254 A TEXTBOOK OF THEORETICAL BOTANY 



The sexual organs arise as lateral branches from the mycelium (Fig. 246). 

 The ascogonium arises from a cell which divides to form a stalk cell and an 

 oogonial cell, the latter being uninucleate. The antheridium is smaller 

 and is also borne on a narrow stalk cell. It also contains a single nucleus. 





Mi 



jps 



i4 -Ji 





m 



|4^ 



Fig. 245. — Sphaerotheca humuU. Asexual reproduction. 

 A to D, Stages in the development of the conidiophore. 

 E, Septation of conidiospores. {After Foex.) 



Antheridium and ascogonium stand side by side in close apposition. The 

 wall between the two sex organs breaks down, and the male nucleus enters 

 the ascogonium and unites with the female nucleus. Nuclear division follows 

 and the zygote cell divides into a row of cells, the penultimate cell of which 

 contains two nuclei. This row of cells is equivalent to an ascogenous 

 hypha. From the penultimate cell the ascus is produced. While this is 

 going on, hyphae form around the developing sex organs, producing the 

 wall of the perithecium. The two nuclei in each ascus then fuse, and 

 immediately the zygote nucleus divides into four, during which reduction 

 occurs. This is followed by a further mitotic division which, according 

 to some workers, is also reductional. Thus eight nuclei are formed in each 

 ascus and around them walls are formed producing the eight ascospores. 

 It will be seen from this account that there may be two fusions, one in the 



