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. NOTES TO CHAPTER XIV. 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. Jahrb. 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. CHAPTER XV.