456 Comparative Vegetable Chromatology. [October, 



exposed to the sun, decreases in the same order as they are 

 more and more rapidly decomposed by the action of light, 

 and in proportion as the leaves or fronds are exposed to more 

 and more light. There is thus established a sort of equi- 

 librium, varying with these different conditions, and easily 

 explained, if we suppose that the different coloured sub- 

 stances are being constantly formed by the internal con- 

 structive energy of the plant, and constantly decomposed 

 in varying proportion by the destructive action of the oxygen 

 of the atmosphere, intensified by the influence of light. 

 There are, however, well-marked exceptions to this rule, 

 which require us to suppose that the constructive force 

 varies qualitatively as well as quantitatively, when it is 

 much reduced by the absence of light or other causes, so 

 that some of the different compounds are formed in very 

 different proportions. The development of fructification 

 also sometimes produces a certain amount of alteration, as 

 though the colouring matter formed in the organs of re- 

 production were abstracted from the fronds. In the case of 

 the lichen Peltigera ca7iina, when it grows in a very damp 

 and shady situation, there is a greater relative deficiency of 

 certain colouring matters, which I have named lichno- 

 xanthine and orange lichnoxanthine, than seems likely to be 

 due to the decomposing action of light on the other con- 

 stituents of the specimens more exposed to the sun, and 

 the relative amount is again decreased by much more ex- 

 posure. On the whole it appears more probable that the 

 deficiency is mainly due to their imperfect development 

 when there is too little or too much light for the 

 healthy growth of the plant, and this fact is of much in- 

 terest, when we know they are the characteristic colouring 

 matters of the fructification, and that it is imperfectly, or 

 not at all, developed in very exposed or in very shady 

 situations, perhaps because these requisite substances are 

 not formed in sufficient quantity. I have also found that 

 there is a most remarkable alteration in the relative amount 

 of the different coloured substances characteristic of 

 Oscillatoricz, when the light is very feeble, evidently due to 

 the weak constructive energy, as will be more fully considered 

 in the sequel. 



Having thus learned what is the character and the extent 

 of the changes produced by varying conditions on the 

 colouring matters found in the same, or in closely allied, 

 species of plants, we are in a better position to understand 

 the variations corresponding to the difference in the general 

 organisation of different classes, and to distinguish and 



