THE FUNGI 



245 



Structure. The yeasts are composed of minute spherical or 

 slightly elongated cells varying from ^ ^ o" to loTTo ^ an mc ^ 

 in diameter. They usually occur as single cells, but may adhere 

 to form cell chains or loose temporary colonies. The yeast cell 

 (Fig. 131) has the same general structure as that of plant cells 

 with which we are familiar. The cell wall is a thin membrane 

 composed of two or more layers, and the cytoplasm is filled with 

 granules of various sizes, 

 some of which are sup- 

 posed to be composed of 

 fatty material, probably 

 useful as a reserve food. 

 The nucleus can only 

 be observed in material 

 which has been specially 

 stained for that purpose, 

 and is found in close 

 proximity to the large 

 central vacuole which 

 fills the greater part of 

 the cavity of the mature 

 yeast cells. 



Reproduction. If yeast 

 plants are placed in a 

 sugary solution, such as 

 beer wort, at a temperature of from 25 to 30 centigrade, they 

 begin to reproduce new plants by the process known as bud- 

 ding. Budding consists in an outgrowth, or protrusion, of the 

 cell wall of the yeast cell into which the cytoplasm flows, thus 

 forming a new cell. The outgrowth is very minute at first, but 

 by growth it finally reaches the size of the mother cell from 

 which it grew. Finally a cell wall is formed between the mother 

 cell and the bud, and the two cells split apart, thus forming two 

 new yeast plants (Fig. 131, d and e). Division of the nucleus 

 accompanies the budding process in such a manner that each new 

 bud which is formed into a new yeast plant is furnished with a 

 nucleus. Since the budding process goes on with extreme rapidity 



FIG. 131. Yeast cells highly magnified 



a and b, showing vacuoles and granular cyto- 

 plasm; c, showing nucleus; d, budding yeast cell 

 with dividing nucleus; e, the bud cut off from 

 the mother cell with its portion of the nucleus. 

 After Conn 



