MODE OF GROWTH 441 



takes no further part in metabolism ; the ratio of leaf area 

 to dry weight becomes less. But more important is the fact 

 demonstrated by Kidd, West, and Briggs (1921), and Briggs 

 (19236) that the metabolic activity of the protoplasm, as 

 measured either by respiration or by assimilation, decreases 

 with age. Thus if we put the assimilating capacity of the 

 young leaf of a young sunflower at 100, the capacity of 

 a young leaf of a half-grown plant is only 40, and of a 

 young leaf of a full-grown plant is only 30. The active 

 metabolic matter is therefore less efficient. Towards the 

 end of development the rate of increase slows down still 

 more and the growth-time graph becomes a curve concave 

 to the abscissa axis. This final portion is probably connected 

 with the formation of reproductive organs, in which dissimi- 

 lation is relatively more important, and with a much 

 diminished formation of new leaves. 



The graph representing growth throughout a vegetative 

 period is therefore an S curve. Such a graph for the 

 growth of the sunflower, constructed from the data of 

 Reed and Holland (1919), is given in Fig. 67, with another 

 for the growth of pea roots from the data of Pearsall (1923). 



The same mode of growth is found if we examine indi- 

 vidual organs instead of the whole plant. Sachs (1887) 

 showed that the daily increment in length in a shoot of 

 Fritillaria increased up to the sixth day and slowly fell away 

 to the twentieth ; the same thing was true of a zone of tissue 

 just behind the root tip of the bean, the rate of growth 

 increasing up to the fifth day and falling to zero on the 

 eighth. He called this passing through a " Grand Period of 

 Growth," and it is of course just another expression of the 

 S graph. 



Priestley and Pearsall (1922) found that the whole root 

 system formed on cuttings of Tradescantia went through 

 a series of 3 growth curves. The depression at the end of 

 each corresponds to the time of formation of the roots of 

 next higher order, and is referred to the changes of meta- 

 bolism, especially increases in respiration, which take place 

 when the new meristems are laid down. The flat portion, 



