128 PLANT MORPHOLOGY 



The sex organs arise from uninucleate cells formed at the tips of special 

 branches of the mycelium, all the (;ells of which are uninucleate (Fig. 

 104). The antheridium, slightly smaller than the oogonium (ascogo- 

 nium), comes in contact with it. The intervening cell wall is dissolved 

 and the male nucleus passes over to fuse with the female nucleus. Ster- 

 ile hyphae, arising from the cell beneath the oogonium, form a closed 

 ascocarp. Following fertilization, the fusion nucleus gives rise to three 



Fig. 103. Ascocarp of Microsphaera alni with characteristic appendages, crushed slightly 

 so that three asci, each with eight ascospores, have appeared, X 250. 



to five (often more) free nuclei and then transverse walls come in, form- 

 ing a short row of cells. All of these are uninucleate except the penul- 

 timate cell, which is binucleate. In Sphacrofheca and Podosphaera this 

 cell directly forms a solitary ascus in which the two nuclei fuse, while 

 in the other genera it gives rise either to a row of cells, each of which 

 becomes an ascus, or to ascogenous hyphae that, in turn, produce the 

 asci. Although in Sphacrotheca and Podosphaera the ascocarp has only 

 one ascus, in the other genera it contains a basal layer of several parallel 

 asci. The development of the ascus takes place in the regular way, 

 except that it frequently contains less than eight ascospores. Eight 

 nuclei are formed as usual, but some are not organized into spores. The 

 asci are generally not intermixed with sterile hyphae. 



If the male and female nuclei actually fuse in the oogonium, the fusion 



