THALLOPHYTA: FUNGI 



129 



in the young ascus involves two diploid nuclei, necessitating a double 

 reduction of chromosomes in the two meiotic divisions that immediately 

 follow. This beha\ior has been disputed by some investigators, who 

 assert that the only nuclear fusion occurs in the young ascus and involves 

 haploid nuclei, some claiming that the male and female nuclei remain 



F G 



Fig. 104. Sphaerothera castagnei. A, antheridial and oogonial branches in contact; B, 

 antheridial branch cut off by a wall; C, antheridial cell separated from stalk cell; D, union 

 of male and female nuclei in oogonium; E, oogonium with zygote nucleus and two layers 

 of investing hyphae derived from cell just below; F, multicellular ascogonium, the penulti- 

 mate cell, with two nuclei, becoming the ascus; G, young ascus with fusion nucleus and two 

 ascogonial cells below it. {After Harper.) 



distinct in the oogonium, others that the antheridium is nonfunctional 

 and a male nucleus does not enter the oogonium. If these views are cor- 

 rect, the fusion nucleus in the young ascus is diploid and divides meiot- 

 ically in the usual way. 



5. Pezizales 



The Pezizales, or cup fungi, grow mostly on decaying wood or humus, 

 but some are parasitic on seed plants. They are a large order of approxi- 

 mately 5,000 species. The principal genera include Pyronema, Peziza, 

 Ascoholus, Lachnea, and Sclerotinia. 



Pyronema. This is a saprophyte on soil, especially where it has been 

 burned over. The mycelium grows as a white fluffy layer on the surface. 



