304 PLANT GROWTH SUBSTANCES 



Thimann (40,41). This species in pure culture gave only 8 to 12 per cent 

 conjugation while in cultures contaminated with Aspergillus m'ger the 

 incidence of conjugation was increased to 65 to 75 per cent. This effect 

 could be readily duplicated by filtrates of the culture fluid in which 

 the contaminating Aspergillus had grown. The yeast itself was shown to 

 contain the active substance but in low quantity. The incidence of 

 conjugation in slant cultures was shown to be much higher in the thin 

 end of the slant and this could be correlated with the higher percentage 

 of dead cells in this region as shown by staining with Methylene Blue. 

 This points to the possible role of diffusible substances originating from 

 moribund or dead cells in aging cultures in the normal reproductive 

 process of the yeast. The stimulating activity of the filtrate of A. ?iiger 

 proved to be organic and to consist of two fractions, an organic acid and 

 a member of the vitamin B complex. Each alone was active but together 

 gave activity greater than the sum of their single activities. The stimu- 

 latory effects of these two substances could be duplicated by glutaric 

 acid and riboflavin. Whether these two compounds are identical to 

 the active substances secreted by A. niger and contained in the cells 

 of Zygosaccharomyces has not been determined. 



In Table i is presented a summary list of the demonstrated cases 

 of interspecific stimulation of sexual processes and of the few cases where 

 known chemical compounds exert a significant effect on sexual reactions 

 . in the fungi. 



Intra- specific chemical regulation of sexual processes. — The first demon- 

 stration of the initiation and coordination of sexual reproductive processes 

 by diffusible, autosecreted substances was given by Burgeff in 1924 (14) 

 for Mucor mucedo. In matings of (+) and ( — ) strains of this species 

 on an agar medium a restraint area was formed along the line of inter- 

 mingling of the two myceha and into this region only a few hyphae 

 penetrated from each mycelium. At the tips of these hyphae corraloid 

 swellings developed before any contact was established and there were 

 produced upon them what appeared to be progametangia. This was 

 considered to be the initial stage in the sexual reaction and since it 

 occurred before contact of (+) and ( — ) hyphae it indicated the presence 

 and activity of diffusible chemical messengers. To test this hypothesis 

 Burgeff performed the classical experiment of placing a block of agar 

 containing (-|-) hyphae on the surface of a ( — ) mycelium growing upon 

 an agar medium with a permeable collodian membrane interposed be- 



