1873.] Comparative Vegetable Chromatology . 465 



classes of plants. On the whole they are very slowly changed by the 

 action of light. 



Chrysotannin Group. 



I have also described this in the above-named papers. The various 

 substances included in the group are all soluble in water, and of a more 

 or less pale yellow colour ; they are very slowly changed by the action of 

 light. The connexion between them and the different classes of plants is 

 still very obscure. They are interesting chiefly in connexion with the 

 production of the tints of autumnal foliage, some of which are due to 

 their oxidization. 



Comparative Chromatology. 



Having thus described the principal groups of colouring-matters, and 

 those particular species which appear to be of the greatest importance in 

 connexion with the present subject, I now proceed to the consideration of 

 their distribution. The individual species are the elementary coloured 

 constituents which exist in some and are absent from other plants, or 

 occur in a very different proportion, according to the kind of plant or 

 the conditions in which it grows. In my opinion it is only by a careful 

 qualitative and quantitative determination of the coloured constituents 

 that we can arrive at any satisfactory conclusions respecting them. Mere 

 general colour is altogether deceptive : it might lead us to conclude that 

 there was a complete difference in the colouring-matter, when there is 

 merely a difference in the relative amount of the same substances ; whilst, 

 at the same time, it might appear to indicate that two cases were alike, 

 when they really differ in most essential particulars. On the contrary, 

 by such a method of qualitative and quantitative analysis the exact 

 differences and the exact character of the connexion may be seen at a 

 glance ; and I cannot but think that by carrying out such principles our 

 knowledge of certain questions connected with plants will be very greatly 

 extended. The whole subject, however, is in its infancy ; very much re- 

 mains to be learned respecting nearly every thing connected with it ; and 

 what I now describe must be looked upon as very little more than a few 

 general conclusions, which appear to follow from what is already known. 

 One may fully expect that further research will much modify the ex- 

 planations which now seem the most plausible ; but it appears to me un- 

 desirable not to point out the general bearings of what is known, merely 

 because they may be modified, or even disproved, by what is still un- 

 known, since a mass of unconnected facts would be of very little interest. 



The subject of comparative vegetable chromatology may be divided 

 into two parts, viz. — first, that in which different specimens of the same 

 plant growing in different conditions are compared, in order to learn the 

 effects due to external agencies ; and, secondly, that in which different 

 plants growing in similar conditions are compared, in order to ascertain 

 the effects due to internal organization. 



