180 GROWTH 



var. condensa, Cucumis Melo, C. Sativus, and Lyopersicum 

 esculentiim. 



On the other hand, van de Sande-Bakhuyzen * considers 

 that the formula is but an approximation, and that it is in- 

 correct to regard growth as a simple autocatalytic process, 

 whilst Hooker,f considers that growth is best considered as a 

 consecutive reversible monomolecular reaction. 



Returning to the terms in which increase in growth may 

 be expressed, allusion has been made to offspring especially 

 of unicellular organisms in which the unit grows to a certain 

 size and then divides. The yeast plant may be selected for 

 illustration, more especially as a consideration of its repro- 

 duction rate will introduce other aspects of the subject of 

 growth. 



A young yeast cell on being placed in a suitable medium 

 increases to a certain size, the magnitude of which depends to 

 no small extent upon the osmotic strength of the medium, % 

 and then reproduces itself by gemmation. The phenomenon 

 may be illustrated by the accompanying Fig. II which is 

 based upon observations made by Slator.§ This figure repre- 

 sents the offspring of a single cell up to and including the fourth 

 generation. The cycle was completed in 232 minutes from 

 the second generation, the average time for the interval between 

 one generation and the next, that is the generation time, being 

 seventy-six minutes. 



Elaborating this general statement, the growth of the yeast 

 exhibits a sequence of phases the conspicuousness and duration 

 of which varies with the conditions. The " seed " on being 

 sown in the wort may remain inactive for a time ; this is the 

 lag phase, the duration of which depends in the main on the 

 age of the seed, old plants showing a longer quiescent period 

 than young plants grown from spores which may show no lag 

 phase. When once growth has started, it is unrestricted and 

 follows the logarithmic law, for which reason this phase often 



* Van de Sande-Bakhuyzen : " Science," 1926, 64, 653. 

 t Hooker : " Proc. Nat. Acad. Sci. Wash.," 1925, n, 710. 

 % See Drabble, E. and H., and Scott : " Biochem. Journ.," 1907, 2, 

 221. 



§ Slator : id., 1918, 12, 248. 



