56 PLECTOMYCETES [CH. 



trichogyne and the zygote (oogonium) after septation gives rise to numerous 

 asci which arise from its several cells. 



In none of the Plectomycetes are the more complicated forms of sexual 

 apparatus reached ; the trichogyne and oogonial region are never multi- 

 cellular and the antheridium never becomes a spermatium. 



The structure of the reproductive branches thus bears out the inference 

 drawn from the characters of the ascigerous stage that the species included 

 under the Plectomycetes are simple and for the most part presumably 

 primitive. The group is a collection of forms at a somewhat similar level 

 of development, and may or may not prove to be a natural arrangement ; 

 in this it resembles the Discomycetes and Pyrenomycetes. 



It is within this group that the vegetative sporophyte has been developed 

 and the fusion in the ascus established, and it is probably among these forms 

 that the explanation of the peculiarities of ascomycetous morphology is to 

 be sought. 



The group Plectomycetes includes some 1200 species and may be sub- 

 divided as follows : 



Asci irregularly arranged Plectascales. 

 Asci parallel 



Ascocarp developed Erysiphales. 



Ascocarp lacking Exoascales. 



Plectascales 



The Plectascales include all those Ascomycetes in which the asci are 

 arranged irregularly, at different levels and diversely orientated. In the 

 better developed forms the ascogenous hyphae are enclosed in a definite 

 peridium made up of an inner nutritive and an outer protective sheath, or 

 they are surrounded, as in Gymnoascus, merely by an open weft of hyphae. 

 The latter arrangement suggests a connection with the Endomycetaceae, 

 a series of simple species in which the asci are quite unprotected ; it has 

 been shown, by the recent studies of Guilliermond and others, that these in 

 turn lead to species in which a mycelium is seldom developed, and finally 

 to the typical yeasts, where the endosporogenous cells are scarcely recogniz- 

 able as asci. 



To these may be added a few other simple forms of uncertain relationship 

 so that the Plectascales include the Plectascineae, the Protascineae, and 

 some of the Hemiasci of the authors of Engler's Naturlichen Pflanzen- 

 familien, and contain some of the simplest members of the Ascomycetes. 



Among these the Saccharomycetaceae have presumably been derived 

 by reduction from such forms as the Endomycetaceae where a mycelium is 

 normally developed. In both groups a single fusion occurs in the life-history 

 and immediately precedes the formation of the ascospores. It might be 



