THALLOPHYTA: FUNGI 



121 



are branched and the sporangia uninucleate. Sexual reproduction seems 

 to be absent in Emqmsa muscae but, in several other species, as in the 

 Mucorales, it occurs by the conjugation of multinucleate protoplasts, each 

 representing the whole contents of a gametangium. 



c@ 



©■) 



\®( 



"t; 



B 



Fig. 94. Development of the sporangium of Empusa muscae, X600. A, hyphal body 

 elongating to form a sporangiophore; B, migration of nuclei to apex; C, formation of 

 multinucleate sporangium at tip of sporangiophore. 



4. ASCOMYCETES 



The Ascomycetes, or sac fungi, constitute the largest group of fungi. 

 They differ from the Phycomycetes in having a septate mycelium, that is, 

 one divided by cross walls into cells. They are also characterized by the 

 production of spores in a sac-like structure called an ascus. This is a cell 

 that at first contains two nuclei. These fuse and the fusion nucleus 

 typically gives rise to eight nuclei by three successive divisions, the first 

 two of which are reductional. From these haploid nuclei, eight walled 

 ascospores are then organized. In all except the lowest orders, the asci are 

 enclosed by a definite fruiting body, the ascocarp, composed of interwoven 

 hyphae. The relationships of the Ascomycetes are obscure. They maj^ 

 have been derived either from the Phycomycetes or from the Rhodophy- 

 ceae. The group is of immense economic interest, many members causing 

 serious plant diseases, such as peach leaf curl, brown rot of stone fruits, 

 black knot of plum, apple scab, and bitter rot of apple. There are about 

 25,000 species of Ascomycetes. These are included in nine main orders: 

 Protoascales, Protodiscales, Plectascales, Perisporiales, Pezizales, Helvel- 

 lales, Tuberales, Pyrenomycetales, and Laboulbeniales. 



