136 THE BIOLOGY OF FLOWERING PLANTS 



extent. This may be illustrated by Willstatter's results 

 for Quercus Robur given in Table XXVIII. 



TABLE XXVIII 

 Change in Assimilating Capacity of the Oak 



The rate of assimilation for unit quantity of chloro- 

 phyll increases up to a point, and then decreases till maturity 

 is reached. This does not cover the complete history of 

 the leaf. It -would be necessary to begin with a stage having 

 no chlorophyll. This has been done for seedlings of the 

 barley by Miss Irvine (1910), and of Phaseolus, the French 

 bean, and other plants by Briggs (1920, 1923). 



The remarkable fact -was brought out that in Phaseolus, 

 for example, for about ten days after the first leaves have 

 unfolded, assimilation has a low value even when the leaf 

 is fully green. By ingenious experimental methods Briggs 

 maintained the chlorophyll content of the leaves at a constant 

 low value for eleven days, and showed that, during this 

 time, the rate of assimilation constantly rose ; that is, 

 assimilation increased though chlorophyll did not. Some 

 factor in the process, other than chlorophyll, is therefore 

 increasing during this early stage of development. The 

 increase in this factor depends on the supply of food sub- 

 stances, for in the sunflower, the marrow, and the maple, 

 in which the food-store of the seed exists in the cotyledons, 

 which are also the first assimilating organs, assimilation 

 starts in these at a high value as soon as chlorophyll is 



