114 GROWTH AND WORK OF PLANTS 



living substance, and the carbon is replaced in the ordinary* res- 

 piration of plants by the carbon which is obtained from the 

 carbon dioxide absorbed in the making of sugar and starch. 



192. Fermentation of yeast. f Yeast as used in bread- 

 making to produce the " rising" of the bread, causing it to 

 become spongy and light, is composed largely of microscopic 

 plants mixed in with some food substances, etc. This micro- 

 scopic plant is called the yeast plant or yeast. It is a very inter- 

 esting plant, and the work it performs in bread " rising" is very 

 instructive in connection \vith this subject of respiration. 



The tube and the lower part of the bulb of a fermentation 

 tube is filled with a solution of grape sugar or an infusion of 

 potato, $ and a small piece of yeast cake is put in the bulb. This 

 is kept in a warm place. As the yeast plant grows, bubbles of 

 gas will rise in the tube and displace the solution which is forced 

 into the bulb. If some pieces of caustic potash are placed in the 

 bulb, the potash solution absorbs the gas in the tube and the 

 solution rises again. This shows, as we have learned above, 

 that the gas is CO 2 which is an index of the respiration of 

 the yeast plant, or rather of the fermentation of the sugar in the 

 solution. 



This fermentation has gone on largely in the absence of air, 

 but it is somewhat at the expense of growth and reproduction of 

 the yeast plant, for when air is present growth and multiplication 

 of the yeast is more rapid. During the fermentation of the 

 sugar by the yeast, alcohol is also formed. These two products 

 of fermentation by the yeast plant are of great importance. The 

 evolution of the gas carbon dioxide in bread making forms 



* By ordinary respiration is meant aerobic respiration. Aerobic respi- 

 ration is carried on by plants which require air. Anaerobic respiration is 

 respiration in the absence of air. All plants can carry on anaerobic res- 

 piration for a short time, but most plants are injured and soon die or rest 

 if air is not accessible. Some plants are active either as anaerobes or 

 aerobes (the yeast plant), while others only as anaerobes (certain bacteria). 



f Saccharomyces cervisese. 



} If possible it is well to sterilize the tube after filling with the grape sugar 

 or infusion. Also a second tube for a check is desirable. 



