3o8 PLANT GROWTH SUBSTANCES 



of inorganic salts in the medium; he interpreted these results as affecting 

 the conditions under which de Bary's postulated substance would be 

 secreted or effective. Couch (i6) suspected specific chemical substances 

 as correlative in the mating of male and female strains of Dictyuchus 

 monosporus on the basis of the following observations: i) oogonial initials 

 and antheridial hyphae were formed on reacting mycelia at locations far 

 removed from the region of actual contact; 2) the directional growth 

 of antheridial hyphae to the oogonial initials; and 3) the marked stimula- 

 tion, including sexual organ formation, in intergeneric matings with 

 Thraustotheca primoachlya. He devised experiments, including the effects 

 of extracts and filtrates, membrane matings, and matings in which 

 certain of the partners were not in actual contact, but the results were 

 in each case negative. 



Bishop (11,12) found definite evidence for the activity of diffusible 

 hormones in Sapromyces Reinschii of the Leptomitaceae. He demon- 

 strated the production of antheridial hyphae and oogonial initials on 

 male and female mycelia before contact and the induction of greatly 

 increased branching of male hyphae under the influence of the filtrate 

 of a female culture. These effects as well as the directional growth of 

 antheridial hyphae to the exact distal ends of oogonial initials he at- 

 tributed to diffusible hormones. No attempt, however, was made to 

 define the minimal mechanism necessary to explain these effects. 



The hormonal mechanism regulating the sexual process has been more 

 completely worked out by Raper and associates in two heterothallic 

 species of Achlya, A. ambisexiialis and A. bisexualis, than elsewhere 

 in the fungi. The various phases of the sexual process up to and including 

 the differentiation of oogonia and the delimitation of oospheres (eggs) 

 has been shown to be initiated and coordinated by specific diffusible 

 hormones (44). A number of lines of evidence indicated hormones as 

 the coordinating agents: i) the unvarying sequence of events and the 

 pattern of temporal relationship between the successive stages; 2) the 

 different and characteristic stages of the sexual progression attained in 

 the reciprocal interspecific matings between the two heterothallic species; 

 and 3) the effects of variations in the nutrient content of the medium 

 upon the sexual reaction (45). Conclusive proof of the hormonal mecha- 

 nism as well as the means of determining the number of hormones in- 

 volved, the loci of their secretion, and their specific activities has been 



