INTRODUCTION 



375 



inherited from the Ascomycetous ancestors but whose function is no 

 longer indispensable. 



Mile. Bensaude and Kniep were among the first to show the existence 

 of different sexual phases (or as they called it "heterothallism") in the 

 Basidiomyceteae. Miss Mounce (1922) and Miss Newton (1926) and 

 various other investigators have shown that many of this class are " homo- 

 thallic," i.e., will produce the dicaryon mycelium in culture from a single 

 basidiospore while other species are always "heterothallic." Rarely a 

 monocaryon mycelium of a heterothallic species after a considerable time 

 begins to produce dicaryon hyphae in a manner not yet satisfactorily 

 explained. In the homothallic species in which the basidiospore is uni- 

 nucleate it must be assumed that the genetic factors for incompatibility 

 are absent or are both present and mutually cancelling in the same 

 chromosome. Sass (1929) studied the behavior of the nuclei in certain 

 homothallic forms in which the nuclei are two in number in the basidio- 

 spore. Thus in Coprinus ephemerus Fr. there exist forms in which four 

 uninucleate basidiospores are produced on each basidium ; these forms are 

 heterothallic. In C. ephemerus forma hisporus only two basidiospores are 

 produced each with two nuclei. Mostly these give rise to homothallic 

 mycelia but sometimes they show heterothallism. Sass found that from 

 ninety per cent of such binucleate spores there is produced a coenocytic 

 mycelium which as it grows begins to become septate until the apical 

 portions of the hyphae consist of uninucleate cells. Some of these hyphal 

 branches on coming into contact with other uninucleate hyphae from the 

 same mycelium fuse with them to form typical dicaryon mycelium (with 

 clamp connections) and from this mycelium arise the normal fruiting 

 bodies. Evidently the two nuclei of the basidiospore in this case repre- 

 sented opposite sexual phases. In about ten per cent of the basidia the 

 two nuclei of the spore are clearly of the same sexual phase. Such spores 

 produce a mycelium consisting from the first of uninucleate cells. Only 

 when two such mycelia of opposite sexual phases come into contact is the 

 secondary, fruiting mycehum produced. The situations described by Sass 

 are very similar to those described by Ames (1932) in Schizothecium (see 

 Chapter 10). 



The four basidiospores may represent two or sometimes four sexual 

 phases which are mutually fertile by twos. Because of the presence of male 

 sexual cells (oidia) on the mycelium of both uniting mycelia and of the 

 fact that each mycelium or its oidia can diploidize the other mycelium it is 

 manifest that we can not look upon these as representing opposite sexes. 

 As in homologous cases in the Ascomyceteae it is a question of self- 

 incompatibility, comparable in a way to that occuring within a given 

 horticultural variety of pear which may be sterile to its own pollen but 

 fertile to pollen of another (but not of every other) variety. Since the term 



