482 On Comparative Vegetable Chromatology. [June 19, 



kinds of xanthophyll is often rather difficult ; but in some cases^ at all 

 events, the relative quantity of the yellow xanthophyll is greatly reduced 

 in yellow varieties of leaves, so that, as in other circumstances, it seems 

 to vary in the same manner as yellow chlorophyll. This question, how- 

 ever, requires further examination. The very great relative amount of 

 the xanthophylls is readily explained by supposing that they are formed 

 when the constructive energy is too low to give rise to chlorophyll, and 

 that they may remain in faded leaves when the energy is too weak to 

 prevent its decomposition. 



Condition of Chlorophyll in Leaves, 



By carefully studying the position of the principal absorption-band in 

 the red, seen in the spectra of the leaves themselves, by means of a com- 

 pound prism of considerable dispersive power, and a bright dot, seen by 

 reflection from the upper surface, made to move over the spectrum by 

 a micrometer-screw, as proposed by Mr. Browning and modified by 

 myself, I find that there is every reason to believe that in normal healthy 

 green leaves the chlorophyll is chiefly in a free state. On boiling for a 

 short time in water, the oils or wax present in the leaf combine with the 

 chlorophyll, and raise the band somewhat further from the red end, 

 especially if the relative amount of chloropl^ll is small. Of course I 

 here refer to leaves which have little or no free acid in the juice to 

 decompose the chlorophyll into the well-known product of the action of 

 acids. When dissolved in the common fixed oils the band is still further 

 raised. The small quantity of chlorophyll in very yellow leaves appears 

 to be chiefly combined naturally with some oil, fat, or wax, so that the 

 spectrum resembles that of a green leaf that has been boiled. This may 

 perhaps be due in part to the total amount being so small in comparison 

 to that of the oil or wax, but at the same time it is interesting to find 

 that a relatively large quantity is thus combined in olive Algce ; and it 

 will be a subject for further inquiry to ascertain whether, when chloro- 

 phyll occurs in animals, it is not similarly combined, or even dissolved, 

 in a fat or oil. 



Taking, then, every thing into account, leaves which are normally very 

 yellow appear to be characterized by a very low constructive energy — so 

 low, in fact, that a plant could not live if all its leaves were of the 

 extreme condition of that type ; and this, like what was described in 

 connexion with Oscillator-ice, seems to point to the conclusion that those 

 classes of plants (like the red and olive Algce) which more or less 

 closely resemble these yellow leaves in certain important particulars are 

 lower in the scale as regards some peculiarity on which the production 

 of the colouring-matters depends, which, for want of a better name, I 

 have called constructive energy. We might thus speak of constructive 

 energy as being weak or strong in the same plant in different conditions, 

 or as being of a low or high type in different classes of plants ; and such 



