BREAD MOLD (RHIZOPUS NIGRICANS) 



361 



over 



upright growth bear the sporangia, while others running 

 the surface of the substratum produce at certain places a new 

 set of both penetrating and upright hyphae. These runner-Kke 

 hyphae are called stolons, and serve to spread the mycelium over 

 the substratum. The hyphae which penetrate the substratum 

 are able to change the elements 

 of the substratum into soluble 

 forms and absorb them. 



The sporangia occur singly on 

 the hyphae and contain numer- 

 ous aerial spores, which when 

 mature are liberated by the 

 breaking of the sporangial wall. 

 The spores are nearly always 

 present, floating about in the air 

 and resting on objects where 

 they happen to fall. It is prob- 

 able that they can live for many 

 years in the dormant state and 

 then germinate when they come 

 in contact with suitable food 

 material. 



The Bread Mold has no sex 

 organs, but there is a sexual 

 process which reminds one of 

 the sexual process in Spirogyra. 

 Sometimes, as shown in Figure 

 314, tips of hyphae approach 

 each other and finally meet. From each hyphae an end cell is cut 

 off, and these end cells fuse to form heavy walled zygospores. 

 Upon germination the zygospore produces an erect hypha bearing 

 a sporangium of the ordinary type, and the aerial spores developed 

 therein are capable of starting a new series of plants. 



Conjugation is only occasionally obtained in Rhizopus nigricans 

 unless the cultures are made in a certain way. It has been found 

 that in Rhizopus nigricans there are two kinds of plants, which, 

 although looking just alike, behave differently. They are called 

 strains, one being known as the plus (+) and the other as the 

 minus (— ) strain. When either of these occur alone in a culture 

 then no conjugation takes place, but if both are present then 



Fig. 312. — Methods of repro- 

 duction in the Phytophthora cacto- 

 rum, which attacks Ginseng. A, sex 

 organs consisting of oogonium (0) 

 and antheridium (a). B, conidi- 

 ospore forming zoospores above, 

 and a group of zoospores below. 

 C, conidiospore producing hyphae 

 directly. Much enlarged. From 

 Bulletin 363, Cornell University 

 Agr. Exp. Sta. 



