SUBCLASS HETEROBASIDIAE 437 



basidium. In the Heterobasidiae the basidium may be (1) elongated and 

 transversely divided by three septa into four cells from each of which a 

 sterigma arises and produces a single basidiospore, or (2) it may be 

 rounded and divided into four cells by vertical septa, each cell producing a 

 sterigma and a basidiospore, or (3) elongated and forked into two prongs, 

 at the apex of each of which a sterigma bears one basidiospore, or (4) the 

 rounded basidium produces (usually) four rounded cells at its upper end, 

 these usually being separated by a septum from the collapsed basidium 

 and frequently falling free from it. Each of these cells produces a sterigma 

 and a basidiospore. In all of these four types, and in the Eubasidiae, the 

 basidia arise as enlarged binucleate cells on mycelium of the secondary 

 (dicaryon) type. Mostly these cells are terminal to the hyphae bearing 

 them but sometimes (Sirohasidium) several cells at the terminal portion 

 of the hypha become transformed into a chain of basidia. In a few cases a 

 single intercalary cell may become a basidium. 



In this subclass the author recognizes four orders, based upon the four 

 types of basidia. It must be noted that Martin (1944) considers these to 

 be more closely related and unites them into one order, Tremellales. The 

 orders recognized by the author are: 



Auriculariales : with elongated basidium divided into a row of four cells by 

 cross septa. Nuclear divisions stichobasidial. 



Dacrymycetales : with nonseptate forked basidium ("tuning fork" type) bear- 

 ing two basidiospores. Nuclear divisions stichobasidial. 



Tremellales: with rounded basidium divided cruciately into four cells by ver- 

 tical septa. Nuclear divisions chiastobasidial. 



Tulasnellales : with rounded basidium bearing large rounded epibasidia, usually 

 separated from the hypobasidium by septa and often falUng free. Nuclear 

 divisions chiastobasidial. 



In these four orders there is a marked parallelism of evolution of spore 

 fruit structure from effused, sometimes almost film-like, structures to 

 cushions, shelves, stipitate pilei, etc. 



The arrangement followed below is not to be considered as represent- 

 ing a single progressing line of evolution. On the contrary the groups have 

 been produced by a more or less parallel evolution from primitive forms 

 that have been lost. The orders Auriculariales and Tremellales are some- 

 times set apart from the other orders as the Phragmobasidiae, since the 

 basidium is divided into four cells. The Dacrymycetales and Tulasnel- 

 lales with undivided basidia are then united with the remaining orders of 

 the class under the name Holobasidiae. Gilbert (1921) has studied the 

 nuclear behavior in the Heterobasidiae and finds that in their essentials 

 they are identical with those occurring in the Eubasidiae. 



Within this subclass the basidium in many genera has been con- 

 sidered as being made up of two parts, hypobasidium and epibasidium 



