48 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



fungi whose general characters are a unicellar mycelium, at least in the 

 vegetative stage, and quite generally a well-developed form of sexual 

 reproduction (Figs. 3 1 and 32). In the mucors, the mycehum is usually 

 richly developed within and often also on the surface of the substratum; 

 asexual reproduction is accomphshed by spores borne as conidia or 

 borne within sporangia; and sexual reproduction is accomplished by 

 the conjugation of special branches from the mycehum forming zygo- 

 spores (Figs. 31 and 32). The typical mucors produce sporangia as 

 capsule-like dilations at the ends of erect fertile hyphse, each con- 

 taining many spores. Septa are commonly developed iri the mycehum 

 when sporangia begin to appear. These fertile hyphae may be micro- 

 scopic or attain a length of several centimeters. 



Important Species. — Perhaps the commonest form is Rhizopus 

 nigricans (syn. Mucor stolonifer), the black mold of bread, a cosmo- 

 poHtan species associated with the decay of many kinds of food stored 

 in wet condition or in humid situations. Typical clusters of spor- 

 angiophores are borne on stolons or runners, which are hyphae extending 

 radially from the center of the colony and fastened to the substratum 

 or to the support at intervals by root-hke outgrowths. Abundant 

 growth of this species is found only under very moist conditions or 

 in substrata with high water content. Rhizopus is a very common 

 contamination in laboratory cultures. 



There are many common species of the genus Mucor, very few of 

 which are identifiable without critical study. The specific names as 

 commonly cited, often designate groups of species or varieties rather 

 than sharply marked forms. Certain of these may be briefly considered. 



Mucor mucedo L. is a common form upon dung, characterized by 

 heads (sporangia) upon long sporangiophores,* at first yellow then 

 becoming dark brown or black and studded upon the surface with 

 needles of lime. 



Mucor racemosus, Fresenius, is characterized by the production of 

 chlamydospores or cysts in the mycelium within the substratum, as 

 elliptical thick-walled cells. The sporangiophores typically branch to 

 make racemes of sporangia. The racemose mucors are active agents in 



• The term stOK^ngiophore is composed of the word sporangium combined with the suiSx 

 phore, meaning bearer. In sympodial branching the first fruit is on the tip of the original hypha", 

 the first bra;ich arises below this fruit and is terminated by the second fruit. , Each successive ' 

 branch and fruit originates in similar manner. 



