Jott 27, 1906.] 



SCIENCE. 



121 



gested by experiments with zygosporic ma- 

 terial which was part of a spontaneous growth 

 on bread exposed in the laboratory of Leland 

 Stanford University. Professor Campbell 

 writes that he has almost always gotten zygo- 

 spores in this way since 1892, yet an investi- 

 gation showed that the culture contained but 

 the two sexually opposite strains which were 

 alone incapable of zygospore formation. 

 Bhizopus shares with Penicillium the doubt- 

 ful distinction of being the most common 

 fungus weed in laboratory cultures, and it is 

 almost impossible to keep a favorable sub- 

 stratum such as bread in a moist atmosphere 

 for a week without obtaining an abundant 

 growth of this characteristic ' bread mould.' 

 When (+) and ( — ) spores are in the labo- 

 ratory, one is very likely to get in spontaneous 

 cultures a growth producing zygospores from 

 the germination of sexually opposite spores 

 which may chance to be together on the bread 

 and, if a mixed sporangial transfer is made 

 from a culture already producing zygospores, 

 one is almost certain to obtain them. That 

 spontaneous zygosporic cultures thus often 

 occur has frequently been experimentally dem- 

 onstrated by the writer in the Harvard labo- 

 ratory. 



The zygospore of the heterothallic species 

 Phy corny ces, in germinating, typically pro- 

 duces a sporangium containing both male and 

 female spores.' If the germ tube be forced 

 to grow into a mycelium before the differen- 

 tiation of sex in the germ sporangium, a homo- 

 thallic mycelium results. The condition in 

 these homothallic mycelia is unstable and the 

 delayed segregation of sex takes place at the 

 formation of spores in the sporangia which 

 they eventually produce. Until this homo- 

 thallic character becomes fixed, one can not 

 speak of a homothallic race of Phycomyces; 

 but the facts already obtained are such that 

 one would not be justified in saying that, in 

 other heterothallic forms, homothallic races 

 may not exist. The experiments with 

 Rhizopus, however, render it extremely im- 

 probable that this species is other than strictly 

 heterothallic. A few zygospore germinations 

 of Rhizopus have been secured, but not as yet 



in a condition to enable one to analyze the 

 sexual character of the germination. 



Although up to the time of the discovery of 

 heterothallism in the mucors the zygospores 

 of Rhizopiis are found reported in the litera- 

 ture but five times, they can hardly be rare. 

 They are generally covered by the sporangial 

 growth and, to one unfamiliar with their ap- 

 pearance, are distinguishable only with the 

 aid of a microscope. The writer has found 

 them in spontaneous bread cultures in Eisen- 

 ach and Halle, Germany, and Professor Lloyd 

 and Miss Watterson have obtained them in the 

 same manner in Teachers College, New York. 



Rhizopus is not so sensitive to the influence 

 of external conditions in regard to the forma- 

 tion of its zygospores as many other of the 

 heterothallic mucors. The ordinary substrata 

 used in the laboratory are few upon which 

 zygospore formation can not be induced if 

 proper regard be had to conditions of mois- 

 ture. Carbohydrates are especially favorable 

 and bread makes an admirable culture me- 

 dium. The writer has in progress a series of 

 experiments to determine the conditions under 

 which zygospore formation is possible in the 

 various species. The fact that in agar cul- 

 tures with 1 per cent peptone zygospores have 

 been obtained by the addition of varying 

 amounts of grape sugar from 0.1 per cent, up 

 to 40 per cent., will be sufficient to show that 

 zygospore production in this species is not 

 limited to any great extent by the composition 

 of our ordinary culture media. The tem- 

 I)erature is not a very important factor in this 

 instance. Up to at least 27° C, an increase 

 in temperature accelerates the growth and 

 increases the production of zygospores. More 

 or less moisture in the surrounding air, on 

 the other hand, is necessary to the formation 

 of zygospores and, as the writer has already 

 shown (Z. c), their production may be sup- 

 pressed before that of sporangia by sufficient 

 desiccation. 



The writer' has already described the 



" ' Zygospore Germinations in the Mucorinese,' 

 Annates Mycologici, 4: 1-28, pi. 1. 1906. 



« ' Two Conidia-bearing Fungi, Gunninghamella 

 and Thamnocephalis,' Bot. Gazette, 40: 161-170, 

 pi. 6. 1905. 



