BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 291 



forms in which the thallic condition is known or strongly suspected 

 will show that the homothallic group undoubtedly comprises a very 

 small minority of the species in which zygospores have ever been found. 

 These forms may be recognized by the fact that they can be induced to 

 develop zygospores constantly on the same substrata when grown from 

 a single spore, and, moreover, by the fact that through the zygophoric 

 hyphae the opposed progamctes may be traced to the same branches of 

 the mycelium. 



The members of the homothallic group may be conveniently divided 

 into two subsections according to the presence or absence of a constant 

 morphological difference in the gametes and zygophoric branches. The 

 first division comprises Sporodinia, Spinellus, and Mucors i and ii, 

 and the second comprises Zj'gorhynchus and Dicranophora, both of 

 which have been classed together as heterogamic on account of the 

 morphological differentiation in their gametes. The homothallic forms 

 will be considered in the order just given. 



Sporodinia grandis. 



Inasmuch as Sporodinia grandis is by far the most abundant and 

 widely distributed homothallic form among the Mucorineae, and there- 

 fore the only one from which earlier workers could be sure of obtaining 

 zygospores, it has been the subject of considerable investigation, as may 

 be seen by reference to the literature cited under this species (p. 231). 

 By Klebs ('98) the fungus could be made to produce zygospores on 

 plain bread if tlie relative humidity in the surrounding air was suf- 

 ficiently high. For Falck ('01), zygospores would form only when the 

 concentration of the nutrient was sufficiently increased by the addition 

 of certain solutions to the bread, and the relative humidity of the air 

 seemed to be of slight importance. 



Sporodinia is very common in this country on a great variety of 

 Basidiomycetes of various groups. Dr. Farlow has been accustomed 

 to use in his classes the zygospores of this species, and has sometimes 

 obtained them from sowings on plain bread ; and from all of several 

 different collections the writer has been able to obtain an almost ex- 

 clusive production of zygospores on the same substratum. A pure 

 culture of this species was obtained in the summer of 1902 fi'om material 

 collected at Cold Spring Harbor, L. I., and has been kept running since 

 in the laboratory. It has been noticed in general that in tube cultures 

 sporangia have been exclusively present except in freshly made tubes 



