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Mr. H. C. Sorby on 



[June 19, 



The fact of some kinds of leaves never turning red is analogous to what 

 occurs in the case of particular solutions of chlorophyll. It is not at all 

 improbable that in some cases certain kinds of erythrophyll may be 

 formed independent of chlorophyll. 



Waste and supply of the Colouring-matters in Plants. 



As is well known, when plants grow in the dark very little or no chlo- 

 rophyll and other colouring-matters are formed ; they are, as it is said, 

 blanched. Hence it appears that the production of such substances is 

 some direct function of the action of light. At the same time chlorophyll 

 when in solution is very rapidly decomposed by light, and the other 

 colouring-matters are similarly affected, but in less and varying degree. 

 The power with which they resist decomposition when in living plants 

 is very remarkable, and may in part be due to their not being in solution, 

 and to the unknown force which I have called constructive energy ; 

 but it appears to me very difficult or impossible to explain many facts, 

 unless we suppose that light does to some extent exert this decomposing 

 action on the colouring-matters, even when they exist in the leaves. 

 Perhaps, indeed, it may be one of the most important changes that take 

 place in them, and quite essential to plant life. It may seem strange 

 to suppose that any thing can be formed by the action of light and also 

 decomposed by it, but this corresponds with what does certainly occur in 

 artificial experiments. The red substances formed from chlorophyll by 

 the action of light are themselves afterwards decomposed by it, as I 

 have already described ; and if we suppose that chlorophyll and other 

 colouring-matters are in some way or other formed through the agency 

 of light from other constituents of the plants, and are afterwards decom- 

 posed by further action, we can easily explain a number of remarkable 

 facts. It appears to me that their development is some direct function 

 of light, until a certain quantity of each has been formed and a sort of 

 equilibrium established, varying in its character according to the parti- 

 cular plant, but that at the same time the amount of each decomposed 

 continues to increase directly as the intensity of the light ; so that the 

 equilibrium between the different substances is not the same for light of 

 different intensities, but, after having reached a maximum, the quantity 

 of each more or less decreases with increased light. This view of the 

 subject seems justified by such an experiment as the following : — I 

 selected two perfectly similar and contiguous leaves, nearly a year old, 

 growing in the month of May on the outside of an Aeuba japonica, 

 where much exposed to the sun, and covered part of one with black cloth. 

 After only a week the part of the leaf thus covered up and protected 

 from the fight had become most decidedly greener, and after three weeks 

 it was a moderately deep green ; whereas the part of the same leaf exposed 

 to the light was still, as at first, the same very yellow-green as the whole 

 of the other leaf which had not been covered up, the line of junction 



