Disease in Plants

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."

CHAPTER XIV.

NATURE OF DISEASE (Continued)

Actions of poisons in small doses—Results of killing a few cells—Malformation—Enzymes—Secretions and excretions—Acids, poisons, etc.—Chemotactic phenomena—Parasitism—Epiphytes and endophytes—Symbiosis—Galls.

Physiological research has shown that the respiratory activity of cells may be increased by small doses of poisons, and even that growth may be accelerated by them—e.g. chloroform, ether—and, still more remarkable, that fermentative activity may be enhanced by minute doses of such powerful mineral poisons as mercuric chloride, iodine salts, etc., and that the cells may be gradually accustomed to larger doses without injury. Unfertilised eggs of insects have been started into growth by treatment with acids and those of frogs with mercury salts, and the germination of beans quickened by various poisonous alkaloids. In other words, graduated doses of poison may alter the physiological activity of living cells, inducing pathological phenomena, while larger doses kill them.

Now we know at least one parasitic fungus which poisons the cells of its host, and kills them, with similar symptoms to those resulting from excessive doses of the above-named toxic agents. Botrytis hyphæ, living in the cell-walls of plants, but not entering the cells, excretes a poison which kills the protoplasm, and the fungus then feeds on the debris. Numerous other fungi form powerful poisons, but we do not know whether or how they employ them—e.g. Ergot.

It is obvious that if all the young cells of a root-tip or of the apex of a shoot, or those of a young leaf, are growing and dividing regularly, the killing of one or a few cells at one point on the side of the organ must result in irregularities—in malformation—of the adult organ. This has been proved experimentally by destroying a few cells with a needle. It can also be done by planting a minute mycelium of Botrytis laterally on a young organ—e.g. a very young lily-bud. The fungus adheres to the surface, kills a few epidermis cells, and forms a foxy-red spot, which becomes concave as the dead cells lose water and dry. Since the rest of the bud goes on growing, however, while this dead point remains stationary, the latter gradually becomes the centre of a concavity, the growing tissues having grown round it: the bud is deformed. Numerous cases of malformed organs are explained in this way; a minute insect has bitten or pierced the young tissue, or a fungus has killed a minute area, or a drop of acid condensed from fumes in the air is the lethal agent, and so forth. And even on a much larger scale we see the same kinds of agents at work. Wherever a patch of cells is killed whilst those around go on growing, there must result some deformation of the resulting organ, since had the injury been withheld the number and sizes of the cells now fixed in death would have increased and covered a larger area: they now serve to pull over to their side the still living and growing cells. The same results follow on any lateral wound: the killed spot of tissue serves as a point round which the continued growth of other parts of the organ turns. Hence the malformation is in these cases a secondary effect, and not, as in simple hypertrophy, a direct effect of the action of the cells involved in the injury.

There is another class of bodies secreted by fungi, however, which act directly on cells, viz. enzymes—that is, soluble bodies which are able to dissolve cellulose (cytases), starch (diastases), proteids (proteolytic enzymes), and other substances, by peculiar alterations in their constitution. It is by means of its cytase that Botrytis hyphae pierce the cellulose walls of plants, and no doubt in all cases where fungi pierce cell-walls it is by the solvent action of such a cytase, and similarly when haustoria penetrate into the cells. It is also by means of these starch-dissolving enzymes (diastases) and proteolytic enzymes, etc., that the hyphae inside the cells are enabled to make use of the starch, proteids, etc., they find there.

All living cells form materials, resulting from the activity of the protoplasm, which we may compare with the refuse or by-products formed in any great manufacturing industry: these by-products have to be got rid of if they are injurious or noisome (excretions), and if not—i.e. if they are capable of further use (secretions)—they have to be stored away till required. Some of the most prominent of these bodies excreted by fungi are, as we have seen, poisonous acids, such as oxalic acid, enzymes, and organic poisons, such as those in ergot. But similar enzymes, acids, poisons, etc., to those found in fungi are also found in the cells of other plants and animals; for only by means of their solvent actions can processes like digestion and assimilation of the starchy and other materials into the body-substance be accomplished, and we have seen that it is a general property of living cells to form acids, and other excretions and secretions.

