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.