BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 215 



ent on external conditions in connection with which they have been 

 developed. 



We are indebted to de Bary ('66) for the first discussion of this ques- 

 tion, in his investigation of the common heterotiiallic form Rhizopus 

 nigricans. Zygos()ores were found to form abundantly in tightly closed 

 cultures, but less abundantly and only in the lower parts of cultures open 

 to the air. "The fungus," according to this author, " takes in oxygen 

 and gives off carbon dioxide ; therefore in a closed tube, and below the 

 network of hyphae in a gross culture the oxygen content of the air must 

 be reduced. This circumstance, in that it retards the process of oxyda- 

 tion in the fungus, is apparently influential in the formation of zygospores, 

 since the composition of the substratum,' the water-supply, and the 

 temperature, were the same in the open as in the closed cultures." In 

 direct opposition to this conclusion, it may here be mentioned that 

 Bainier ('SSb. p. 342) gives the exclusive presence of oxygen as a cause 

 previously assigned for the production of zygospores, but the writer has 

 been unable to discover who is responsible for this theory. 



By following de Bary's suggestion, van Tieghem ('75) obtained the 

 zygospores of Rhizopus, and appeared to confirm his theory by his results 

 with other forms. In his Troisieme Memoire {p. 324) he concludes : 

 '' That which determines the formation of zygospores upon a mycelium 

 still provided with a sufficient quantity of protoplasm is the impoverish- 

 ment of the nutritive medium in one or more of its elements, an impover- 

 ishment which places in danger even the existence of the plant," The 

 total nutrient he divides into (1) air, or its oxygen at the pressure of 

 one-fifth of an atmosphere ; (2) water ; (3) the soluble substances, or 

 food proper. Reduction in oxygen is the cause assigned for the occur- 

 rence of zygospores in Sporodinia and Rhizopus ; desiccation for the 

 occurrence of those found by Cornu in Ahsidia sej)tata; while for the 

 apj)earance of the zygospores of Piptocephalis and Sporodinia obtained 

 by Brefeld on bread soaked with beer wort, the cause is supposed to be 

 the unfavorable condition of the food supply of these naturally parasitic 

 plants. Although van Tieghem adds that he gives the above generaliza- 

 tion with great reserve, since it is necessary to determine in each case 

 the special nature of the impoverishment which is effective, yet his last 

 word on the subject ('91, p. 1076) is a reiteration of his impoverishment 

 theory with the mention of cold as an additional factor. 



From his experience in obtaining zygospores of Ahsidia septata, Spo- 

 rodinia and Spinellus, Cornu ('76) concludes, '• It is when the sub- 

 stratum becomes more and more dry that the zygospores appear.'' 



