18/3.] Comparative Vegetable Chromatology. 471 



For equal weights of the fronds of Fucus serratus, in spring, I found :— 



«i ^}l X °\ Chlorofucine. Oiange Fucoxanthme. 

 chlorophyll. xantnophyll. 



In the shade . . 100 100 100 ' 98 



In the sun 79 87 90 100 



Change in Equilibrium due to the Action of Light. 



The principal conclusions to be drawn from the above facts are, that 

 leaves exposed to the sun contain nearly (or, at all events, more nearly) 

 the same quantity of the colouring-matters least changed by the action 

 of light as those grown in the shade, but that when grown in the sun 

 those substances which are more and more readily decomposed by the 

 action of light are more and more reduced in amount in the same order. 

 This difference in the equilibrium could be imitated artificially by so con- 

 triving that a certain quantity of each colouring-matter should be added 

 at a uniform rate to a solution exposed to the sun. In that case the 

 relative proportion in which they would remain would depend on the rate 

 at which they were introduced and the intensity of the action of the 

 light. Thus, making use of the same sort of rough illustration as that 



already adopted, we might have : — 



Added in a given time 10a + 10& + 10c 



Decomposed in the same time by a moderate 1 



light } 3a+2b+ •• 



Equilibrium for a moderate light 7a + 86+ 9c 



Decomposed by equal extra light 3a+ 26 + c 



Equilibrium for much light 4a + 66+ 8c 



In the above illustration the amount supposed to be added at a 

 uniform rate of course represents the quantity of each colouring-matter 

 formed in the leaves in the time required to produce the corresponding 

 decomposition. The results correspond so well that it seems to me a 

 good provisional hypothesis ; and the facts are of much interest, as 

 showing that very important changes occur in living plants in accordance 

 with the same laws as regulate the changes in the same colouring-matters 

 when treated as simple dead compounds in experiment tubes. 



Changes in Peltigera canina. 



I now proceed to a more complicated case. The fronds of Peltigera 

 canina contain at least seven different colouring-matters ; but in my 

 analyses I did not attempt to distinguish the two kinds of lichnoxanthine, 

 and therefore give them as united. The apothecia are coloured by these 

 latter substances, with little or no trace of any others. The results of 



2 o 2 



