120 PLANT MORPHOLOGY 



upon germination, gives rise to a short hypha bearing a terminal sporan- 

 gium. This contains many aerial spores. Meiosis occurs during the 

 first two divisions of the fusion nucleus in the zygote. 



Although all the mycelia of Rhizopus appear to be ahke, gametic repro- 

 duction does not take place unless two sexually differentiated myceha, 

 designated as plus and minus strains, come together. This may happen 

 very infreciuently for, when a mycelium produces spores, all the resulting 

 mycelia belong to the same strain and conjugation does not take place 

 between them. Molds with sexually differentiated strains are said to be 

 heterothaUic, while those without such differentiation are homothaUic. In 

 homothallic species conjugation may take place between any two hyphae, 

 even those of the same mycelium. In some of the heterothaUic Muco- 

 rales, when a sporangium is formed at the end of a hypha arising from the 

 zygote, a segregation of strains occurs, so that some of the spores in the 

 sporangium produce plus myceha and others minus myceha. In other 

 heterothaUic species this sporangium contains spores of one kind or the 

 other, but not both kinds. In Rhizopus nigricans it is not known where 

 the segregation of strains takes place. 



Pilobolus, which lives on barnyard refuse, is an interesting mold with a 

 peculiar method of spore dispersal. As the sporangium ripens, the por- 

 tion of the sporangiophore just below it enlarges and becomes very 

 turgid. Finally it bursts suddenly, shooting out the entire sporangium 

 with considerable force, sometimes to a distance of 2 m., and always 

 toward the brightest source of light. 



Summary. The Mucorales are largely saprophytic fungi with a well- 

 developed mycelium. They produce no zoospores, asexual reproduction 

 occurring by aerial spores borne in sporangia. Sexual reproduction is 

 isogamous, conjugation occurring between the entire contents of two 

 multinucleate gametangia. 



7. Entomophthorales 



The Entomophthorales constitute a small group of fungi, most of 

 which are parasitic on insects. The order includes 6 genera and about 50 

 species, the best-known genera being Empusa and Entomophthora. A 

 common species, Empusa muscae, attacks the housefly. The mycelium, 

 which is feebly developed, penetrates the body of the host and eventually 

 kills it. Then it sends out numerous sporangiophores, from each of 

 which a single multinucleate sporangium is cut off (Fig. 94). This is 

 forcibly discharged into the air and, upon coming in contact with an unin- 

 fected fly, produces a new mycelium. Although it becomes detached and 

 functions directly as a spore, the sporangium of Empusa corresponds to 

 the sporangium of the Mucorales. In Entomophthora the sporangiophores 



