I] REACTIONS TO STIMULI 27. 



1905 Ward, H. Marshall. Recent Researches on the Parasitism of Fungi. Ann. Bot. 



xix, "p. I. 

 1907 BlFFEN, R. H. Studies in the Inheritance of Disease Resistance. I. Journ. Agr. Sci. 



ii, p. 109. 

 1907 Marryat, D. C. E. Notes on the Infection and Histology of two Wheats immune 



to the Attacks oi Puccinia glumarum, yellow Rust. Journ. Ag. Sci. ii, p. 129. 

 191 1 Freeman, E, M. and Johnson, E. C. The Rusts of Grains in the United States. 



U.S. Dept. Ag. Bureau of Plant Industry. Bull. 216. 



191 1 Pole Evans, I. B. South African Cereal Rusts, with Observations on the Problem 

 of Breeding Rust-resisting Wheats. Journ. Ag. Sci. iv, p. 95. 



1912 Biffen, R. H. Studies in the Inheritance of Disease Resistance. II. Journ. Ag. Sci. 

 iv, p. 421. 



1914 Varilov, N. I. Immunity to Fungous Diseases as a Physiological Test in Genetics 

 and Systematics, exemplified in Cereals. Journ. Genetics, iv, p. 49. 



1918 Stakman, E. C, Piemeisel, F. J., and Levine, M. N. Plasticity of Biologic Forms 

 oi Puccinia graniinis. Journ. Ag. Research^ xv, p. 221. 



1919 Wormald, H. The 'Brown Rot' Disease of Fruit Trees, with Special Reference 

 to two Biologic Forms oi Monilia cinerea, Bon. I. Ann. Bot. xxxiii, p. 361. 



REACTIONS TO STIMULI 



Among the fungi, response takes place to a large number of external 

 stimuli, most of which are concerned with nutrition and the distribution of 

 the spores. A special series of reactions which demands further investigation 

 takes place in relation to the formation, approach and fusion of the sexual 

 organs. The stimulus in question may be effective very early in development 

 for de Bary found that the presence of the oogonium in Pythiuni de 

 Baryanum stimulates the formation of antheridia and Blakeslee observed 

 directive growth or " zygotaxis " in morphologically undifferentiated hyphae 

 of Mitcor Mucedo. In Mucor and its allies the formation of the gametangia 

 follows on the contact of appropriate branches. The stimulus inducing 

 directive growth is presumably chemical, as in other and better known cases 

 of the approach of gametes or gametangia; but we have at present no 

 knowledge of the substances concerned or of much more than the fact that 

 the reaction occurs. In some of the higher fungi, where the antheridium is 

 liberated as a spermatium, it would appear to be carried passively to the 

 female structure, but in Zodiomyces among the Laboulbeniales and perhaps 

 in a few other Ascomycetes {Ascobolus carbonarius, Lachnea creted) the tri- 

 chogyne moves towards the male organ. 



Chemotropism. The most marked chemotropic reaction of vegetative 

 hyphae and germ-tubes is a negative one; they tend to grow away from the 

 products of their own metabolism or so-called " staling " substances. Clark, 

 in 1902, concluded that Rhizopus nigricans'^ is negatively chemotropic to 

 some secretion of its own mycelium and that the negative response is much 



^ Rhizopus nigricans, 'Ehitnb. — M7icor slolonifer, Ehrenb. 



