136 BOTANICAL GAZETTE [august 



from either strain is able to call forth the formation of progametes 

 only in the other. Since he considers opposite strains to be zygo- 

 tactic, he is thus able to find the sex of an unknown strain by 

 growing it on a medium with a strain whose sex has previously 

 been determined. Thus, if a strain whose sex is not known is 

 grown on an agar plate with a plus strain, and if a sexual reaction 

 is obtained, the new strain is considered to be minus. If no 

 sexual reaction takes place, however, the strain is then contrasted 

 with a minus strain; if sexual reaction is obtained, the new strain 

 is considered plus; and if there is no sexual reaction with either 

 the plus or minus strains, it is considered neutral. During his 

 work Blakeslee found several strains of different species of 

 Mucorineae which showed no reaction to either of his test species 

 Mucor V. plus and minus. Those inactive strains he called 

 neutrals, and believed them to be produced by the environment, 

 because strains under artificial cultivation are sometimes found 

 to lose their power of conjugation. Hagem also found sexually 

 inactive strains which he isolated from the soil. From 52 different 

 strains of Mucor he found that 20 were minus, 3 were plus, and 29 

 were neutrals. If the Mucors are dioecious, as he and Blakeslee 

 maintain, this seems to be a remarkably large percentage of neu- 

 trals to be found in a natural environment. 



There are other conditions which may account for neutrals 

 as well as the unfavorable environment. Burgeff contrasted a 

 plus and a minus strain of Phycomyces nitens, and from the sporan- 

 gium of the germinating zygospore he obtained some spores which 

 were plus, others which were minus, and a third kind which were 

 neutral. The mycelium arising from the plus spores was also 

 plus and produced plus spores. From the minus spores was 

 obtained a minus mycelium, which in turn produced minus spores. 

 On the other hand, from the neutral spores a mycelium was 

 obtained which gave 3 kinds of spores, plus, minus, and neutral. 

 In the neutral spores he believed the plus and minus nuclei to be of 

 equal numbers. Thus the zygotactic stimulus of one kind of nuclei is 

 counterbalanced by the nuclei of the opposite sex in the same hyphae. 



In Cunninghamella there is exhibited a neutrahty toward 

 dift'erent strains which is unlike either of the neutral conditions 



