ORDER ERYSIPHALES (PERISPORIALES OF MOST AUTHORS) 



313 



Fig. 99. Erysiphales, Family Erysiphaceae. Sphaerotheca castagnei Lev. Stages 

 in sexual reproduction. (A) Oogone and antheridial branch. (B) Antherid set off by a 

 septum. (C) Male nucleus has passed into oogone through an opening. (D) Row of 

 ascogonial cells, the penultimate one binucleate. (E) The nuclei of the penultimate 

 cell have united to form the primordium of the ascus. (F) Young perithecium showing 

 three layers of cells surrounding the young ascus whose nucleus has not yet divided. 

 (A-E, after Harper: Jahrb. wiss. Botan., 29(4):655-685. F, after Hem: Bull. Torrey 

 Botan. Club, 54(5):383-417.) 



Already, therefore, it is apparent in this family as in other cases in the 

 Ascomyceteae that a substitution has occurred of uniting vegetative 

 hyphae for specially formed antherid and oogone. This tendency is one 

 that appears to become more pronounced in the higher groups of fungi 

 where (as in most Basidiomyceteae) no organs can be distinguished as 

 definite oogonial structures or in many cases as antherids. (Fig. 99.) 



Harper's accounts indicate two chromosome reductions during the 

 nuclear divisions in the ascus, as would be necessary if there were nuclear 

 fusions both in the oogone and in the ascus. Dangeard, on the contrary, 

 who denies the occurrence of a nuclear fusion in the oogone, admits but 

 one reduction division in the ascus. In the other genera of the family the 

 binucleate penultimate cell of the row arising from the oogone undergoes 

 further division and produces a number of short ascogenous hyphae made 

 up of dicaryon cells. The terminal cell of each ascogenous hypha enlarges 

 and the nuclei fuse and undergo division, thus giving rise to an ascus with 

 ascospores. The latter develop as do those of Sphaerotheca. In a few genera 

 the ascospores are two- or four-celled (see Key to the genera of this family 

 on p. 353). The number of asci may vary from 5 to 8 in some species up 

 to 20 to 30 in others. As the asci enlarge the layer of nurse cells is gradually 

 destroyed, leaving only the usually two-layered cortex, (Fig. 100 A.) 



