338 



PLANTS WITHOUT CHLOROPHYLL 



lect enough of the gas in these bubbles to make a test, we find it is 

 carbon dioxide. 



Evidently something changed some part of the apple or grape, 

 namely, the sugar (C6H12O6), into alcohol (C2H5OH) and carbon 

 dioxide (CO2). This chemical process is known as fermentation. 

 Yeast. — If a small piece of compressed yeast cake is shaken 

 up with some molasses and water and the mixture allowed to 

 stand over night, a similar fermentation will take place. Exam- 

 ination of a drop of the settlings from the mixture shows that the 



common compressed yeast cake 

 contains millions of tiny yeast 

 plants. In its simplest form a 

 yeast plant is a single cell, 

 ovoid in shape and usually 

 containing one or more vacu- 

 oles. The nucleus is difficult 

 to find. Many of the cells 

 have others attached to them, 

 sometimes several in a row 

 (figure, page 223). Such a 



In three fermentation tubes were placed illustrates their method 



equal amounts oi a compressed yeast , 



cake. Then tube A was filled with dis- of reproduction by budding, 

 tilled water, tube B with a solution of a_ T?„r„T.»v»« /»««o«o ir^^^^^^ 



, J 4. J 4. ^^ n --t-x. An H/nzyme causes r ermen- 



glucose and water, and tube C with a •' 



nutrient solution containing nitrogenous tation. — It haS been proved 



matter as well as glucose. The quantity ^j^^^ -f g^ ^^^^^ ^^^ grOUnd 



of gas (CO2) m each tube is an index oi . "^ ^ 



the amount of growth of the yeast cells, up Until they are destroyed, the 



take'^piacef^' '^^^ ^^^ ^'^^*''* ^''''^^^ juice filtered from them is able 



to cause fermentation. Sim- 

 ilar experiments have been made with bacteria, showing that 

 enzymes formed within the cells cause fermentation. These en- 

 zymes are called zymases. 



Commercial Yeast. — • Cultivated yeasts are now supplied in 

 compressed or dried yeast cakes. In both cases the yeast plants 

 are mixed with starch and other substances and pressed into a 

 cake. The compressed yeast cake must be used fresh, as the yeast 

 plants begin to die rapidly after two or three days. The dried 

 yeast cake contains a much smaller number of yeast plants, but 

 is probably more reliable if the yeast cannot be obtained fresh. 



