ORDER SACCHAROMYCETALES 341 



According to Varitchak (1931) two of the many nuclei in the young ascus 

 enlarge and then unite. From the division of this zygote nucleus it is 

 supposed that the nuclei of the ascospores are formed. After the ascospores 

 are set free by the dissolution of the apex of the ascus the basal septum 

 arches upwards and a new ascus is produced by proliferation. This may 

 occur many times as in the formation of sporangia in Saprolegnia. Miss 

 Walker (1935) was unable to confirm Varitchak's report of the union of 

 two privileged nuclei and believes that the ascus develops partheno- 

 genetically. She noted earlier (1931) that the ascospores frequently fuse 

 by twos after their escape and produce mycelium originating from the 

 conjugation tube connecting the two ascospores. It should be noted that 

 in Dipodascus uninucleatus frequently the ascus may be produced par- 

 thenogenetically. If that should occur also in Ascoidea it may be that 

 both Varitchak and Miss Walker were correct in their reports. (Fig. 111.) 



The true yeasts, i.e., those forms of this order that are normally not 

 hyphal, may be divided into the families Schizosaccharomycetaceae in 

 which the cells divide by fission and Saccharomycetaceae in which they 

 divide by budding. Many of them are of great industrial value because 

 of their power of fermenting various sugars and producing alcohol and 

 CO2, but many are unable to ferment the commoner sugars. Some of the 

 alcohol-producing yeasts grow at the top of the solution to be fermented, 

 the so-called "top yeasts," while others are more abundant where the 

 oxygen supply is less, in the lower portion, the "bottom yeasts." 



Sexual reproduction is quite varied. Perhaps the more common mode 

 is the union of two cells, more often equal in size but sometimes of un- 

 equal size, by means of short tubes. The intervening walls are dissolved 

 and the two nuclei unite, usually in the conjugation tube. The number of 

 ascospores is more often four but in many cases is eight. Probably by 

 failure of some of the nuclei to function in the production of ascospores 

 the number may be less than four, in a few cases only one. The mature 

 ascospores may be distributed evenly or unevenly between the two con- 

 jugating cells or may be found in only one. They are more often ellipsoidal 

 but may be hat-shaped or spherical or needle-shaped. A modification of 

 the foregoing type is the conjugation of a mother cell with a bud produced 

 from it. Both cells may produce conjugation tubes or the partition be- 

 tween the larger cell and the bud may dissolve away. This process has 

 been called pedogamy. It is clear that this is a modification of the pre- 

 ceding type. In both types the vegetative cells may be assumed to have 

 haploid nuclei with immediate meiotic division of the zygote (diploid) 

 nucleus to bring about the ascospore production. 



In some yeasts the four ascospores unite by twos within the ascus. 

 From the conjugation tube a process pushes through the ascus wall and 

 produces a daughter cell outside. This cell multiplies by budding or fission 



