34 TEXTBOOK OF BOTANY 



The protoplasm contains many small bodies or granules, some of 

 which often are yellowish in color ; these latter are drops of oil. There 

 is commonly also a large, round, clear space ; this is a, vacuole — a 

 part of the protoplasm which contains chiefly water with small amounts 

 of various substances dissolved in it, and which because of its watery 

 nature is clearer and more transparent than the rest of the protoplasm. 

 Sometimes, instead of a single large vacuole, a cell contains two or more 

 smaller ones. There is also a nucleus, which cannot be seen in the living 

 cell, but which can be made visible by careful staining of a killed cell. 



50. Food. — From the fact that the yeast reproduces so 

 rapidly in the molasses solution, we may conclude that it finds 

 there the substances necessary for its food. Since it has no 

 chlorophyl, it cannot make use of as simple food materials 

 as can Protococcus. We have seen that Protococcus can 

 build up sugars out of carbon dioxid and water. The yeast 

 cannot do this, so must obtain sugar or some similar food 

 ready-made. Sugar, of course, it finds in the molasses solu- 

 tion. But sugars contain no nitrogen, nor do they contain 

 certain other elements (especially sulphur and phosphorus) 

 that are needed in building up living matter. For this reason 

 the yeast could not long continue to live and grow in a pure 

 sugar solution — that is, in one that contained only sugar and 

 water. But in reality our molasses solution is not pure ; it 

 contains small amounts of substances other than sugars, and 

 these other substances supply the additional elements that the 

 yeast must have. 



51. Respiration. — Like all other hving organisms, the 

 yeast requires oxygen, which plays its part in breaking down 

 the living matter and other substances within the cell arid so 

 in furnishing a supply of energy. Like Protococcus and 

 most other organisms, excepting some bacteria, the yeast 

 must obtain oxygen from the air. It can, however, get along 

 for a time if air is excluded, and in this respect it resembles 

 those bacteria that can do without oxygen from the air; 

 but the yeast must have access to the air from time to time 

 if it is to continue to thrive. 



