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.
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. fahrb.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.