Now we know very little about what may happen when an organism—say a fungus—secreting especially one kind of enzyme or poison or other active substance, comes into intimate contact with another—say a leaf-cell—which secretes predominantly others, but what we do know points to the certainty that various complications will occur.

For instance, if certain bacteria which prefer an alkaline medium, and yeasts which prefer an acid environment are mixed in a saccharine solution, it depends on the reaction of the liquid which organism gains the upper hand: if the liquid is acid the yeast may dominate the bacteria; if alkaline it may be suppressed by them.

That a parasite may be prevented from successfully attacking a particular plant is shown by the failure of Cuscuta to establish its haustoria in poisonous plants such as Euphorbia, Aloe, etc., and it has been pointed out that poisonous secretions in the cells of the plant protect them against the penetration of fungi. This cannot be taken as meaning that any poison protects against any parasite, however, for Euphorbia is itself subject to attacks of Uredineae, and Pangium edule, which contains prussic acid and is extremely poisonous to most animals, is eaten with avidity by several insects, while nematode worms can live in its tissues. This is no more remarkable, however, than the fact that Fontaria, a myriapod, secretes prussic acid in its own tissues, or than that certain glands of the stomach secrete free hydrochloric acid, and Dolium forms sulphuric acid in its glands.

There is yet a further point to notice here. It has been proved that certain substances formed in plant-cells, not necessarily nutritive, attract the hyphae of parasitic fungi or repel them, according to the kind and degree of concentration. So clear has this proof been made that it was possible in experiments conducted apart from a host plant, to make the hyphae on one side of an artificial membrane—e.g. collodion—penetrate it by placing one of these attractive (chemotropic) substances in suitable proportions on the other side. The hyphae dissolved holes in the membrane by means of enzymes and plunged into the attractive substance on the other side.

The foregoing sketch gives us a glimpse into the causes at work in parasitism.

Suppose a fungus on the outside of the epidermis of a young organ—say a leaf. It may be unable to penetrate into the plant, and finding no suitable food outside it dies: or it may be satisfied with the traces of organic matter on the epidermis and then lives the life of a saprophyte. Or it may be able to establish a hold-fast on the tender epidermal surface, but without entering the cells, and irritate the developing organ by contact stimulation, inducing slight abnormalities; if in its further, purely superficial growth such an epiphyte covers large areas of the leaf, and especially if the hyphae are dark coloured—e.g. Dematium and other "Sooty Moulds"—injury may be done to the leaf owing to the shading action which deprives the chlorophyll below of its full supply of solar energy. Some epiphytes, however, are able to fix their hyphae to the epidermis by sending minute peg-like projections into the cuticle—Trichosphaeria, Herpotrichia—while others send haustoria right through the outer epidermal walls—e.g. Erysiphe—and thus supplement mere contact-irritation and shading by actual absorption from the external cells. Here the fungus is a parasitic epiphyte.

A stage further is attained in those fungi which enter the stomata and live in the intercellular spaces—e.g. many Uredineae and Phytophthora—and many such intercellular endophytes increase their attack on the cells by piercing their walls with minute (Cystopus) or large and branched (Peronospora) haustoria, or even eventually pierce the cells and traverse them bodily (Pythium). In all these cases it is clear that conflicts must occur between poison and antidote, acid and alkali, attractive and repellent substances, enzyme and enzyme, etc., as was hinted at above; and the same must take place when the parasite is endophytic and intracellular from the first, as in Chytridiaceae, etc., the zoospores of which pierce the outer cell-walls and forthwith grow into the cells. There are also fungi which, while able to pierce the outer cell-walls, and grow forward in the thickness of the wall itself, cannot enter the living cells themselves—e.g. Botrytis. In the example mentioned, the fungus excretes a poison, oxalic acid, which soaks into and kills the cells next its point of attack: into these dead cells it then extends, and, invigorated by feeding on them, extends into other cell-walls and excretes more poison, and so on.

