*873-J Comparative Vegetable Chromatology. 455 



properly developed ; but I have found by careful com- 

 parative quantitative analysis that, when plants are exposed 

 to more light than is requisite for their healthy growth, 

 the amount of chlorophyll and other colouring matters is 

 diminished sometimes to even one-third of the maximum 

 quantity. I have also found that, when a leaf is partially 

 covered up and screened from the light, the amount of 

 chlorophyll increases in the shaded part. In the case of a 

 leaf of Aucuba japonica, chosen for the experiment because 

 it is much influenced by light, the increase was no less than 

 at the rate of two percent per diem. Chlorophyll separated 

 from the leaves is rapidly decomposed by light, and it could 

 scarcely be supposed that a similar change would not to 

 some extent occur in the living plants. In fact the power 

 with which it then resists such a change seems to require 

 special explanation. The general connexion of all the facts 

 I have observed leads me to conclude that some, if not all, 

 the coloured constituents of growing leaves, like the 

 constituents of the bodies of animals, are in a constant 

 state of transformation, new being formed and the old 

 destroyed, the apparently uniform composition being due 

 simply to the establishment of an equilibrium, which 

 remains nearly the same when the conditions are the same, 

 but is very soon changed when they are altered. This sup- 

 position explains in a satisfactory manner many facts which 

 would otherwise be unintelligible, and probably one result is 

 that the endochrome* is thus constantly maintained in a 

 young and vigorous condition. According to this view of 

 the subject we may suppose that, in the above-named case, 

 when the amount of chlorophyll apparently increased at the 

 rate of two per cent per diem, the relative increase was 

 due, not to more being developed when the light was 

 excluded, but to more being decomposed in a corresponding 

 portion of leaf left exposed to the sun. The equilibrium of 

 the constituents was thus partially changed from that found 

 in leaves when growing much exposed to the sun to that of 

 leaves growing in the shade. 



On comparing the relative amount of the other con- 

 stituents of various plants, when more or less exposed to 

 the sun, I have found that equal weights of the leaves or 

 fronds contain almost the same amount of those colouring 

 matters which are the least changed by the action of light, 

 and that the relative quantity of the others in those leaves 



* I think it would be found very convenient to adopt this term as the name 

 for all kinds of simple or complex coloured substances found in the cells of 

 plants. 



