320 



GENETICS OF FUNGI 



the heterozygous zygospores were (Arb arb + — ). When these 

 were germinated, half of them gave four different haploid deriva- 

 tives with the respective constitutions (Arb +), (Arb — ), 

 {or}) _)_) ? an d (arb — ). The other half yielded only two different 



Fig. 50. Plew-age cmser'ma. A. Normal perithecium, external appearance. 

 B. Normal ascus, bearing four ascospores. C. Occasionally asci are found 

 bearing five ascospores, two of which are smaller than normal. Mycelium 

 from small ones bear ascogonia, D, and spermatia. E. The mycelium from 

 normal spores can produce perithecia; the spermatia and ascogonia borne 

 on mvcelia from small spores are self-incompatible and hence self-sterile, 

 but compatible and fertile if reciprocally crossed. F. Spermatia borne by 

 phialide-like lateral branches. G. Trichogync with empty spermatium 



attached. (Adapted from Ames.) 



