398 BOTANICAL GAZETTE [November 



* 



r 



of 25 : 1. The ratio in the Oberlin group was 20 : 1 ; in the Sullivan 

 group, about 48:1; and in the Cleveland group, 40:1. 



However, in making any objections to the theory on the ground 

 that B. dissectum occurs too frequently to be a mutant, it must 

 be remembered that mutation in plants has been studied almost 

 exclusively in angiosperms, which are heterosporous and which have 

 comparatively low chromosome numbers. We are assuming that 

 mutations occur in the mitotic mechanism, probably during the 

 reduction divisions, so that the mutant, which one . recognizes in 

 the sporeling stage, is merely the result of a preceding phenomenon. 



At first thought, someone might suggest that B. dissectum 

 is a hybrid. What species could cross to give such characters j 



as we find in B . dissectum ? The mere question seems a sufficient 

 answer, especially since B. dissectum is found when no other species 

 except B. obliquum occurs in the vicinity. When we remember 

 that the prothallia are of the tuberous, subterranean, saprophyte 

 type, and not closely associated, the possibility of crossing seems | 



very remote. ! 



We believe the evidence is sufficient to raise a strong pre- 

 sumption that B. dissectum is a sterile mutant from B. obliquum. 



University of Chicago ! 



