GROWTH OF THE YEAST PLANT 115 



reactions in that it is independent of hypothetical mechanisms 

 of growth. For it is obvious that if growth is an expression 

 of the activity of some catalytic agent, that agent must be 

 sought out ; according to Robertson * the lipoids may sub- 

 serve the required function. With regard to other observations 

 in this aspect of the subject, the work of Reed and Holland,! 

 who found that the rate of growth of the sunflower closely 

 approximates the course of an autocatalytic reaction, may be 

 mentioned. Reed J also observed that the rate of increase in 

 the height of walnut and pear trees showed growth cycles 

 throughout the growing season. In each cycle the growth 

 rate corresponded to an autocatalytic 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 in illus- 

 tration, more especially as a consideration of its reproduction 

 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 gemmatioa The phenomenon 

 may be illustrated by the accompanying Fig. 10 which is based 

 upon observations made by Slator.H This figure represents 

 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 dura- 

 tion 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 



* Robertson: "Arch. Entwicklungsmech. Org.," 1913, 37, 497. 

 fReed and Holland: " Proc. Nat. Acad. Sci.," 1919, 5, 135. 

 JReed: "Journ. Gen. Physiol.," 1920, 2, 545. 



See Drabble, E. and H., and Scott : " Biochem. Journ.," 1907, 2, 221. 

 ||Slator: Id., 1918, 12, 248. 



8* 



