270 CLASS ASCOMYCETEAE 



there are formed eight ascospores, each with a single nucleus. Four of these 

 represent one sexual strain and the other four another sexual strain. As 

 with the uninucleate ascospores of A^. tetrasperma and of Schizothecium 

 anserinum the mycelia formed by their germination will form perithecial 

 primordia and "microspores" (sperm cells) which are self incompatible 

 but mutually fertile if both sexual strains are present. In N. tetrasperma 

 and G. tetrasperma Dowding the ascospores normally contain two nuclei 

 which with rare exceptions represent both sexual strains. As a result when 

 the spores germinate the mycelial cells contain both types of nuclei and 

 at the formation of the perithecial primordium no receptive hyphae or 

 trichogynes are produced and no fertilization of these by microspores or 

 sperm cells can occur. On mycelia developed from the occasional uni- 

 nucleate ascospores of these species only one kind of nucleus is present 

 and the contact of the mycelium or spores from two mycelia of opposite 

 phase is necessary. In N. tetrasperma this may occur in any of the three 

 following ways: (1) Microspores (sperm cells) from the mycelium of one 

 phase are brought into contact with trichogynes that have grown out of 

 the perithecial primordia of the other phase and fertilize these. This is 

 the same method by which the perithecia of A^. sitophila are brought to 

 production. (2) The large conidia or mycelium from them may similarly 

 fertilize the trichogynes. (3) The mycelia of the two sexual phases may 

 come into contact and fuse and mutually diploidize one another so that 

 all the cells of either mycelium come to contain nuclei of both types. Then 

 as the perithecial primordia arise both types of nuclei being present fer- 

 tilization from outside is unnecessary. In Gelasinospora tetrasperma this 

 third method seems to be the usual one if mycelia from uninucleate asco- 

 spores are concerned as there are no microspores (sperms) nor conidia 

 (Dowding, 1933). In this species Miss Dowding and A. H. R. Buller (1940) 

 have demonstrated that the nuclei pass from cell to cell in the mycelium 

 through the central pore found in every septum. These proceed at the 

 rate of 4 to 5 mm. per hour which is nearly twice to over twice the rate at 

 which the mycelium itself grows. Normally in Schizothecium anserinum 

 the first method is the usual one even when the mycelium grew from a bi- 

 nucleate spore for the perithecial primordia and the antherids often arise 

 from hyphal branches that contain only one kind of nucleus. Dodge (1936) 

 showed that in this species there are various races some of which produce 

 no sperm cells. When cultures of the two sexual phases of these are mated 

 perithecia are formed indicating that probably the third method was the 

 one that was effective in perithecial formation. 



Keitt and Palmeter (1938) and Keitt andLangford (1941) have shown 

 that also in Vcnturia inaequalis (Cke.) Winter there are two sexual phases, 

 four of the ascospores representing one phase and four the other. Segrega- 

 tion of the genes that determine the two phases may occur in the first 



