CLASSIFICATION OF THE MOLDS 59 



hypha, called a stolon, grows on from this cluster of rhizoids 

 until it again attaches itself at some other point. From 

 each cluster of rhizoids there arises a group of sporangio- 

 phores which grow into the air, developing at the tip the 

 large dark brown or black sporangia. These sporangia are 

 readily recognized under the microscope. When mounted 

 in a drop of water the ripe sporangium usually collapses, 

 the outer wall disappearing and the spores go free. Th6 

 columella in this form is partly adherent at its base to the 

 outer wall of the sporangium. Usually when the sporan- 

 gium collapses it also collapses, the lower side invaginates, 



Fig. 39. — Rhizopus. 1. Growth habit of the mold, showing stolons, 

 rhizoids, sporangiophore.s and sporangia. 2, 3. Columella. 



and together with the stalk or sporangiophore gives rise to 

 a structure having the appearance of a small toadstool or 

 umbrella. 



Some species of Rhizopus are used commercially in the 

 transformation of starchy materials into sugar, precedent 

 to the fermentation of the sugar into alcohol in the" com- 

 mercial manufacture of this material. 



The most common of the species of Rhizopus is Rhizopus 

 nigricans, the common black bread mold. The species 

 used in hydrolysis of starch are Rhizopus oryzae and R. 

 japonicus. 



Mucor. — The genus Mucor is not as abundant as Rhizopus 

 but is found under the same general conditions. Usually 

 the mold is not quite so coarse. It does not develop stolons. 



