802 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In Spoi'odiiiia, although the aerial hyjjliae which bear sporangia and 

 those which bear zygospores are somewhat alike in their method of 

 branching, yet they are distinct, and each form of fructification is pro- 

 duced alone on individual filaments. In Dicranophora the zygospores 

 are not aerial, but are formed between special short branches of the 

 superficial or even slightly immersed mycelium. In Mucors i and ii 

 and Zygorhynclius, however, both zygospores and sporangia may be 

 formed from the same aerial hyphae. 



In so far as is known, external conditions have a marked influence on 

 the kind of fructification produced only when the sporangia and zygo- 

 spores are developed separately upon differentiated hyphae. Even in 

 such forms as Sporodinia, in which external factors are most effective, 

 under ordinary conditions zygospores and sporangia occur side by side, 

 and it is only under extreme conditions that it is possible to obtain an 

 exclusive production of either form. 



The homothallic species thus exhibit a type of sexual reproduction 

 which seems exceptional, and is distinctly different from that which is 

 predominant in the Mucorineae as a whole ; but although in several 

 cases this process is not associated with any differentiation of the con- 

 jugative apparatus, such a differentiation is distinctly indicated in other 

 instances, as has already been pointed out. This differentiation, the 

 sexual nature of which can hardly be disputed, will be further discussed 

 in connection with the phenomena of hybridization which afford further 

 evidence in support of this view. 



All the homothallic species studied have been kept under cultivation 

 for a considerable time, — in the case of Zygorhyiichus Moelleri for 

 about ten years, — and no change in their zygosporic activity has been 

 observed. No neutral strains of these forms, moreover, have been found, 

 and homothallism therefore may be assumed to be a fixed condition in 

 the forms in which it is known to occur. 



It is a noticeable fact that no homothallic species have been found 

 growing siDontaueously on animal excrement, which is the most pro- 

 lific source for forms of the Mucorineae. Sporodinia, Spinellus, and 

 Dicranophora, in which alone a differentiation is apparent between the 

 sporangiophores and zygophoric hyphae, have their natural habitat on 

 the fructifications of certain fleshy fungi. Zygorhynchus has been found 

 growing spontaneously on bread and on cultures from soil origin. The 

 forms represented by Mucors i and ii were obtained from Hymeno- 

 mycetous fungi and from the schleimfliiss of recently cut birch; and 

 from Hansen's ('02) investigation it seems probable that soil is not an 

 uncommon source of homothallic forms. 



