9 



CLASS ASCOMYCETEAE: LABOULBENIALES 

 AND DISCOMYCETES 



Introduction 



THE members of the two extremes of this class have Httle in common 

 beyond the production of the ascus. The type of sexual union (which 

 is often absent), the plan of the spore fruit, even the nature and size of 

 the vegetative mycelium vary tremendously. At the one extreme we find 

 the Yeasts (Order Saccharomycetales) in some of which the unicellular 

 plant becomes transformed directly into an ascus, while near the other 

 extreme are the "Discomycetes" (Lecanorales, Pezizales, etc.) in which 

 there is a well-developed mycelium and in some species a sexual union of 

 a nonmotile sperm with a trichogyne, leading to the production of a 

 well-organized apothecium with many asci. 



It is therefore essential to study the ascus, as being the one structure 

 common to all Ascomyceteae. Omitting the apogamous forms for the 

 present, we find the young ascus to be a binucleate cell well supplied 

 with food. The two nuclei are usually considerably larger than those of 

 the vegetative mycelium. They fuse, forming a diploid nucleus with 

 double the number of chromosomes found in each of the original pair. 

 This nucleus usually enlarges quite considerably. It divides meiotically 

 to form four nuclei which in the great majority of cases divide again. In 

 a few species the nuclear division may be repeated until 16, 32, 64, or 

 more nuclei are present in the young ascus. In one species of Schizothecium 

 (Pleurage) the number, according to L. M. Ames,^ is 512, while in Thele- 

 bolus stercoreus Tode ex Fr. the number is over 1000. A part of the ascus 

 cytoplasm gathers around each nucleus and is soon set off from the re- 

 maining cytoplasm (epiplasm) by a cell wall thus forming the ascospore. 

 The epiplasm may assist in the formation of the outside layer of the 

 ascospore wall (epispore) which is often beautifully sculptured. The 



1 In a letter to the author. 



200 



