246 



GENERAL BOTANY 



at the proper temperature, new plants are formed in almost 

 incredible numbers in a comparatively short time when yeast is 

 placed in a favorable medium, such as bread sponge or beer wort. 

 When young, vigorous yeast cells are grown at the proper tem- 

 perature and moisture but are lacking in an adequate food supply, 

 they sometimes form internal spores (Fig. 133) by division of the 

 nucleus and aggregation of the cell cytoplasm around each new 

 nucleus. From 4 to 8 spores are usually formed in each cell, 

 each spore consisting of cytoplasm and nucleus surrounded by a 

 cell wall. Finally the mother-cell wall breaks down, and the lib- 

 erated spores form new 

 yeast plants. The proc- 

 ess of spore formation 

 in yeast is apparently 

 designed to preserve the 

 yeast during inclement 

 conditions, rather than 

 as a method for more 

 rapid reproduction or 



The figure shows how a cell group (d) may be dissemination, 

 formed in actively growing yeast if successively 



formed buds remain attached. After Conn Fermentation. The 



importance and interest 



of yeasts for man is due largely to their power of causing alco- 

 holic fermentations in sugar solutions, since it is upon this power 

 that the modern baking and brewing industries are based. 



If yeast plants, in the form of commercial yeast cakes or in 

 a free state, are placed in a sugar solution under the proper 

 temperature (25-30 C.), bubbles of carbon-dioxide gas will 

 soon rise to the surface of the liquid, indicating that the process 

 of fermentation has begun. Proper chemical tests will also 

 reveal the presence of alcohol, which is formed in the fermenting 

 solution at the same time as the carbon dioxide. It has been 

 found that the carbon dioxide and alcohol are produced by 

 the splitting up of the sugar molecule by the yeast ferment 

 called zymase. 



The carbon dioxide which rises to the surface is utilized in 

 the process of bread-making to make the bread light, while the 



FIG. 132. Growing yeast with buds and 

 cell chains 



