symbiosis—results.
The dissemination of plant parasites is favoured by many circumstances not always obvious, whence an air of mystery regarding epidemics was easily created in earlier times. The spores of Rusts, Erysipheae and other Fungi may be conveyed from plant to plant by snails; those of tree-killing polyporei, &c., by mice, rabbits, rats, &c., which rub their fur against the hymenophores. Bees carry the spores of Sclerotinia as they do the pollen of the bilberries, and flies convey the conidia of ergot from grain to grain. Insects, indeed, are largely concerned in disseminating Fungi, either on their bodies or via the alimentary canal. Worms bring spores to the surface of soil, ducks and other birds convey them on their muddy feet, and, as is well-known, wind and other physical
