234 STRUCTURAL BOTANY 



wall, and forms a quantity of oil in its contents. We 

 see that the process is one of perfectly typical conjuga- 

 tion, the two cells concerned taking an exactly equal 

 part in the production of the zygospore. It has recently 

 been shown that in many of the Mucorineoe (the family to 

 which Pilobolus belongs) there is a physiological differ- 

 entiation of sex, definite sexual " strains " existing, which 

 will only conjugate with one another, while hyphse of the 

 same strain are sterile among themselves. 



After a period of rest, the zygospore, if moistened, 

 germinates. The germination is best known in an allied 

 genus, Mucor. If food enough is to be had, it simply grows 

 out at once into a new mycelium ; if, however, supplies 

 are scanty, it proceeds without delay to form an asexual 

 sporangium, thus increasing the chances of survival. 

 These differences are quite analogous to those which we 

 found in the germination of the oospores of Pythium. 



The Zygomycetes, so far as their sexual reproduction is 

 concerned, stand on a lower level than the last group. On 

 the other hand, they are more fully adapted to a terres- 

 trial mode of life, and so far are more perfect, as Fungi, 

 than the Oomycetes. We saw in the case of Peronospora 

 how a transition can be traced from the sporangium, to a 

 single conidium, germinating directly. A somewhat 

 similar gradation is to be followed among the immediate 

 relations of Pilobolus. Some of these produce, in addition 

 to the typical large sporangia, very small sporangioles, 

 containing very few spores, or even only one. In other 

 species sporangioles only are known, and these become 

 detached bodily from the supporting hypha, and behave 

 like single conidia. Thus in the Zygomycetes, as in the 

 Oomycetes, a succession of steps leads from the typical 

 sporangium to the simple conidium, the most char- 

 acteristic form of reproductive cell in the Fungi. 



