THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 247 



As in the previous genera we can trace the gradual evolution of a conidio- 

 spore from the sporangium of Mucor. The conidiospore condition seems to 

 have been reached by the gradual reduction in the number of spores produced 

 in a sporangium until there is only one, when the body may be truly regarded 

 as a conidiospore, since it germinates by a germ tube. 



Heterothallism 



The phenomenon of heterothallism is so important in the Fungi that we 

 must say something more about it. Though the process was first discovered 

 in the ^Nlucorales by Blakeslee in 1904, it has now been shown to occur in 

 many families of Fungi. In many species there is nothing in the structure 

 of the mycelia to distinguish them from one another, but where they meet a 

 line of zygospores is formed (Fig. 240). In such cases it is customary to 

 speak of one strain as plus ( -^ ) and the other as minus ( - ). 



Fig. 240. — Mucor hietnalis. Culture of 

 ( + ) and ( - ) strains showing forma- 

 tion of zygospores at line of meeting. 



In a few species of Mucor it has been found that zygospores are produced 

 by the interaction of hyphae in a mycelium produced from one spore, and 

 such species are said to be homothallic. In some instances imperfect 

 hybridization mav occur between opposite strains of different heterothallic 

 species and between both ( 4- ) and ( - ) strains of heterothallic species on 

 the one hand and homothallic species on the other. In such cases only the 

 early stages are gone through and the mature zygospore is never produced. 



In certain other genera the gametangia of homothallic species are either 

 not quite alike, or they may even differ markedly from one another. Such 



