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.