SUBCLASSES OF ASCOMYCETES 



25 



head of grain (Fig. 18). It is these ergot grains which contain the 

 poison. 



These ergot grains fall to the ground and remain dormant over 

 winter. In the spring, they revive and sprout a number of little stalks 

 which develop rounded masses of tissue at their 

 tips (Fig. 19). These are called stromata. In 

 each stroma a number of pear-shaped areas, the 

 perithecia, are formed (Fig. 20) . One of these is 

 shown in section in Fig. 21. Each perithecium 

 contains a number of asci, and each ascus con- 

 tains eight needle-like ascospores. When mature, 

 these are forcibly discharged into the air and 

 carried by the wind. If they lodge on grain they 

 germinate, forming a mycelium which invades the 

 growing grain and starts the cycle over again. 



Subclasses of Ascomycetes. The Ascomycetes 

 are subdivided into two main series according to 

 the way in which they are borne in the ascocarp. 

 The first series, the subclass Protoascomycetes, 

 contains the more primitive forms, in which the 

 ascogenous hyphae are lacking. In Endomycopsis 

 and the yeasts like Schizosaccharomyces men- 

 tioned above, the fusion cells give rise to asci at 

 once and hence ascogenous hyphae are lacking. 

 In many of the yeasts, after fusion of two cells, 

 and presumably also of nuclei, the resulting dip- 

 loidal cell proliferates by budding. Here also there are no asco- 

 genous hyphae. In the second series, the Euascomycetes, the fusion 



Fig. 18. A head of 



grain affected with 



ergot showing a 



sclerotium. 



Fig. 19. "Germination" of a sclerotium of Claviceps purpurea showing the 



formation of stromata. 



cell gives rise to ascogenous hyphae which are now usually thought to 

 have paired nuclei. By far the greatest number of species falls into 

 this group which is often given the rank of subclass. It is some- 



