BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 253 



Tieghem cell culture of potato agar acidulated with orange-juice, — a 

 nutrient which has the advantage of preventing the growth of hacteria 

 and at the same time favoring the formation of zygospores in this species. 

 Two similar cell cultures were made containing only the (+) and (— ) 

 strains respectively, and in these sporangia alone were formed. In the 

 culture, however, in which the (+) and (— ) strains were contrasted, from 

 fifty to one hundred zygospores developed, and in several cases the two 

 suspensors could be traced with absolute certainty to the two different 

 (+) and (— ) mycelia. 



The heterothallic condition of Rhizopus was thus established, and from 

 these two original suspensors a series of (+) and (— ) tube cultures have 

 been run to the twenty-seventh generations, and form a certain means of 

 obtaining zygospores at will, whenever spores from the (+) and the ( — ) 

 strains are sown, either in a mixed condition or side by side on any 

 suitable substratum. 



The two strains having been at last obtained, it seemed a matter of 

 some interest to endeavor to discover an experimental explanation of the 

 early failures to obtain zygospores by contrasting sporangial transfers in 

 the paste cultures above described, some twenty-eight of which still 

 remained in the laboratory in five crystallizing dishes. From each dish 

 a mass sporangial transfer was opposed in separate stenders against the 

 (+) and (— ) strains respectively obtained in A. The five contrasts of 

 groups of uniform strains against the A( — ) strain gave zygospores, while 

 those against the A(+) strain produced only sporangia, thus proving 

 that the twenty-eight transfers were all ( + ) in character. It is further 

 probable that the same was true in regard to the remaining eighteen of 

 the forty-six original transfers, and that they were (+) in character. 

 The absence of zygospores from the mutual contrasts of these eighteen 

 transfers shows that all in the same culture dish must have been either 

 ( + ) or (— ), and in view of the uniform (+) condition of the twenty- 

 eight transfers tested, it seems unlikely that a (— ) condition was present 

 in every case. 



By the experiments performed with culture A just discussed, it had 

 been demonstrated that zygospores were produced only through the inter- 

 action of the hyphae of the diverse strains A(-|-) and A(— ), Only the 

 two (-h) and ( — ) strains from this one culture were then known, and 

 though such a condition was hardly to be expected, it was perfectly 

 possible that other strains might be found which, on account of having 

 grown under dissimilar conditions, would be sufficiently different in 

 physiological character to form zygospores with both the (-f-) and (— ) 