On the basis of the foregoing it seems possible to sketch a general view of the nature of parasitism. In order that a fungus may enter the cells it must be able to overcome not only the resistance of the cell-walls, but that of the living protoplasm also: if it cannot do the latter it must remain outside, as a mere epiphyte, or at most an intercellular endophyte. If it can do neither it must either content itself with a saprophytic existence or fail, so far as that particular host-plant is concerned. Its inability to enter may be due to there being no chemotropic attraction, or to its incapacity to dissolve the cell-walls, or to the existence in the cell of some antagonistic substance which neutralises its acid secretions, destroys its enzymes or poisons, or is even directly poisonous to it.

Moreover when once inside it does not follow that it can kill the cell. The protoplasm of the latter may have been unable to prevent the fungus enemy from breaking through its first line of defence—the cell-wall, but it may be quite capable of maintaining the fight at close quarters, and we see signs of the progress of the struggle in hypertrophy, accumulation of stores, and other changes in the invaded cells and their contents.

Finally, the invested or invaded cell may so adapt itself to the demands of the invader that a sort of arrangement is arrived at by which life in common—Symbiosis—is established, each organism doing something for the other and each taking something from the other. In this latter case, which is often realised—e.g. lichens, leguminous plants and the organisms in their root-nodules, mycorrhiza, etc.—we leave the domain of disease, which supervenes indeed if the other symbiont is lacking.

Some interesting facts bearing on the matters here under discussion, have been obtained from the study of Galls, the curious outgrowths found on many plants and due to the action of insects.

A typical gall exhibits three distinct and characteristic layers of tissue surrounding the hollow chamber in which the larva of the insect lies, viz., an outer layer of soft cells forming a parenchyma covered with an epidermis, and frequently also with a layer of cork; an inner stratum consisting of very thin-walled delicate cells filled with protoplasmic and reserve food-materials on which the larva feeds; and between the two a more or less definite layer of thick-walled sclerenchyma cells which serve as a protection against accidents to the larva as the outer layer shrivels or rots, or if it is exposed to the attack of marauders. This layer may be absent from galls which have a short life only. Vascular bundles run into the outer layer from the leaf-veins or the stele of the shoot, etc. Such galls abound in tannin, and are frequently of use in the arts on this account: they also contain starch, and proteid substances and crystals of calcium oxalate. When the larva has consumed the stores of food material and reached the adult stage it eats its way out and escapes.

The growth of such a gall is preceded by the laying of an egg on or in the embryonic tissue of a leaf, stem, or other young part, and it is interesting to note that only organs in the meristematic stage can form galls, and that it is by no means necessary that the tissues should be wounded. Moreover, the egg as such is incapable of stimulating the plant tissues, but when it hatches, the resulting larva, beginning to feed on the cells, irritates the tissues and rapid growth and cell-division occur, as in the case of other wounds or of fungus attacks. The actual wound made by the ovipositor heals up at once. It is evident from numerous recent researches that these true galls are not due to any poisonous or irritating liquid injected by the parent, but that the stimulus to the tissue formation is similar to that exerted by a wound. The young gall is in fact a callus enclosing the living larva, and it is the continued irritation of the latter which keeps up the stimulation. The final shape and constitution of the gall depend on mutual reactions—not as yet explained in detail—between the species of plant and the species of gall-insect concerned, as may readily be seen from the extraordinary variations in size, shape, colouring, hairiness and other structural peculiarities of the galls on one species of, for instance, the common oak. From what we have learnt about fungus parasites, however, there can be little doubt that reactions between the cells and the larva of the insect occur, resembling those which take place between the cells and the hyphae of the fungus, and this is borne out by the study of other hypertrophies due to animals; e.g. Nematode worms in roots, and the remarkable galls—the simplest known—on Vaucheria, caused by the entrance into this alga of a species of Notommata, which induces a different gall on each of the various species of its host plants.

It must be concluded that the formation of the Vaucheria gall is induced by the mechanical irritation which the Rotifer causes in the protoplasm. These galls are comparable to the hypertrophies in Pilobolus caused by the presence of Pleotrachelus.

