PROTOPHYTA. 111 
and larger, and finally a cell-wall forms between the two, 
which sooner or later separate from one another (a and 3, 
Fig. 47). Under certain circumstances new plants form 
internally, as in ¢ 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 ,,¥1¢. 47,—Yeast-plants in various 
stages of growth, a and b. Ate 
* . and d new cells have formed by 
the formation of alcohol; hence internal cell-formation. a and b 
the growth of yeast-plants in a 7Agnified 400, ¢ and d magnified 
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. 
Practical Studies.—(a) Fill a strong bottle half full of active yeast, 
cork tightly, and keep for an hour or two in a warm room. Draw 
the cork and notice the violent escape of gas (carbon dioxide). 
(by Place a small drop of the yeast 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 the yeast-plants 
yellowish. Many of the latter will be found in process of budding, 
as in a and 4, 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 ¢ and d, Fig. 47. 
