32 MORPHOLOGIC VARIATION 



of growth was roughly proportional to the concentration of the 

 medium, as was the final yield. 



Graham-Smith has contributed some valuable information on 

 the relationship between the concentration of foodstuff and the 

 growth of bacteria, though his observations are confined to the 

 later stages of growth and are therefore not useful in determining 

 the relative effects upon the lag and logarithmic phases. Figure 

 9 is copied from his paper, and shows curves for the number of 

 cells in five cultures in media containing graded concentrations of 

 meat extract. It will be noted that the maximum number attained 

 is proportionately greater with increasing concentrations of nutrient, 

 and also that growth is continued for a longer period with increas- 

 ing amounts of extract. Further experiments showed that after 

 growth had come to a standstill and the number of cells was de- 

 creasing, the addition of new meat extract in concentrated solution 

 led to the development of a new cycle of growth, and that this could 

 be repeated a number of times. By adding small quantities of ex- 

 tract in concentrated solution daily to the cultures, the death phase 

 could be postponed for some time, the number of living cells remain- 

 ing at a constant level. The addition of distilled water, diluting 

 the medium, decreased the number of cells per unit volume but not 

 the total number of cells in the flask. 



It would appear, therefore, that the concentration of available 

 foodstuff is a factor of prime importance in determining not only the 

 rate of growth but also the final yield, the duration of the growth 

 period, and possibly also the rate of death ; and that the concentration 

 of nutrients bears a reciprocal relation to the initial density of the 

 population in determining the form of the growth curve, in con- 

 formity with Pearl's "law of growth" stated at the beginning of this 

 chapter. 



Some information is available on the influence of various other 

 factors upon the rate of growth of bacteria. Salter found that the 

 inhibitory effect of crystal violet on B. coli was more pronounced 

 on young cells than old ones, and acted more in prolonging the lag 

 phase than in reducing the rate during the logarithmic phase. Cur- 

 ran found that a large series of compounds added in small quanti- 

 ties tended to prolong the lag phase. Graham-Smith similarly found 



