BASIDIOMYCETES 415 



the Tremella type, they are inserted apically and the cruciate basidium is 

 an acrosporous phragmobasidium. It is noteworthy that the difference 

 between the acrosporous and pleurosporous phragmobasidium is not much 

 sharper than that between the stichobasidium and chiastobasidium. In 

 some orders, acrosporous basidia appear in a pleurosporous basidial group 

 and vice versa. These relationships are significant for the systematic 

 arrangement of Basidiomycetes. 



The essential facts in the development of the basidium are the fusion 

 of the dicaryon to a diploid nucleus in the young basidium and the reduc- 

 tion of the chromosome number and the genotypic separation of unit 

 characters and of sex factors as it matures. Consequently, at this stage 

 the basidium is both zeugite and gonotocont as in the ascus. 



In some forms fruiting directly on the mycelium, there is a tendency 

 for the caryogamy to take place in the terminal cells of the hyphae and to 

 await particular conditions in the environment for further development. 

 Meanwhile the terminal cell swells more or less, stores up large quantities 

 of foodstuffs and the primary basidial nucleus enters into synapsis. 

 Under favorable conditions, the enlarged terminal cell forms a basidium 

 which completes meiosis in the usual way and proceeds to spore formation. 

 The terminal cell of the hypha, which in the previously described forms 

 is itself enlarged to a basidium, is used here only as a preliminary stage 

 in basidial formation; i.e., basidial formation is deferred and the functions 

 of zeugite and gonotocont, which were originally joined in the same 

 organ, are divided between two organs, the enlarged hyphal cells and the 

 basidium. This enlarged hyphal cell which functions as zeugite and 

 thereby forms the first stage of the basidium (in Neuhoff's sense an 

 epibasidium) is called probasidium (Tieghem, 1893) or hypobasidium 

 (Neuhoff, 1924). 



One can hardly go astray — and in the Auriculariales (p. 415) we 

 will discuss this question — if one seeks the original cause for the forma- 

 tion of this new organ mainly on biological grounds. Since imper- 

 fect stages are usually wanting, these fungi are dependent upon the 

 basidia as the only organs of fructification, and those individuals can 

 best survive selection which are able to wait for the favorable moment 

 to form basidiospores. 



Subsequently, probasidia and basidia still further diverge morpholog- 

 ically. The wall of the probasidium thickens, encysts (to a certain degree) 

 and becomes a resting cell, very resistant to external conditions (Fig. 

 364, 9 and 10). This gemma-like structure is called a sclerobasidium 

 by Janchen (1923). 



In parasitic forms, the direct connection between sclerobasidium 

 and basidium gradually disappears. These scerobasidia arise at the 

 beginning of unfavorable conditions, e.g., in temperate climates, in the 

 fall; they pass the winter in this state and in the spring germinate to a 



