442 THE BIOLOGY OF FLOWERING PLANTS 



representing a period of steady increase, in the middle of 

 each curve may be due to the limits imposed on the supply 

 of material by the conducting system. Gregory (1921) 

 found the same mode of growth followed in the increase 

 in area of the leaves of Cucurbita in favourable conditions. 



Fig. 67. — Growth curves : left, growth of sunflower in height through 

 twelve weeks from data of Reed and Holland ; right, dry weight of root- 

 system of pea through seventeen days from data of Pearsall. 



The decline at the end we may here refer to the leaf 

 reaching maturity, or full size. But we must note that this 

 leaves unexplained what factors, presumably controlled 

 largely by the inheritance of the plant, set the limit. 



The S growth curve has been compared by Robertson 

 (1924) to that of an autocatalytic reaction (cp. Bayliss, 1920), 

 and the close agreement has been pointed out, too, by Reed 

 and Holland. Robertson holds that the rate of growth is 

 controlled by a catalyst produced by the organism. 



