146 



PLANT BIOLOGY 



B. Yeast (Optional) 



154. Microscopical appearance and size of yeast. A small 

 piece of a cake of compressed yeast, mixed in a spoonful of water, 

 forms a milky fluid that is much like so-called bakers' or brewers' 

 yeast. If we examine with the microscope a bit of this mixture 

 in the same way in which the bacteria were studied, we find that it 

 consists of innumerable bodies of minute size. These are yeast 

 cells. Each cell is more or less egg-shaped, and is composed of color- 

 less protoplasm inclosed within a wall 

 of cellulose. By the use of special 

 stains, a nucleus becomes visible. 

 (The spherical dots seen in fresh yeast 

 cells are known as vacuoles and are 

 filled with a colorless. liquid.) Yeast, 

 like bacteria, is regarded as one of the 

 lowest forms of plant life. 



155. Reproduction of yeast. Most 

 of the cells that we are looking at 

 are not separate individuals, but are 

 FIG. 74. Yeast cells, showing strung together in little chains. This 



Sf.tT5Sit:JS fact leads us to a disc of the 



shown. Clear spaces are vac- method of reproduction of yeast. When 

 uoles. there is a sufficient supply of food, 



moisture, and oxygen, and when the temperature is favorable, 

 these living plant cells begin to feed and to grow. They soon reach 

 their full size, and then the cell wall is pushed out at the side by the 

 growing protoplasm. In this way a bud is formed. This continues 

 to grow and soon becomes a daughter cell, closed off from the mother 

 cell by a wall of cellulose. Meanwhile, one or more buds may be 

 forming on the outside of the daughter cells. If all these cells 

 cling together, a colony is formed which consists of a mother cell 

 (largest in size), one or more daughter cells, and several tiny grand- 

 daughter cells. The individual cells are easily separated from one 

 another. This method of reproduction is known as budding (Fig. 74) . 



