PLECTOMYCETES 



[CH. 



the usual row of three to five cells is formed. The penultimate cell regularly 

 contains more than one nucleus ; the rest, as a rule, are uninucleate. 



Just after fertilization the sheath begins to grow up (fig. 44 o), springing 

 in this case from the stalk cell of the antheridium, as well as from that of 

 the oogonium, and developing into the three layers described above. 



The ascogenous hyphae arise as lateral branches from the septate 

 oogonium (fig. 44 c), all or most being derived from the penultimate cell 

 about which they are crowded and intertwined. They are at first multi- 

 nucleate, and, as development proceeds, push up vertically within the peri- 

 thecium (fig. 45); septation then takes place. The asci, of which there are 

 several in each perithecium, arise as lateral outgrowths from the intercalary 

 cells, or are formed directly from the terminal cells of the ascogenous hyphae. 

 Each young ascus contains two nuclei, but the remaining cells are almost 

 without exception uninucleate. Fusion takes place in the ascus (fig. 46) and 

 is followed by three nuclear divisions ; as a rule only two spores are formed. 



F'g- 45- Phyllactinia Corylea (Pers.) Karst.; peri- 

 thecium containing uninucleate asci;after Harper. 



Fig. 46. Phyllactinia Corylea 

 (Pers.) Karst.; a. b. fusion in 

 ascus; after Harper. 



Eight chromosomes (fig. 47) have been observed throughout the life- 

 history. 



In Phyllactinia Corylea and also in Microsphaera A Ini (Sands, 1907) and 

 various species of Erysiphe (Harper, 1905), the organization of the resting 

 nucleus is very characteristic. A deeply staining central body lies against 

 the nuclear membrane and to this the chromatin threads are attached. 

 From it they extend into the central cavity of the nucleus forming a sheaf 

 of divergent rays connected laterally by delicate fibrillae (fig. 46). 



