CHAPTER V 



ORIGIN OF THE YEASTS, THEIR POSITION IN CLASSI- 

 FICATION OF THE FUNGI AND THEIR 

 SYSTEMATIC RELATIONSHIPS 



LET us now consider the morphological, cytological, and physi- 

 ological characteristics of the yeasts. It is interesting to con- 

 sider the place which they hold in the classification of the fungi. 

 It has been stated, at the beginning of this book, that the sporangium 

 of the yeasts is comparable to the asc in the Ascomycetes. It now re- 

 mains for us to discuss the reasons for wishing to incorporate the 

 yeasts under the Ascomycetes. Although this is definitely settled to- 

 day, this question has been the object of such polemics that they are 

 worthy of our attention. 



(A) Historical 



The question of the position of the yeasts in classification of the 

 fungi has remained unsolved for quite a period of time. Do the yeasts 

 make up an autonomous species or do they simply represent a stage 

 in the development of the filamentous fungi, more advanced, which 

 exist during the fruit season as yeasts, and during the winter as myce- 

 lial fungi? It is easy to observe the different stages in the life history 

 of yeasts, the stages of budding and sporulation; but it has riot been 

 shown that the culture in artificial media presents the whole life cycle 

 and that it may not be more complex in nature. Thus, we have seen 

 in the early part of this book that many fungi present yeast-like 

 structures during some stage in their life cycles. Such is the question 

 that arose in the days when Pasteur worked and which ought to be 

 answered in our day. 



The subject is rendered more complex by the fact that little is 

 known about their origin and life cycles. It is known that beer 

 yeast has been handed down from brewery to brewery from time im- 

 memorial, and that other industrial yeasts used today may have their 

 beginning in early Egyptian history. The domestic yeasts by con- 

 tinued cultivation by man may have been reduced to a constant form 

 of a yeast. Such is not the case with wine yeasts. These exist 

 naturally on the surface of the grapes and it is only necessary to 

 press out the juice which will soon ferment. But where does this 

 yeast come from which is on the surface of the grape? 



131 



