122 



MEDICAL MYCOLOGY 



groups the fructification is initiated by the formation of the sexual organs. 

 In many of the higher forms, external stimuli cause the fructifications to de- 

 velop independent of sexual organs which are later formed within the fruc- 

 tification. 



Plasmogamy is not followed directly by caryogamy but one or several 

 dicaryons are formed. In the lower forms, the dicaryon migrates directly 

 into an ascus which is the product of the plasmogamy. In the higher forms, 

 plasmogamy is increasingly retarded and the fertilized gametangium develops 

 into one or more hyphae. These take up the dicaryon and by conjugate divi- 



Fig-. 11. — Pyronema confluens. 1, Two antheridia arising from a dichotomous hypha, a 

 trichogyne is in contact with each. 2, An ascog-onium showing fusion in pairs of the sexual 

 nuclei. S, An older stage, showing the beginning of ascogenous hyphae. i, Young ascogenous 

 hypha. 5, An older hypha in which wall formation is in progress. 6, Older hvphae in which 

 the binucleate cells are building out to form croziers. (i and 3 X660, 2 Xl.060, Jf-S Xl,230.) 

 (After Gwynne Vaughan & Williamson 1931.) 



sion, branch and form asci. Such dicaryotic hyphae are therefore called 

 ascogenous hyphae; biologically they offer the advantage that one gametan- 

 gium can create a number of asci. 



In most of the higher Ascomycetes, the asci develop from croziers at the 

 end of the ascogenous hyphae. Each of the ascogenous hyphae arising from 

 the ascogonium contains a number of dicaryons and develops by repeated 

 forking, more or less vertically toward the top of the future fructification 



