564 Introduction to the Study of Science 



These small bodies are yeast plants, microscopic organisms 

 that live and thrive in such conditions as have been provided. 



Growth and reproduction. The living, growing yeast plant 

 is a single-cell organism, with a nucleus or " knot " near one 

 end, and the whole enveloped in a cell wall of cellulose. As 

 the cell grows, it develops within it one or more small drops of 

 an oily substance, or " cell sap " ; these drops are called 

 vacuoles. The cells also begin to multiply by a process known 

 as budding. The first appearance of a new cell is as a bud on 

 the side of the parent plant, where it increases until it is fully 

 grown. It may remain thus attached in a time of rapid growth 

 or it may separate. Either the parent cell or the new cell may 

 produce a new bud, and this process continues with the plants 

 appearing in oddly formed groups as long as the nutritive con- 

 ditions are favorable. If the conditions become unfavorable, 

 the plants separate and become inactive. When favorable 

 conditions are restored, the plants resume active growth and 

 multiplication. The inactivity of the yeast may be due to a 

 lack of nutritive material or to toxic (poisonous) waste products, 

 which the plants themselves have thrown off. But an inter- 

 esting fact is that just when this condition is brought about, 

 another variety of plant begins to develop and produce fermen- 

 tation of a different character. The dead yeast plants may be 

 distinguished easily from the active and resting plants by their 

 opaqueness. 



Size of yeast plants. Yeast plants are much smaller than 

 molds, so minute, in fact, that a hundred or more placed side 

 by side would hardly equal the thickness of the paper upon 

 which this is printed. The size varies in different conditions, 

 such as the nature of the food solution and the temperature. 

 But the size is not an important item in identifying-* yeasts, 

 molds, or bacteria. A compressed yeast cake is made up of 

 yeast plants, perhaps several million, and may have a small 

 percentage of starch to dry and preserve it. The yeast is 

 alive, but inactive, as it cannot grow under such conditions. 



