TILLETIA TRITICI 237 



from a type which resembles the basidium of one of the Hymeno- 

 mycetes to a type which appears to be like the basidium of one of 

 the Hymenomycetes but very greatly modified. 



Granted that the primary conidia of Tilletia tritici are in reality 

 sterigmata, we must assume that these sterigmata are highly 

 specialised. The evidence for this specialisation is indicated by the 

 following facts : (1) the sterigmata are extremely long and slender ; 

 (2) the sterigmata conjugate in pairs by means of a bridging hypha, 

 thus differing from the sterigmata of the Hymenomycetes and the 

 Uredineae ; (3) not every sterigma but only one of each pair of 

 sterigmata produces a basidiospore ; (4) the basidiospores do not 

 develop at the apices of the main shafts of such sterigmata as bear 

 them ; but (5) each conjugated H -shaped pair of sterigmata sends 

 out laterally a short conical branch on which the basidiospore is 

 developed, so that the sterigmata are branched. 



In the heterothallic Hymenomycetes, e.g. Coprinus lagopus, the 

 process of conjugation takes place between two mycelia derived 

 from two spores of opposite sex. In Tilletia tritici the conjugation 

 process has been put back a stage, so that it takes place not between 

 two haploid mycelia or two haploid spores but between two haploid 

 sterigmata, and it is therefore accomplished before the basidiospores 

 are formed. The sterigmata are specialised for the sexual process : 

 (1) by being very elongated and curved, so that sterigmata of 

 opposite sex often come near enough to one another to allow of 

 conjugation ; and (2) by having the power of sending out bridging 

 hyphae toward one another through the air. The production of 

 one basidiospore only from two conjugated sterigmata is evi- 

 dently correlated with the sexual process. The nucleus of one 

 sterigma passes via the bridging hypha into the other sterigma and 

 thus the two nuclei become associated as a pair. To receive this 

 pair of nuclei only one basidiospore is required. Since the sterig- 

 mata are so very long and slender, and since conjugation takes place 

 usually near the middle of each pair, it is perhaps not surprising that 

 the basidiospore is developed not at the tip of one of the main shafts 

 of the two sterigmata but on a short branch or secondary sterigma 

 which has a lateral origin from one of the two conjugated primary 

 sterigmata. 



