10 
E. M. Cutting 
HETEROTHALLISM AND SIMILAR 
PHENOMENA 
By E. M. CUTTING 
I n 1906 The New Phytologist published a critical resume of work ( 5 ) 
by A. F. Blakeslee on the presence of two kinds of mycelia in 
certain forms of Mucorine fungi. Since that date there have been 
accumulating a certain number of new facts bearing on the problem 
presented by the communication mentioned above. It is the purpose 
of this note to review shortly these disconnected observations and 
to attempt to indicate certain points of contact between them. 
Of these, one of the most interesting is contained in a preliminary 
note by Burgeff (12), followed by a fuller illustrated account, in Flora , 
1914(13). This author worked mostly on Phycomyces nitens, a species 
that had previously been investigated by Blakeslee, who found that 
it possessed + and — forms, or in other words, was heterothallic. 
By means of a very delicate and difficult technique “grafting” was 
accomplished between a + and a — form of mycelium. The mycelial 
outgrowths resulting from this operation formed sporangia, which 
contained three kinds of spores, those which gave rise to + mycelia, 
those that formed —, and those that formed neutral hyphae. These 
results were regarded as in keeping with the view, which is the one 
held by Burgeff himself, that the “plusness” or “minusness” resides 
in the nuclei themselves. The spores of Phycomyces , it will be remem¬ 
bered, are multinucleate and it is held that the neutral strain is 
formed when — and + nuclei are present in about equal numbers. 
Blakeslee(6) has stated that in the heterothallic Mucors, so far as 
his investigations carried him, the zygospores germinated to form a 
sporangium which contains either + or — spores, so that a segre¬ 
gation occurs before the formation of sporangial spores. There was 
no evidence to show the method of this segregation. In Phycomyces 
nitens, however, the germ-mycelium was neutral and bore sporangia 
containing spores which gave rise to plus, minus and neutral mycelia; 
and the neutral mycelium had the power of forming abortive or 
imperfect zygospores with either — or + mycelia. 
The experimental results of Burgeff fill in a gap in the observa¬ 
tions given above. We have here, seemingly, a vegetative segre¬ 
gation caused by a differential nuclear distribution. 