Attempts to induce the development of galls artificially by injecting formic, acetic and other vegetable acids, poisons and other substances into the tissues have, however, failed, and even the substances contained in the insect or gall itself only produced negative results. Nothing further was obtained than slight callus formations in some cases. Nor have experimenters succeeded in obtaining more than slight distortions by fixing insects on the growing leaves in such positions that they could scratch the epidermis.

We must therefore conclude that very complex interactions between the plant and insect are here concerned, among which may be the infiltration of some liquid from larva to plant—many of these gall larvae are strongly scented, and Kustenmacher says that fluids excreted by the larva are absorbed by the gall-tissue apparently as nutriment. This would point to the symbiotic character of galls and their guests.

Notes to Chapter XIV

With regard to the action of poisons in small doses see further Johannsen, Das Aether-Verfahren beim Fruhtreiben, Jena, 1900, and, for Botrytis, see Marshall Ward, "A Lily Disease," Annals of Botany, Vol. II., 1889, p. 388.

The subject of enzymes has been exhaustively treated by Green, The Soluble Ferments and Fermentations, Cambridge, 1899, to which the reader is referred for literature. I have taken the statements regarding Fontaria and Dolium from Kassowitz, Allgemeine Biologie, p. 182. The two most important works on chemotactic phenomena are Pfeffer, "Uber Chemotaktische Bewegungen," etc., Unters. aus dem Bot. Inst. zu Tubingen, B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen durch Pilzfaden," Pringsh. Jahrb. f. Wiss. Bot., B. XXVIII., 1895, p. 269, and from these the further literature can be traced. As regards the nature of parasitism see Marshall Ward, "On Some Relations between Host and Parasite," etc., being the Croonian Lecture delivered before the Royal Society, Proc. Roy. Soc., Vol. 47, p. 393. On Symbiosis, see Marshall Ward, "Symbiosis," Annals of Botany, 1899, Vol. XIII., p. 549, where the literature is collected. For a general account of galls the reader may consult Kerner, The Natural History of Plants, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler, Alternating Generations, A Biological Study of Oak Galls, etc., 1894.

CHAPTER XV.

SPREADING OF DISEASE AND EPIDEMICS

Dissemination of fungi by the aid of snails, rabbits, bees, and insects—Man—Distribution in soil, on clothes, through the post, etc.—Worms, wind—Puffing of spores—Creeping of mycelia—Lurking parasites—Spread of insects and other animals—Losses due to epidemics.

The dissemination of plant diseases is a subject which has been far too much neglected, but our knowledge of it is slowly increasing. The spores of fungi such as Rusts and Erysipheae are often carried from plant to plant by snails; those of root-destroying and tree-killing Polyporei by rabbits, rats, and other mammals which rub their fur against the hymenophores. Bees have been shown to carry the spores of Sclerotinia and infect the stigmas of Bilberries, etc., with them; and flies convey the conidia of Ergot from grain to grain. Insects, indeed, of all kinds are great disseminators of disease—as witness also the part played by mosquitoes in transferring the malaria parasite to man—and beetles, bees, flies, etc., of all sorts probably play more active parts in this work than has yet been proved, since they not only carry spores attached like pollen to their hairy bodies, but in many cases in their alimentary canal, to be spread later in the dung.

The part played by man in conveying fungi from plant to plant counts for much. Not only do gardeners and farm labourers carry spores on their boots and clothes as they pass from infected to non-infected areas, but carted soil and manure are frequently infested with spores of Smuts, Fusarium, Polyporus, and the sclerotia or rhizomorphs of Sclerotinia, Agaricus melleus, Dematophora, etc. Man also sends diseases through the post, and by rail and ship, by spores or mycelia attached to seedlings, bulbs, fruits, flowers, etc., as shown in several cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every time a carpenter saws a piece of fresh timber with the saw which has been used previously for cutting wood attacked with dry rot, he risks infecting it with the fungus. Similarly in pruning: every cut with a knife which the gardener has used on infected branches may infect the tree.

Cuttings made with a soil-contaminated knife and stuck into ordinary soil in dirty boxes covered with equally dirty glass, present every chance for infection by soil organisms; bacteria and fungi obtain access to the vessels, and derive plenty of food from the juices, and the wonder is not that so many cuttings "damp off," but that any are raised at all under ordinary conditions.