38 STUDIES IN PHYSIOLOGY 



soon becomes a daughter cell, closed off from the mothei 

 cell by a wall of cellulose. Meanwhile, one or more buds 

 may be forming on the outside of the daughter cells. If all 

 these cells cling together, a colony is formed which consists 

 of a mother cell (largest in size), one or more daughter 

 cells, and several tiny granddaughter cells. The individual 

 cells are easily separated from one another. This method of 

 reproduction is known as budding. 



Spore Formation is a second way in which yeast reproduces 

 itself. When conditions become unfavorable for further 

 growth, the protoplasm within each cell separates into spores 

 (usually four), and since these bodies are well protected by 

 thick spore walls, they may be blown about in the dust of 

 the air. If they happen to settle upon a surface that supplies 

 food and moisture, these spores develop into yeast cells. 

 For this reason, it is very difficult to keep sweet substances 

 from fermenting, unless they are heated to a high tem- 

 perature and then carefully closed from the air. Alcoholic 

 fermentation is always due to the action of yeast. 



Uses of Yeast. The action of yeast in bread-making will 

 be discussed in the next chapter. These minute organisms 

 are also of great commercial importance in the manufac- 

 ture of alcohol and of all kinds of liquors. We have 

 learned that yeast cells are found commonly in the air. 

 As different kinds of fruits ripen, they are usually more 

 or less covered with yeast or its spores. When, there- 

 fore, grapes are gathered and their juice is pressed out, 

 the sweet liquid is soon alive with the busy cells, and fer- 

 mentation begins at once. Wines are produced in this way. 

 In the so-called light wines the percentage of alcohol is 

 small (5 to 12%); in heavy wines it is 15 to 25%. Cider 

 is produced by the fermentation of apple juice. 



In the manufacture of beer and of other malt liquors, bar- 

 ley is commonly used. The grain is soaked and allowed to 

 sprout for a short time, until the starch is changed to grape 

 sugar. The barley kernels are then killed by heat to pre- 



