ORDER MUCORALES 163 



that the zygospore is formed solely from the female gametangium. (Fig. 

 50D-F.) 



In a few species of Mucorales the gametangia and supporting sus- 

 pensors are very unequal so that the product of the union of the two 

 gametangia might be called an oospore. In general practice, however, 

 they are called zygospores since their mode of origin is similar to that in 

 isogamous species. In Dicranophora the one gametangium is many times 

 larger than the other. In Zygorhynchus also, the difference between the 

 two gametangia is marked. Both these genera are homothallic. (Fig. 

 53A-J.) 



In Piloholus, Mortierella, Piptocephalis, Phycomyces, and other forms 

 two hyphae coming into contact may become attached by finger-like 

 processes or wind around each other once or twice and then grow parallel 

 and in contact. Very soon they curve away and bend so that only the 

 tips are in contact, much as in a pair of tongs. In Piptocephalis and Endo- 

 gone the zygospore does not form in the space occupied by the two 

 gametangia but buds out from these to form an external zygospore. (Figs. 

 51B-C, 54C-D, 55F-I, 59A-F.) 



The zygospore remains naked in the majority of species. In Phyco- 

 myces nitens branches grow out from the suspensors, surrounding the 

 zygospore loosely with stiff, black, more or less dichotomously branched 

 processes. In Ahsidia glauca these protective structures are curved and 

 hooked. Often they arise more abundantly from the female suspensor in 

 both these species. In Mortierella a dense hyphal mass several layers in 

 thickness is formed around and closely appressed to the zygospore. (Figs. 

 49C-E, 57A-B, H-J.) 



The order Mucorales is divided into several families. Mycologists are 

 not in accord as to their number or limits. Probably these differences of 

 opinion will remain until greater agreement is attained as to the probable 

 course of phylogenetic development within the order and as to the rela- 

 tive importance to be ascribed to the evolutionary processes involving 

 the sporangia and those concerning sexual reproduction. If the group is 

 primitively isogametangic then the heterogametangic genera represent a 

 later modification while if heterogamy is considered primitive then the 

 isogametic condition found in Mucor is a modification of this ancestral 

 condition. The customarily used classification is based largely upon the 

 asexual reproductive structures and development and is probably artificial 

 in many particulars. It can be replaced by a more natural system only 

 when the above-mentioned questions are settled satisfactorily. In this 

 book eight families are recognized, following Fitzpatrick (1930) in the 

 main. These agree in most points with the six families into which Zycha 

 (1935) divides this order. Naumov (1935, 1939) recognizes eight families. 

 He divides many of the older genera so that he includes 38 genera and 



