24 o RESEARCHES ON FUNGI 



We have seen * : that two nuclei, presumably of opposite eex, 

 fuse together in a young chlamydospore ; that the fusion nucleus 

 undergoes division producing a number of haploid nuclei ; that these 

 nuclei pass up into the primary sterigmata ; and that, when two 

 primary sterigmata have conjugated, two nuclei, presumably of 

 opposite sex, pass through the secondary sterigma into a primary 

 basidiospore. 



The conjugation process just described, resulting in primary 

 basidiospores containing two nuclei, indicates that Tilletia tritici, 

 in the stage of its life-history under discussion, is homothallic. 



According to Boss, 2 the two nuclei which enter a primary basidio- 

 spore do not fuse and do not long remain together ; for, when a 

 primary basidiospore germinates, the two nuclei separate from one 

 another and divide separately. Of the crop of haploid nuclei so 

 produced in the mycelium, a single nucleus enters each secondary 

 basidiospore, so that secondary basidiospores differ from primary 

 basidiospores in being haploid instead of diploid. 



Boss also found that, when an H -shaped pair of sterigmata 

 germinates directly (c/. Fig. Ill, p. 226), the two nuclei which go 

 into the germ-tube separate and that the secondary basidiospores 

 produced on the mycelium, like those described in the preceding 

 paragraph, contain one nucleus only. 



From Boss's nuclear studies it is to be concluded that some 

 secondary basidiospores are (+) and others (— ) in sex. 



A secondary basidiospore, on germinating, gives rise to a uni- 

 sexual mycelium. When one considers the nuclear condition of the 

 secondary basidiospores and of the mycelia which they produce, we 

 are obliged to regard Tilletia tritici, at the stage of its life-history 

 under discussion, as heterothallic. 



It thus appears that we must regard Tilletia tritici as homothallic 

 in one stage of its life-history and heterothallic in another stage. 



There is the possibility, even the likelihood, that two haploid 

 mycelia derived from two haploid secondary basidiospores of opposite 

 sex, (+) and (— ), may meet in the tissues of the host plant, fuse 



1 Vide supra, p. 218. 



2 G. Boss, " Beitrage zur Zytologie der Ustilagineen," Planta, Bd. Ill, 1927, 



pp. 619-622. 



