42 



ASCOMYCETES 



[CH. 



second fusion in the ascus, and in the following year he recorded the same 

 process in Erysiphe Polygoni. 



Fig. 9. Sphaerotheca Humull (DC.) Burr.; a. and b. antheridium and oogonium ; c. entrance of male 

 nucleus; d. fusion in oogonium, antheridium without nucleus; e. fusion nucleus in oogonium; 

 f. and g. septation of oogonium ; h. two nuclei in ascus ; i. ascus after nuclear fusion ; after Harper. 



In 1900, Harper observed fertilization in the oogonium of Pyronema 

 confluens; here, however, the gametangia are both multinucleate and fer- 

 tilization consists of the fusion in pairs of a large number of male and female 

 nuclei. Many asci are produced, each from a recurved filament in the pen- 

 ultimate cell of which a second nuclear fusion occurs. 



Development of the Ascus. Though the young ascus, so far as obser- 

 vation goes, is almost invariably binucleate and the seat of a nuclear fusion, 

 the details of its formation are not always the same. In 1905 Maire showed 

 that in Galactinia succosa and occasionally in other forms a series of three 

 or four binucleate cells is produced at the end of each ascogenous hypha. 

 The nuclei of the terminal cell undergo simultaneous division and two are cut 

 off at the apex by a transverse wall. These fuse and the cell containing them 

 becomes the ascus. Harper in Erysiphe, and other authors in various other 

 plectomycetous fungi have found that any cell of an ascogenous hypha, if it 

 contains two nuclei, may give rise to an ascus without preliminary nuclear 

 division. 



In some of those species in which the ascus is derived from a binucleate 

 penultimate cell the curvature of the hypha is so great that the uninucleate 

 terminal cell lies in contact with the stalk cell of the ascus. When this 

 happens the terminal and stalk cells sometimes fuse, a nucleus wanders from 

 one to the other, and the cell thus provided with two nuclei grows out as a 



