34 VEGETABLE HISTOLOGY. 



one species, but ordinary commercial yeast seldom consists of 

 this species alone. 



Sow some fresh baker's yeast in Pasteur's fluid and keep in a 

 warm place. As soon as the solution begins to froth and the 

 yeast is manifestly increasing in quantity it is ready for study. 

 Fermentation is most active between 28 and 34 C. At 

 38 C. growth ceases. 



Mount a drop of the liquid and examine with low power, 

 minute specks will be seen. With high power numerous 

 rounded or ellipsoidal bodies will be seen, either single or loosely 

 united into short chains. The diameter of the cells varies from 

 Vasoo to VTOOO incn (average, Vsooo inch). Each torula con- 

 sists of a well-defined homogeneous transparent sac or cell-wall 

 of cellulose material, enclosing a semi-fluid granular substance 

 called protoplasm, within which there is often a space full of 

 a more watery fluid than the rest, termed a vacuole. The whole 

 structure is known as a cell. The cell-wall is comparatively 

 tough, but may easily be burst and the contents thrown out ; it 

 is thicker in old cells than in young actively growing ones. 

 Minute shining dots, thought to be fat globules, may be seen 

 in the protoplasm, but there is neither chlorophyll nor starch 

 present. By the use of special reagents, a nucleus has been 

 demonstrated to be present in the cells, but it is never seen in 

 the living cells. 



Torulse break down sugar mainly into alcohol and carbon 

 dioxide gas, and at the same time increase in number. Multi- 

 plication takes place in this way. A 

 small protuberance begins to form on 

 the parent torula, which grows larger, 

 forming a bud. The bud increases un- 

 til it attains the size of the parent 

 torula and eventually becomes detach- 

 ed, though generally not until it has 

 developed other buds on itself arid these 

 still others. The torulse produced thus 

 by gemmation or budding are apt to 



adhere to each other for a long time and thus produce heaps 

 and strings (Fig. 27). 



Mount a drop of the yeast culture in a small drop of fuchsin 

 stain. Note which cells stain most rapidly and deeply. 

 Actively-growing protoplasm is not stained readily by many 

 dyes, while dead or passive protoplasm is colored quickly by the 

 same dyes. The cell-wall is unaffected and the vacuole also, 

 although the latter may appear purplish, because it is seen 

 through a colored layer of protoplasm. 



Make another mount and apply iodine solution at the edge of 

 the cover glass. As the iodine diffuses under the glass the 



