MECHANISM OF COMPETITION IN YEAST CELLS 67 



generally speaking, with the factors limiting growth. Therefore the 

 equations of the struggle for existence ought to be expressed in tern^s of 

 masses of the species concerned and not in terms of the numbers of 

 individuals, which are connected by more complex relations with the 

 factors limiting growth. 



In order to pass on from the number of yeast cells of the first and 

 second species counted at a definite moment to the masses of these 

 species, we must take into account that: (1) the cells of the first 

 species differ in their average volume from those of the second, (2) 

 this average volume of the cell in each species can change in the course 

 of growth of the culture. (Richards ('28b) showed that the average 

 size of a cell of Saccharomyces cerevisiae is different at different stages 

 of growth), and (3) the species can be of different specific weight. 

 Therefore, by multiplying the volume of all the cells of a definite 

 species at a given moment of time by their specific weight, we shall 

 obtain the weight of the given organisms enabling us to judge of their 

 mass. Assuming for the sake of simplification that the cells of our 

 yeast species are near to one another in their specific weight, we can 

 measure the volumes occupied by each species of yeast cells in order 

 to obtain an idea of the masses of these cells. 



(6) The volume of yeast was determined by the method of centrifu- 

 gation. The fluid from the test tubes or flasks with the counted number 

 of yeast cells was centrifuged for one minute in a special tube placed 

 in an electric centrifuge making 4000 revolutions per minute (usually 

 in portions of 10 cm 3 each). The liquid was then poured off and the 

 yeast cells that had settled on the bottom were shaken up with the 

 small quantity of the remaining liquid. The mixture thus obtained 

 was transferred by means of a pipette into a short graduate glass tube 

 of 3.5 mm in diameter. The mixture in the graduated tube was 

 again centrifuged for 1.5 minutes, and then the volume of the sedi- 

 ment was rapidly measured with the aid of a magnifying glass. To 

 avoid errors connected with the different degree of compression of the 

 yeast in different cases, the quantity of the mixture poured into the 

 short graduated tube was always such that the sediment did not 

 exceed ten divisions of the graduated tube and, if necessary, the sec- 

 ondary centrifugation was made by several doses. The volume of 

 yeast occupying one division of the graduated tube was taken for a 

 unit. 



The centrifugation method may be criticized as, according to 



LjIubrary 



ysra* 



