252 PHARMACEUTICAL BOTANY 



4. Species which ferment dextrose and maltose, but not saccha- 

 rose: Saccharomyces n. sp. obtained from the stomach of the honey- 

 bee. 



5. Species which ferment neither maltose, dextrose nor saccharose: 

 Saccharomyces anomalus var. belgicus, S. farinosus, S. hyalosporus, 

 S. membranifaciens. 



The two most important yeasts in the fermentation industries are 

 Saccharomyces ceremsice and Saccharomyces ellipsoideus. 



Saccharomyces cerevisia, commonly called Brewer's Yeast, is a 

 cultivated species with many strains. It is used extensively in the 

 brewing and baking industries and in recent years has met with 

 considerable esteem by the medical profession in the treatment of 

 certain skin diseases. 



When examined under the microscope it is found to be somewhat 

 spheroidal to ellipsoidal in outline, 8 to i2/x long, and 8 to lOyi* broad. 

 It consists of an outer cell wall of fungous cellulose enclosing cyto- 

 plasm and a nucleus, the latter invisible without special staining. 

 The cytoplasm is differentiated into a clear outer membrane lying 

 directly within the cell wall and termed the ectoplasm and an inner 

 granular region, the endoplasm. In the young condition of the 

 yeast cell numerous glycogen vacuoles are found scattered more or 

 less uniformly throughout this region but as the cell matures these 

 coalesce, until, in a very old cell, a huge glycogen vacuole may be seen 

 occupying most of the interior, with the cytoplasm and nucleus 

 pushed up against the cell wall and forming there a very narrow 

 layer. 



Yeast plants grow in dilute saccharine solutions containing dis- 

 solved nitrogenous substances such as beerwort, Pasteur's solution, 

 grape juice, etc. Here they are constantly wasting away and as 

 constantly being built up. The question may well arise: "How do 

 they obtain the material necessary for growth and repair?" . The 

 answer, in a general way, is not difficult. ' The fluid in which they 

 live is a solution of sugars and pf nitrogenous and other matters. 

 The cell walls are readily permeable. Food substances diffuse 

 through it into the cell, and by a series of changes (which, indeed, 

 it is no easy matter to understand) are converted into new living 

 substance. The waste products likewise diffuse readily outward. 



