332 GENERAL MORPHOLOGY OF PLANTS 



cases, definite in number, usually eight, but in some cases contain- 

 ing multiples of eight, and in others containing regularly six, or 

 four, or two, and in rare cases one, while there are a few in which 

 the number is variable, being from two to three or four or five. 

 This indicates a more specialized condition than in the Phycomy- 

 cetes. The mycelium is regularly septate. All of these characters 

 indicate an advance over the Phycomycetes. The fruit body of 

 the Ascomycetes, the perithecium or asocarp, recalls the cysto- 

 carp of the red algae, and there are several peculiarities in con- 

 nection with the form of the sexual organs and in fertilization in 

 some of the sac fungi, which resemble those of the red algae. 

 This has given rise to the theory that the sac fungi have had their 

 ancestors among the red algae of the past, and that they have 

 lost chlorophyll and the function of photosynthesis by becoming 

 parasites, or in an adaptation to a saprophytic mode of life. 



485. The Class Basidiomycetes. The basidium is the 

 characteristic fruit structure in this class of fungi. It is a single 

 cell of a specialized form in the higher members of the class. 

 In the lower forms it is divided into four cells by cross divisions, 

 or perpendicular divisions, a; in many of the trembling or jelly 

 fungi. In the latter there is one spore from each cell, while in 

 the former there are usually four spores from the single cell, 

 rarely two or six. In the smuts the number of cells of the 

 promycelium (which is basidium-like) varies. This specialized 

 structure, with the definite and limited number of spores, also 

 indicates a higher stage of development than is found in the 

 Phycomycetes. - The mycelium is septate, often there are buckle 

 joints at the cross walls, while the large and highly specialized 

 fruit bodies are in great advance over the Phycomycetes. The 

 fungi of this class also are believed by some to have originated 

 from some of the higher algae in the past. The cluster cup of 

 the rust fungi may possibly represent the cystocarp, but there 

 is little else to suggest an ancestral relationship, unless the basidio- 

 spores represent tetraspores, and in the rust fungi they do stand 

 in the same position in the life cycle as the tetraspores do in some 

 of the red algae. 



