THE YEASTS ^CUAPMAN 305 



is a provision on the part of nature for securing the persistence of 

 the species under conditions in which active budding is impossible. 

 It appears, at an}^ rate, to play an important part in the hibernation 

 of 3^easts, rendering it possible for them to live through the winter 

 in the soil, or on surfaces from which very little nutriment can be 

 extracted. 



In addition to reproducing by budding and by ascospore forma- 

 tion, yeasts are capable of reproducing by still a third method, 

 name!}', that of true conjugation. In these yeasts, constituting the 

 genus Zygosnccharomyces^ certain of the cells form, instead of ordi- 

 nary buds, long beak-like processes. When the " beaks " of two ad- 

 jacent cells touch one another a union takes place, the tijis of the 

 " beaks " disappear, and a tubular connection is established between 

 the two cells, one or both of Avhich then proceed to produce asco- 

 spores. Of these conjugating yeasts a number of different species 

 have been described, and this sexual process in one form or another 

 appears to be much more common than was until recently supposed. 



Finally, there is a group of organisms, usually included among 

 the Saccharomijcetes^ which are capable of reproducing by the pro- 

 cess of fission. In these so-called Schizosaccharomycetes the fission 

 of the cell, often accompanied by conjugation, is preceded by the 

 formation of a septum, which at once commences to divide into 

 two lamellae. Budding does not occur, but the cells form spores, 

 usually from two to eight. It will be seen, therefore, that in the 

 great family of the yeasts many types of reproduction are exhib- 

 ited — from true conjugation (heterogamic and isogamic) in the case 

 of some, through isogamic conjugation of ascospores formed in the 

 same asc, in oth«rs, to complete parthenogenesis, as in the case of 

 many of the better knoAvn cultivated yeasts. The industrial yeasts, 

 which appear to be entirely asexual, may perhaps be regarded as 

 retrograde forms descended from higher types in which sexuality 

 was quite clearl}^ marked. On this point I do not consider myself 

 qualified to express an opinion. 



As may well be supposed, in the case of a group of organisms 

 which, although presenting some very important differences, are 

 yet so closely allied, and in which there are very many transitional 

 forms, a great deal of confusion exists in respect of their classifica- 

 tion. The system at present generally adopted is one based upon 

 that suggested by Hansen in 1904, but it is customary to include 

 the Schizosaccharomycetes which he excluded, and there has been, of 

 course, a natural tendency to include a number of subdivisions. Tlie 

 great family of the Saccharoniycetes is capable of being subdivided 

 into a number of groups or genera, each of which in turn includes a 

 number of species, considerably more than 100 of which have been 

 described. 



