136 



PLANTS WITHOUT CHLOROPHYLL 



Apparatus to show effect of 

 fermentation. N, molasses, 

 water and yeast plants ; C, 

 bubbles of carbon dioxide. 



Yeast causes Fermentation. — Let us now take a compressed 



yeast cake, shake up a small portion of it in a solution of mo- 

 lasses and water, and fill a fermentation 

 tube with the mixture. Leave the tube 

 in a warm place overnight. In the 

 morning a gas will be found to have 

 been collected in the closed end of the 

 tube (see Figure on page 138). The 

 taste and odor of the liquid shows 

 alcohol to be present, and the gas, if 

 tested, is proven carbon dioxide. 

 Evidently yeast causes fermentation. 

 What are Yeasts ? — If now part of 

 the liquid from the fermentation tube 

 which contains the settlings be drawn 

 off, a drop placed on a slide and a little 



weak iodine added and the mixture examined under the compound 



microscope, two kinds of structures will be found (see Figure below), 



starch grains which are stained 



deep blue, and other smaller 



ovoid structures of a brownish 



yellow color. The latter are 



yeast plants. 



Size and Shape, Manner of 



Growth, etc. — The common 



compressed yeast cake contains 



millions of these tiny plants. 



In its simplest form a yeast 



plant is a single cell. The 



shape of such a plant is ovoid, 



each cell showing under the 



microscope the granular ap- 

 pearance of the protoplasm of 



which it is formed. Look for 



tiny clear areas in the cells ; 



these are vacuoles, or spaces filled with fluid. The nucleus is hard 



to find in a yeast cell. Many of the cells seem to have others 



Yeast and starch grains. Notice that the 

 starch grains around which are clustered 

 yeast cells have been rounded off by the 

 yeast plants. How do you account 

 for this ? 



