4 YEAST. [CH. I 



yeast-containing fluid to a dilute solution of sugar in 

 spring water. The increase of the organism is visible 

 by the increased turbidity of the culture-fluid. With the 

 microscope it can be seen that the increase is due to a 

 process of budding, as shown in fig. 1. The cells begin to 

 bulge or swell in places and the buds so formed break off 

 and begin an independent life. They may however remain 

 attached for some time, and by a series of buds give rise 

 to the chains of cells shown in the figure 1 . 



Nutrition. 



When a small number of yeast plants increase so as to 

 alter the appearance of the fluid in which they float, the 

 fact that a quantity of new protoplasm and new cellulose 

 has come into being forces itself on the observer ; and the 

 question whence and how it has arisen must be met. 

 When an organism grows, the new organic material built 

 on to the old body comes from the food supplied. The 

 food diminishes, while the organism increases; one turns 

 into the other literally, and absolutely. Nearly half the dry 

 weight of cellulose is carbon, it is certain therefore that 

 the yeast has been supplied with carbon in some form in 

 which it can be used as food. In the laboratory carbon is 

 given to yeast in the form of sugar 2 : and if two jars are 

 prepared one (i) with, the other (ii) without, sugar, it will 

 be found that yeast increases rapidly in (i) but not in (ii). 

 In a similar way it can be shown that the increase in the 



1 Another form of reproduction occurs in yeast; it is not described 

 because it is not met with in ordinary cultures. 



2 The formula of Cane Sugar is C^H^Ou , of Grape Sugar C 6 H 12 6 . 



