BESSEY: MYCOLOGY 253 



The third class, Ascomj^cetes, is considered to have arisen from some Zygo- 

 mycete in which tlie union of two gametangia — instead of producing a zygo- 

 spore, out of which later, on germination, a stalked sporangium arises — produces 

 immediately the sporangium, which is specialized to become the ascus. In the 

 subclass Protascales ascogenous hyphae and spore-fruits are lacking. Here come 

 the orders Endomycetales (^ Saccharomycetales of authors) and Taphrinales. 

 The subclass Euascomycetes includes the rest of the Ascomycetes, in which spore- 

 fruits are built and the asci are produced on ascogenous hyphae. The basal 

 group of the Euascomycetes is the order Plectascales, in which the ascogenous 

 hyphae branch through the interior of the spore-fruit so that the asci are scat- 

 tered throughout it. The asci and other tissues dissolve at maturity of the asco- 

 spores so that the latter lie free in the now hollow ascocarp. The ascospores are 

 not expelled from the asci. The spore-fruit shows varying degrees of complexity, 

 from a loose weft of hyphae among which the ascogenous hyphae creep and pro- 

 duce their asci (family Gymnoascaceae) to rather massive structures (e.g., Ela- 

 phomycetaceae). Sexual reproduction by union of antherids with ascogones 

 occurs frequently. From this order branch off two series of orders, the Ascolo- 

 culares, in which the asci are formed in cavities which they dissolve out in the 

 stromatic tissue, and the Ascohymeniales, in which these cavities are formed 

 during the growth of the spore fruit and then become lined by a palisade of asci. 

 The former group includes among other orders the Perisporiales, Myriangiales, 

 Pseudosphaeriales. In the Ascohymeniales are found the Sphaeriales (including 

 Hypocreales), Pezizales (=the operculate Discomycetes), Helotiales (inoper- 

 culate Discomycetes) and Tuberales. The Laboulbeniales are placed at the close 

 of the class with uncertain position as regards relationship. 



The fourth class, Basidiomycetes, is placed highest because of its derivation 

 from the higher Ascomycetes (probably some of the Discomycetes). The basi- 

 dium is looked upon by Gaumann as an ascus from which have emerged four 

 exogenous pockets containing each a single ascospore. This ascospore with its 

 containing wall is the so-called basidiospore. Gaumann recognizes two sub- 

 classes: Holobasidiomycetes, with one-celled basidium, and Phragmobasidiomy- 

 cetes, with the basidium longitudinally or transversely septate. The Holobasi- 

 diomycetes he considers the more primitive type, derived from the Ascomycetes 

 in which the hook or crozier (or its derived form, clamp-connections) is present. 

 The Holobasidiomycetes are divided into the Hymenomycetes, in which the basi- 

 diospores are violently expelled, and the Gastromycetes, in which the basidio- 

 spores are passively distributed. 



In the second subclass, Phragmobasidiomycetes, the basidiospores are violently 

 shot away in most of the species except in Family Ustilaginaceae. The following four 

 groups are placed here : Tremellales, Auriculariales, Uredinales, and Ustilaginales. 



The late Herbert Spencer Jackson, for many years a student of the Uredi- 

 nales, published a memoir (1931) in which he compared the life cycles of the 

 rusts with those of the red seaweeds, suggesting that the similarities might indi- 

 cate relationship between these groups. 



The spermogonium of the Uredinales may be considered to sliow relation- 

 ship of the rusts to those Ascomycetes in which such structures occur. 



The ideas of relationship which the author has inherited and developed in 

 the last half -century (see Bessey, 1942, 1950) may be outlined here. 



