68 



AloUI'IIOLOGY 



Fig. 162. — Mucor: showing the development 

 of a sporangium at the end of a sporophore; in the 

 last member of the series the spores (s) and 

 columella (c) are evident. 



Sexual reproduction. — Sexual reproduction occurs under special 

 conditions, when suitable individuals are brought together. The two 

 kinds of individuals are called strains, and have been distinguished as 



( + ) and (— ) strains, which 

 apparently correspond to 

 female and male individuals. 

 In general, the two strains 

 are not recognizable by sight, 

 but are known to be sexually 

 different by their behavior. 

 Fertile branches (suspensors) 

 are developed by pairing in- 

 dividuals, come into contact 

 (fig. 163), and a terminal cell 

 (gametangium) is cut off by 

 each (fig. 164). These two 

 abutting gametangia are 

 multinucleate, a perforation 

 is developed at the contact, and the two protoplasts fuse, a very 

 large and heavy-walled zygospore being formed (figs. 165, 166), from 

 tvhich a new mycelium is developed (fig. 167). In many cases the 

 pairing suspensors and gametangia differ decidedly in size, in which 

 case a differentiation into male and female individuals becomes visible 

 (figs. 164-166). It has also been observed that the larger gametangia 

 and suspensors arise from more robust mycelia, and these in turn from 

 larger sporangia than do the smaller gametangia and suspensors. It 

 seems possible to arrange a series of pairing gametangia, ranging from 

 such inequality in size that the pair may be regarded as an oogonium 

 and an antheridium (as in Albugo), to absolute equality. Such a series 

 suggests a line of degenerating sexual organs, in which distinct heter- 

 ogamy passes into isogamy; but of course it might be read in the 

 opposite direction. 



Pilobolus. — This mold is abundant on stable manure, and resembles Mucor, 

 but it is remarkable for the method of dehiscence of its sporangium. The sporo- 

 phore becomes very turgid and swollen just beneath the sporangium and finally 

 bursts, hurling the sporangium with considerable force. This curious habit has 

 given to the plant the name squirting fungus. If a bell jar is placed over the 

 plants, the inner surface becomes dotted with discharged sporangia. 



Entomophthorales. — These are parasites fatal to insects, the common house 

 fly often being destroyed by them. The spore (conidium) in germination sends 



