MUCOR AND RHIZOPUS 



85 



Mucor and Rhizopus. Of the various genera of the Mucoraceae, 

 Mucor and Rhizopus contain most of the species of Phycomycetes 

 encountered in bacteriological work. Mucor and Rhizopus may be 

 readily differentiated from each other. Because of runners, or sto- 

 lons, Rhizopus tends to cover the surface of the agar plates rapidly, 

 to climb the sides of the Petri dishes and fill the latter with mycelium. 

 The rhizoids, or holdfasts, attach themselves to the under sides of 

 the lids. Although Mucors may fill up the Petri dish with mycelium, 

 they do not thus attach themselves to the lid. 



Fig. 34. Mucor sp.: (a) young sporulating head; (b) mature sporangium; 

 (c) spores being liberated from sporangium; id) columella after scattering of 

 spores. From S. A. Waksman and R. L. Starkey, The Soil and the Microbe, 



1931. 



The rhizoids or holdfasts of Rhizopus may be seen readily by 

 focusing through the lid of the Petri dish with the low-power lens. 

 The sporangiophores of Rhizopus arise from the nodes of the runners, 

 i.e., at the point where the holdfast or rhizoids are formed. In Mucor 

 the sporangiophores are formed from all parts of the thallus. In 

 Mucor the columella is always either round, cylindrical, or pear- 

 shaped, never hemispherical, and is continuous with the sporangio- 

 phore. In Rhizopus it is hemispherical and rests in a cup-shaped 

 expansion of the sporangiophore called the apophysis (Figs. 34 and 

 39). In Mucor the spores, though varying in form, are always smooth 

 and regular; in Rhizopus they frequently appear to be angular be- 

 cause they collapse readily when mounted in water. Some species 

 of Mucor show a positive phototropism. Sporangiospores of these 

 species bend toward the source of light. This does not occur with 

 Rhizopus. 



