CHLOROPHYLL AND ENERGY 199 



phyll; the internal environment, the arrangement or distri- 

 bution of the chloroplasts and perhaps the quantity of 

 diastases — in brief, the whole specific physiology — seem to 

 assume an unsuspected importance which makes comparison 

 impossible. Investigation therefore has to be confined to 

 leaves of the same species. 



The result of comparing the rate of assimilation of very 

 green leaves and of more or less etiolated leaves on the same 

 plant is rather surprising and would seem to indicate that here 

 again the quantity of chlorophyll is unimportant. Willstatter 

 and Stoll studied the rate of assimilation of normal and 

 etiolated elm leaves. For the same fresh weight the first 

 contained fourteen times more chlorophyll, but assimilated 

 only 1-2 times more, so that per unit area, the etiolated leaves, 

 weighing less, were a little more productive, but, more par- 

 ticularly, their chlorophyll was, at equal weight, nearly twelve 

 times more active. The conclusion must be, not that chloro- 

 phyll is useless, but that it is normally superabundant in the 

 cells and that the leaf possesses much more than is necessary. 



If its production is checked by making iron a hmiting 

 factor, chlorophyll is no longer superabundant and its influence 

 on the rate of assimilation becomes evident — the rate increases 

 in proportion to the quantity of chlorophyll. The same result 

 is obtained if the limiting factor is nitrogen, but not if it is 

 magnesium; this element does not seem to have the unique 

 function of forming part of the molecule of chlorophyll. 



The diastases collaborate with chlorophyll in assimilation 

 and its rate will be retarded by anything which hinders them 

 in their action. Their influence is still not very exactly known, 

 but it may be perceived fairly clearly in connection with light 

 and particularly heat. 



The Influence of Light 



The light intensity received by a leaf depends on its 

 position on the tree and on the angle J;hat its surface makes 

 with the sun's rays falling on it. The leaves at the top, except 

 at dawn and dusk, probably have enough fight, but many 



