ASCOMYCETES 



219 



Perithecia are usually developed during the middle or latter 

 part of the growing season. They are produced directly upon 

 the mycelium, and the development is interesting and instructive. 

 The development, however, can be best followed only by serial 

 sections of properly imbedded material. In brief, it may be 

 described as follows : Two adjacent cells or hyphas give rise to 

 erect branches, one of which is larger and may be designated 

 as the oogonium, the other, smaller branch as the antheridium. 

 After a basal cell is cut off in each case, and further, a terminal 

 antheridial cell in the one case, there is dissolution of a portion 



Fig. 85. Phyllactinia Corylea: Gametes, Fertilization, and 

 Development of Perithecium and Young Asci. (After Harper) 



of the wall between the antheridium and the oogonium, migration 

 of the antheridial nucleus, and fusion of this with the oogonial 

 nucleus (Fig. 85, b). Subsequently the oogonial cell undergoes sev- 

 eral divisions. The last cell but one in this ascogonium contains 

 always two nuclei, and these fuse prior to the development of this 

 cell as an ascus. This is the case when a single ascus is produced, 

 and it is only slightly more complex when many asci result (cf. 

 Fig. 85, d-f). Following the fusion of the two gametic nuclei, 

 hyphal branches arise from the stalk cell of the oogonium. These 

 converge around the oogonium and finally completely inclose 

 it. Within this first layer a second layer of hyphae is produced 

 in similar manner ; and subsequently, by outgrowths from each of 



