THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 263 



cytoplasm becomes increasingly vacuolated. The male nuclei now pass down 

 the trichogyne into the ascogonium and become associated with the female 

 nuclei. 



Whether these male and female nuclei actually fuse in pairs or whether 

 they simply remain associated together is a matter of dispute. According 

 to some workers there is definite nuclear fusion so that the resulting nuclei 

 are diploid. Others consider that the male and female nuclei associate but 

 do not fuse, so that they remain monoploid and subsequently divide in pairs 

 (conjugate division). 



The Ascogenous Hyphae 



After a few hours changes have taken place in the fruiting body. In the 

 first place the ascogonium has become enveloped in a tissue of vegetative 

 hyphae which have originated from the lower cells of the original knot of 

 tissue, thus forming the peridium. At the same time outgrowths have been 

 formed from the wall of the ascogonium, and into each of these a pair of 

 nuclei has made its way. These are the ascogenous hyphae (Fig. 255). 

 They branch repeatedly, and by conjugate division each branch is supplied 

 with a pair of nuclei or dicaryon. Septa now appear, cutting the hypha 

 into a number of cells, each with one dicaryon in it. Finally the terminal 

 cells bend over to form a hook like an inverted V, known as the crozier, and 

 from the penultimate cell, which forms the apex of the hook, there grows out 

 upwards a single elongated cell, the ascus. 



The two nuclei of the dicaryon in the young ascus fuse. This fusion 

 in the ascus is immediately followed by three nuclear divisions whereby 

 eight nuclei are formed. It will be noted that there is no question about the 

 nuclear fusion at this stage, hence it follows that if a nuclear fusion also did 

 occur in the ascogonium then the fusion nucleus in the ascus must be tetra- 

 ploid, seeing that it is formed as the result of two nuclear fusions. Alterna- 

 tively, if there was only an association of male and female nuclei in the 

 ascogonium and the sexual act was only consummated in the ascus, then 

 the nucleus will be a typical diploid. Now, since obviously the nuclei of the 

 ascospores must be monoploid, as they give rise to a monoploid mycelium, it 

 follows that the normal procedure of meiosis will not provide the mechanism 

 for reducing the chromosome complement of a tetraploid to a monoploid. 

 It is postulated therefore by those who hold the view of a double fusion that 

 there is correspondingly a double reduction, and that this second nuclear 

 reduction takes place in the third division in the ascus. The name brachy- 

 meiosis is given to this division. 



In either case the mature ascus contains eight nuclei around each of 

 which a wall is laid down, resulting in the formation of eight separate 

 ascospores which are smooth and ellipsoidal. 



The asci arising from each group of ascogonia are arranged side by side 

 in a concave cluster or disc, around which the peridium forms a marginal 

 layer, thus constituting an open ascocarp or apothecium. 



