GROWTH OF LEMNA 



185 



Richards * also has studied the growth of yeast by ascer- 

 taining the increase in the number of cells and also by finding 

 the volume of the population per unit volume of 1/250 cmm., 

 the separation of the cells from the nutrient medium being 

 effected by the centrifuge with appropriate precautions and 

 a standardized routine. By both methods it was found that 

 the growth curve was asymmetrically S-shaped, attaining a 

 maximum yield within ioo hours at a temperature of 30 C. 

 At first the increase in volume is greater than the increase 

 in cell number, which is presumably due to the increase in 

 size of the individual cells before gemmation begins. After 

 15 hours the increase in the cell volume becomes slower until 

 it equals the increase in cell number at about 90 hours. The 

 complexity of the growth curve is due to a limiting factor, 

 possibly the accumulation of the products of the plant's me- 

 tabolism in a limited environment. 



This survey, incomplete though it be, shows the usefulness 

 of offspring as an index of growth f and emphasizes the im- 

 portance of various conditioning factors in the governance of 

 life processes. It must, however, be pointed out that the 

 number of offspring is not always a true index of growth. 

 This is shown in the accompanying table which relates to the 



growth of Lemna in a normal culture solution and in solutions 

 lacking various essential elements. J It will be seen that the 

 percentage increase in the number of plants is ever greater 



* Richards : '* Ann. Bot.," 1928, 42, 271. 



t A little thought will show that measurements of increase in length 

 and girth of those parts of plants exhibiting merismatic activity are in 

 reality measurements of offspring of the dividing elements. 



% Based on observations made by Messrs. Williams and Cole in the 

 laboratory of the authors. See also Ashby, Bolas and Henderson : " Ann. 

 Bot.," 1928, 42, 771. 



