THE PROCESS OF ASSIMILATION 139 



readily observe that the growth of the " aurea " elders is 

 about as vigorous as that of the green forms. This is 

 true only for high light intensities ; for low intensities the 

 assimilation of the yellow leaves is relatively much poorer — 

 they are less fitted for work in poor hght, and it is possible 

 that here the chlorophyll is working at maximum capacity 

 and limits the rate. 



This gives us some idea of the enormous capacity for 

 assimilation which the chlorophyll present in the ordinary 

 plant would develop if the reactive surface of the plastid 

 were greater. In fact, the chlorophyll content of the ordinary 

 leaf may be regarded as enabling it to make the best of the 

 low light intensities with which it must often work ; it is 

 far greater than is necessary in good illumination. 



The similarity of the assimilation numbers within one 

 class of leaves seems to indicate, however, a close connection 

 between amount of chlorophyll and assimilating powers ; 

 against this is the existence of two classes of plants with 

 exactly the same sort of chlorophyll, yet the one with 

 assimilation numbers twice as large as the other. It is 

 possible that the similarity within a class is due to the 

 association of a definite amount of assimilating plasma 

 (plastid substance) with a unit amount of chlorophyll. The 

 chlorophyll would then be an indication of the amount of 

 plasma, or enzymes, active in assimilation, and this would 

 be the relation expressed by the assimilation numbers. 

 The difference between the two classes might be due to a 

 fundamental difference in the type or extent of reactive 

 surface developed in the individual plastid. 



§ 16. External Conditions and Assimilation 



The chlorophyll acts as an absorber and transformer of 

 light energy ; whether it takes any part in the ensuing 

 chemical changes we do not know. It is clear, in any case, 

 that it is not the chlorophyll alone that is effective ; the 

 protoplasm or its products must also be active. We have 

 seen how these two agents come into play, though we do not 



