THE FUNGI WHICH CAUSE PLANT DISEASE 117 



The oogonium after fertilization gives rise to a more or less 

 complicated system of ascogenous hyphae, very simple in the 

 Erysiphaceae, very complex in some Discomycetes, which produces 

 the asci. The sterile parts of the ascocarp, the paraphyses and 

 enveloping structures, arise from parts below the oogonium and 

 antheridium. 



The very young ascus usually receives two nuclei from the parent 

 strand of the ascogenous hypha. These nuclei unite giving the 



Pig. 79. — Tip of ascus 

 of Erysiphe showing 

 delimitation of asco- Fig. 80. — Later stage than 

 spore from asco- fig. 79, showing well de- 

 plasm by astral fined spore-wall. After 

 rays. After Harper. Harper. 



primary-ascus-nucleus. This by successive mitoses affords the 

 single spore-nuclei. The spores are cut out from the protoplasm 

 of the ascus in a most peculiar manner by reflexion of and union 

 of astral rays which emanate from a centrosome-like organ at the 

 beak of the prolonged nucleus. Figs. 79, 80. 



The significance of two nuclear fusions in the life cycle of these 

 fungi, one following the union of the antheridium with the oogo- 

 nium, the other later, in the asci, is a puzzling phenomenon, the 

 real significance of which is not clear. 



Key to Subclasses of Ascomycetes 

 Asci with varying number of spores, 



usually numerous 1. Hemiascomycetes, p. 118. 



Asci with definite number of spores 



Asci separate or scattered 2. Protoascomycetes, p. 119. 



Asci approximate, usually forming a 



hymenium 3. Euasc(nnycetes, p. 123. 



