1873.] Comparative Vegetable Chromatology . 469 



being sharply denned. I then took exactly equal quantities of the part 

 thus covered up and of the corresponding leaf which had remained the 

 whole time exposed to the light, and determined the relative quantities 

 of the chlorophylls, of the xanthophylls, and of a yellow colouring-matter 

 soluble in water, by the quantitative methods already explained. Taking 

 the amount of each at 100 for the leaf left in its natural exposed state, 

 the relative quantities in that kept shaded were as follows : — 



Chlorophylls. Xanthophylls. Soluble in aq. 



Natural state 100 100 100 



Shaded for three weeks .... 150 75 200 



Here, then, we see that the chlorophyll had increased 50 per cent.' 

 which is an increase of above 2 per cent, per diem ; and this, be it 

 remembered, in a leaf nearly a year old. The increase in the yellow 

 colour soluble in water was still greater, whereas the xanthophyll had 

 diminished. This alteration may be greater than common, because I 

 selected the plant on account of its being unusually sensitive to the 

 action of light. Shading a leaf in the manner described above is of 

 course quite a different thing to so smothering up an entire plant as to 

 cause it to become sickly and fade, or to preventing the development of 

 young leaves by keeping them in the dark. It seems to me that such a 

 striking difference, about which there could be no doubt, must prove 

 either that the absence of light is favourable to the development of chlo- 

 rophyll, which would scarcely agree with other well-known facts, or else 

 that the increase in its amount does in some measure indicate the quantity 

 formed also in the other, but decomposed in the same interval by exposure 

 to the sun. At all events the facts seem to prove that the equilibrium 

 betwean the different constituents of leaves can soon be changed by 

 altered conditions, as though, like the bodies of animals, the apparent 

 permanence were only an approximately uniform equilibrium between 

 construction and decomposition, the actual chemical changes being of 

 course entirely different in the two cases. We may very well believe that 

 the old and effete colouring-matters are thus destroyed, and that the 

 result of this constant change is that the endochrome in the leaves is 

 always in a young and vigorous condition. I use this term endochrome 

 to signify any kind of mixed colouring-matter found in the cells. 



Various comparative quantitative analyses. 



This change of equilibrium is fully established by a comparison of 

 leaves grown under different circumstances. The exterior leaves of 

 Acuba japonica, on the south side, where fully exposed to the sun, are 

 usually very much dappled with yellow patches almost free from chloro- 

 phyll, and even the green parts themselves are pale ; whereas the leaves in 

 the interior of the plant, where much shaded, have very few or none of 

 the yellow patches, and are of a dark green. There is a similar differ- 



tol. xxi. 2 o 



jt. 4 

 % 



