34 MORPHOLOGIC VARIATION 



that the addition of acid and alkali to the medium tended to retard 

 growth during the earlier phases, though subsequently growth pro- 

 ceeded at the normal rate. These results may be explained by the 

 fact demonstrated by Sherman and Albus (1923) that "physio- 

 logically young" cells of bacteria are much more sensitive to various 

 injurious agents than are older ones. Sherman, Holm, and Albus 

 have shown that the accelerating effect of small quantities of neutral 

 salts added to the medium is due to both a shortening of the lag 

 period and an actual increase in rate during the growth phase. 



In considering the growth curves of bacteria, two main problems, 

 more or less interrelated, present themselves, viz: Why do the 

 organisms stop growing, and why do they not start to grow at once 

 with maximum rapidity when introduced into fresh medium? These 

 have both been the subject of much theorizing. 



Three explanations have been offered for the first problem: 

 prowth ceases because the foodstuff has been exhausted and the 

 organisms are starving; or because certain by-products of growth, 

 either excretory materials or specific "autotoxines" are injurious to 

 the organisms; or because actual physical crowding limits them. 



A little thought will show that this last theory is quite untenable. 

 When grown on the surface of agar, the organisms are more closely 

 crowded together at all stages of growth than is ever the case in 

 liquid media, and during the later stages are packed together as 

 closely as they can be, yet with practically all aerobic bacteria the 

 growth rate is higher on agar than in broth, and the final yield is 

 higher than is the case with an equal volume of broth of the same 

 nutrient value. Clark has shown that by growing yeast in a paper 

 filter with the wort continually added, the yield was practically as 

 high as that obtained when allowed to grow free in the wort, though 

 the yeast cells were actually packed in a solid paste. I have simi- 

 larly found that when yeasts are grown in a paper extraction thimble 

 through which they could not pass, suspended in a large flask of 

 dextrose broth, the total yield closely approached that in the control 

 flask where the cells were free to diffuse through the medium; a 

 similar experiment with colon bacilli contained in a collodion sack 

 did not give a yield so closely approaching the expected amount, 

 but still the actual density was much greater than when grown 



