84 MOLDS BELONGING TO THE PHYCOMYCETEg 



Zygospores may be formed by the fusion of neighboring filaments 

 from the same thallus in some cases (as in Zygorrhynchus) or only 

 by fusion of filaments from two neighboring thalli in others (as most 

 of the Mucors and Rhizopus). The first type is said to be homo- 

 thallic, the second heterothallic. As pointed out in Chapter I, in the 

 latter case there is a physiologically distinguishable sex, although 

 the elements are morphologically identical. Since the two sexes can- 

 not be distinguished, they are designated as plus or minus strains 

 rather than as male and female. Sometimes bodies resembling zygo- 

 spores may be formed without fusion of the hypha with another. 

 Such spores are called azygospores. They may be formed by an 

 isolated thallus, or only by hyphae which approach filaments from 

 another thallus of the opposite sign. It is noteworthy that plus and 

 minus strains of different species may exhibit this evidence of sex 

 when grown in contact, even though actual conjugation does not 



occur. 



The zygospores are usually large cells, generally black, and with 

 a rough, w^arty exterior. The filaments which form them become 

 expanded near the spore, forming broad supporting bands called sus- 

 pensors. These are sometimes of characteristic form. Unfortunately, 

 zygospores are seldom formed in cultures on artificial media in the 

 laboratory. They are found most frequently on strains freshly iso- 

 lated from their natural habitat. Naturally, with heterothallic varie- 

 ties no zygospores will be seen unless both plus and minus strains are 

 included in the culture. According to Blakeslee, prune extract and 

 moistened slices of bread are media most favorable to the production 

 of zygospores. 



The following key covers those genera of the Mucoraceae which 

 contain species likely to be encountered by the bacteriologist. 



A. The fungus spreads over its substrate by stolons or runners. 



1. Sporangiophores arise at the nodes of the stolons. RHIZOPUS 



2. Sporangiophores arise at the internodes. ABSIDIA 



B. No stolons or runners are formed. 



1. Sporangiophores are simple or branched; sporangia borne apically on the 

 sporangiophore and its branches. 



a. Zygospores formed from equal gametes, usually heterothallic. 

 aa. Never parasitic on other Mucorales. MUCOR 



bb. Facultative parasite on certain genera of Mucorales. 



PARASITELLA 



h. Zygospores formed from unequal gametes, homothallic. 



ZYGORRHYNCHUS 



2. Sporangia borne only on the lateral circinate branches of the sporangiophore. 



a. Sporangia globular, columella not constricted. CIRCINELLA 



b. Sporangia pear-shaped, columella constricted. PIRELLA 



