PROTOPHTTA. HI 



and larger, and finally a cell-wall forms between the two, 

 which sooner or later separate from one another (a and b, 

 Fig. 4:1). Under certain circumstances new plants form 

 internally, as in c and d, Fig. 47. 



228. Yeast-plants are saprophytes, and live upon the 

 starch of flour. They break up 

 the starch, and in the process lib- 

 erate considerable quantities of 

 carbon dioxide, which appears 

 as bubbles upon the surface of 

 the yeast. Another result of 

 the breaking up of the starch is ^"*- 47.— Yeast-piants in various 



° '^ stages of growth, a and 6. At c 



the formation of alcohol: hence ?°'^ ** new ceils have formed by 



' "'="'^"= internal cell-formation, a and 6 



the growth of yeast-plants in a ^^um^. ^' ' *"** ^ mag^ifled 

 starchy substance is always ac- 

 companied by what is known as alcoholic fermentation. 

 The housewife and baker use yeast-plants for the carbon- 

 dioxide gas which they evolve, to give lightness to the 

 bread, while the brewer and distiller use the same plants 

 for the alcohol produced by their activity. 



Yeast-plants are now considered to be greatly reduced 

 Sac-fungi (p. 175). 



Practical Studies. — (a) Fill a strong bottle half full of active yeast, 

 cork tightly, and keep for au hour or two in a warm room. Draw 

 the cork and notice the violent escape of gas (carbon dioxide). 



(S) Place a small drop of the j'east upon a glass slide, add a little 

 water, cover with a cover-glass, tapping it down gently. After a 

 little examination under a high power of the microscope, add iodine, 

 which will stain the starch-grains blue or purple, and tlie yeast-plants 

 yellowish. Many of the latter will be found in process of budding, 

 as in a and b, Fig. 47. 



(c) Spread a half-teaspoonful of yeast on a fresh-cut slice of potato 

 or carrot; cover with a tumbler or bell-jar to keep it moist; after a 

 few days (4 to 8) examine for cells which are reproducing by internal 

 cell-formation, as in e and d, Fig. 47. 



