INTRODUCTION 



yeasts. These do not present a true mycelium at any time. They 

 reproduce at times by elongation of their cells, which adhere 

 together, forming structures resembling mycelium; but these never 

 offer the complexity of a typical mycelium. In the category of yeasts 

 belong the alcoholic ferments and all of the fungi more generally 

 known under the name of yeasts. 



These yeasts, which are often designated as " true yeasts " in 

 contradistinction to " yeast-like fungi " derived from more highly 



developed fungi, are not distinguishable in 

 any manner from the latter. The general 

 form and the internal characteristics of the 

 cells are the same in both cases. Physi- 

 ologically, certain true yeasts differ only 

 from yeast forms of molds by their resist- 

 ance to anaerobic conditions and excep- 

 tional activity of the fermenting function, 

 but very many yeast-like structures, de- 

 rived from fungi more highly developed, 

 are equally capable of producing alcoholic 

 fermentation, and only differ, from this 

 point of view, from true 3 r easts by a de- 

 creased activity of fermentation. On the 



F'^l.-Dematiumpullulans. ther hand > a C6rtain number f tme 



1 to 4, Mycelium Forming Yeasts; 5, 3*** ^ totally deprived of the ferment- 



->-ocess of i n g function. It is understood then, how 

 the early investigators were much confused 

 when it became necessary to characterize the yeasts. 



In the meantime, an essential difference which did not escape 

 investigators existed between the yeast-like fungi and the yeasts 

 properly so-called. Indeed, most of the true yeasts are distinguished 

 closely from " yeast-forms " by their aptitude to produce resistant 

 endospores at certain stages in their life cycles (unfavorable con- 

 ditions), in the interior of their cells; the cells are then transformed 

 into sporangia. De Bary, Rees, and Hansen first compared these 

 sporangia to ascs of Ascomycetes, considering the true yeasts as auton- 

 omous fungi which live only in the form of yeasts and are incapable 

 of developing a mycelium. 



This conception is definitely admitted today, as we shall see 

 when the origin and systematic relationships of the yeasts are taken up. 

 The autonomy of the yeasts and their incorporation as a group of 

 Ascomycetes have been demonstrated only since Hansen observed their 

 life cycles in nature and since certain investigators have given evi- 

 dence in the origin of the asc of certain yeasts, of the presence of 



