646 



THE PHYLOGENY OF THE FUNGI 



Fig. 208. Diagram showing Linder's suggestion as to the origin of the teliospore 

 and promyceUum of Uredinales from an ascus. (Courtesy, Linder: Mycologia, 32(4): 

 419-447.) 



ascus assumes that the type of ascus was that with a two-layered wall, 

 such as occurs in Pleospora, Leptosphaeria, etc. The outer layer is firm and 

 the inner layer thin and elastic. Upon the softening of the apex of the 

 ascus where there is a germ pore the inner layer expands so that the endo- 

 ascus and contained ascospores project some distance beyond the outer 

 layer and eventually ruptures to set the spores free. He assumes that such 

 an ascus became more thick-walled so as to become a resting spore but 

 with the oncoming of favorable conditions the inner portion pushed out as 

 described above, the meiotic division occurring usually within this pro- 

 jection. The contents of this extruded portion (promycelium or basidium) 

 divide to form four spores by simple cross walls instead of by free spore 

 formation as is usual in the Ascomyceteae. These four spores remaining 

 attached to each other permit their contents to escape by forming ex- 

 ternal sterigmata from which secondary spores (sporidia or basidiospores) 

 are set free. (Fig. 208.) 



Accompanying the gradual loss of obligate parasitism and simplifica- 

 tion of the life cycle the Uredinales according to Linder led to the Auricu- 

 lariales, probably to those forms in which a "probasidium" is present 

 such as Septohasidium and Helicohasidium. With the complete loss of the 

 formation of a distinct probasidium, forms such as the entirely sapro- 

 phytic Auhcularia could be reached. In other more or less similar ways he 

 postulates the origin of the Tremellales and from them the Autobasidio- 

 myceteae (Eubasidiae). 



