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.