BREAD MOLD (RHIZOPUS NIGRICANS) 361 
upright growth bear the sporangia, while others running over 
the surface of the substratum produce at certain places a new 
set of both penetrating and upright hyphae. These runner-like 
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 aérial 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 
Fic. 312.— Methods of repro- 
duction in the Phytophthora cacto- 
rum, which attacks Ginseng. A, sex 
organs consisting of odgonium (0) 
and antheridium (a). 3B, conidi- 
ospore forming zodspores above, 
and « group of zodspores below. 
C, conidiospore producing hyphae 
directly. Much enlarged. From 
Bulletin 363, Cornell University 
Agr. Exp. Sta. 
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 aérial 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 
