FUNGI, SAC FUNGI 



291 



439. The ascus stage, or perfect stage. This is developed 

 after the formation of conidia, as the result of a sexual process 

 ending in the formation of minute brown or blackish fruit bodies.* 

 These fruit bodies are often very numerous, appearing as minute 

 black specks just visible to the eye. The fruit body is provided 

 with appendages of a dark color, consisting of short hyphae which 

 are branched at the end several times in a forked manner (fig. 

 235). The sacs or asci are somewhat elliptical in outline. 

 Several are formed in each fruit body, and may be seen by crush- 

 ing the latter in water and examining with a microscope. Each 

 ascus contains several spores Other genera of the powdery mil- 

 dews have different kinds of appendages. 



Fig. 236. 



Contact of sperm case 

 and egg case (egg case 

 the larger cell); beginning 

 of fertilization. 



Fig. 237- 



Disappearance of con- 

 tact walls of sperm and 

 egg cases, the two nuclei 

 uniting. 



Fig. 238. 



Fertilized egg sur- 

 rounded by the envelop- 

 ing threads which grow 

 up around it. 



Figs. 236-238. Fertilization in Sphaerotheca; one of the powdery mildews. (After Harper.) 



440. The sexual process in the powderymildews. The sexual organs 

 are short branches of the mycelium, a sperm case and an egg case. The 

 processes here described occur in the genus Sphoerotheca. These branches 

 arise close together and their ends come in contact. At the point of contact an 

 opening is dissolved through the walls. The sperm nucleus in the sperm 

 case moves into the egg case and unites with the egg nucleus (figs. 236-238). 

 The egg-case cell now grows into a short branch of five or six cells. In the 

 last cell but one are two nuclei. These fuse into one, and the cell grows out 

 into a large globose cell, the ascus. The nucleus now divides to form eight 

 nuclei which become centers in the protoplasm for the formation of eight 

 ascospores. 



441. The black fungi (Sphaeriales) . The black fungi 

 include a vast number of the sac fungi, with many genera and 



* Called here a perithecium. 



