UNICELLULAR ORGANISMS 79 



A second type of reproduction sometimes occurs in some 

 species of yeast. Under conditions not yet clearly understood, 

 the contents of a yeast cell breaks up into two, three, or four 

 parts which become surrounded by thick walls; Fig. 32 s. These 

 are called spores, or ascospores, because held in an ascus (Gr. 

 ascus = sac) or sac, and eventually they are liberated by the 

 breaking of the sac. Each spore is tnen capable of starting a 

 new series of generations of ordinary yeast cells. The spores 

 can resist drying and therefore serve to protect the yeast from 

 adverse conditions. 



A comparison of Figures 30 and 31 will show that yeast and 

 Pleurococcus greatly resemble each other in structure; but there 

 is one important difference between them, for Pleurococcus is 

 green and yeast is colorless. This difference in color makes 

 a very great difference in their life; see page 131. Whereas 

 Pleurococcus may grow luxuriantly upon a fence post, and even 

 bare rocks, feeding upon the gases of the air, yeast is unable to 

 live and grow unless it is fed upon some organic matter, like 

 sugar. While yeast cells may be found widely distributed in the 

 air, in the soil, and in the water, they grow only where they find 

 organic food to eat, and chiefly in solutions containing sugar, like 

 fruit juices, etc. Elsewhere, in the soil or air, while they may be 

 alive, they are dormant. 



The chief function of yeast in nature is to convert sugars into 

 carbon dioxid and alcohol. Sugar is produced in great quanti- 

 ties by various fruits and vegetables, and is eventually attacked 

 by the numerous yeasts that are floating in the air. After the 

 yeasts have acted upon it, the sugar disappears and in its place 

 can be found a gas, carbon dioxid (COz), and a liquid, alcohol 

 (C 2 H 6 O) . This is called fermentation, and it is used extensively 

 in the fermentative industries which produce alcoholic beverages, 

 like beers, wines, ales, brandies, etc. The fermentation by yeasts 

 is also made use of in the raising of bread. The yeast growing 

 in the midst of bread dough produces bubbles of carbonic acid 

 gas which cause the- solid heavy dough to become light and 



