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 hyphae, 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 Uredinae, 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.