BRIEFER ARTICLES 
HETEROTHALLISM IN BREAD MOLD, RHIZOPUS 
NIGRICANS 
In recent publications the writer has shown that, according to their 
method of sexual reproduction, the Mucorineae may be divided into two 
groups—homothallic and heterothallic: In a homothallic species, zygo- 
spores are formed from branches of the same mycelium and can be obtained 
from the sowing of a single spore. In heterothallic species, which probably 
constitute the large majority of the Mucorineae, zygospores are formed 
from branches which necessarily come from two different mycelia belonging 
to sexually opposite races or strains. By sowing a single spore, therefore, 
one cannot obtain zygospores of a member of the heterothallic group unless 
the culture becomes contaminated with spores of the opposite strain. 
Within the last few months, two communications have appeared which 
claim that the production of the zygospores of Rhizopus is not at all depend- 
ent upon the presence of two sexual races; in other words, that on the 
proper nutrient the zygospores can be obtained from the sowing of a single 
spore. This laboratory weed, the common bread mold, is the best known 
species among the mucors and has been used as a type of the heterothallic 
group. 
A propos of the article by HAMAKER (Science N. S. 232710. 1906), the 
writer has recently summarized the results of a series of investigations on 
the sexual condition in this species covering a period of over six years 
(Science N. S. 24:118-122. 1906). Since this was written, the prelim- 
inary communication of HAMAKER has been reviewed in the BoTANICAL 
GAZETTE (42:77. 1906) where the detailed formula is repeated for making 
the corn muffin bread claimed to be a sure medium for the production of 
the zygospores of this species. It is not the purpose of the writer to repeat 
what has been said already in regard to the inadequacy of this nutrient 
theory of zygospore formation. It seems desirable, however, to consider 
somewhat in detail the paper of NAmysLowskKI (Rhizopus nigricans et les 
conditions de la formation de ses zygospores: Bull. Acad. Sci. Cracovie 
676-692. pl. 21. 1906), since this investigator explicitly casts doubt upon 
the occurrence of heterothallism in any of the Mucorineae and would give 
the appearance of supporting his conclusions by cultural experiments with 
Ss. 
415] [Botanical Gazette, vol. 43 
