THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 279 



rise to much smaller, almost spherical cells, which also multiply by budding 

 and form a " dwarf " strain. These cells cannot produce asci until two of 

 them have conjugated to form a zygote, which by budding produces the 

 normal, large-celled strain again. Frequently many such conjugations occur 

 simultaneously among the dwarf cells so that it would appear as if the dwarf 

 strain had almost wholly turned into the large-celled strain (Fig. 271). 



A \JzYGOTES 



Fig. 271. — Saccharoniyces validus. A, Monoploid 

 spore with three buds. B and C, Twin 

 zygotes formed by fusion. D to G, Develop- 

 ment of diploid colonies from twin zygotes. 

 {After Winge.) 



These facts find their explanation readily enough when viewed as an 

 alternation of generations. The normal large-celled strain is diploid, i.e., 

 is sporophytic, and gives rise to sporangia (asci) in which presumably meiosis 

 occurs at spore formation. The ascospores would therefore, as in other 

 cases, be monoploid. If they germinate singly they give rise to a small- 

 celled gametophytic strain. This forms no definite sexual organs, but 

 conjugation takes place, as in some other Ascomycetes, between vegetative 

 cells, producing diploid zygotes, which reproduce the diploid strain once 

 more. It happens, however, that in most cases the gametophyte phase is 

 short-circuited, and the first cells produced from the germinating ascospores, 

 instead of developing vegetatively, conjugate immediately to form a diploid 

 zygote, so that the ascospores in such a case are the only monoploid cells in 

 the life-cycle. Whether there is any advantage in such a procedure, or w^hether 

 it merely is the result of the propinquity of the ascospores we cannot say, 

 but we know that such short-circuiting of an alternating cycle does in fact 

 occur in very various types of organisms, e.g., in Puccinia among the Fungi ; 



