487 



relatienships, being so far foaud only in particular classes of plants. 

 Fourth, the chrysotannic group, containing a considerable number of 

 yellow colors, some so pale as to be nearly colorless, and others of a fine, 

 dark, golden-yellow. They are soluble in water, in alcohol, and in ether, 

 but not in bisulphide of carbon. Of these there are two sub-groups, 

 one in which a dark color is produced with ferric salts, constituting the 

 tannic acid sub-group, and the other giving no such reaction, and form- 

 ng the chrysophyl sub-group. In both sub-groups the intensity of 

 color is usually greatly increased by partial oxidization, and they are 

 thus alterexl into colors of the following group. Fifth, the xjhaiophyl 

 group, which comprises a number of colors insoluj3le in bisulphide of 

 carbon, and of very variable solubility in water or alcohol. These are in 

 that state of oxidization which has a maximum intensity of color, and are 

 simply decolorized by further oxidization. Our author i^roceeds to state 

 that the numerous tints of foliage depend almost entirely on the relative 

 and absolute amount of the various colors of these different groups, 

 although all their relationships cannot at present be explained. 



The color of green leaves is mainly due to a mixture of chlorophyl 

 and xanthophyl, and the variation in the relative and absolute amount 

 of these easily accounts for the darker and brighter greens. The tints 

 are also much modified by the presence of colors of the erythrophyl 

 group, which, according to circumstances, may give rise to lighter or 

 darker browns, approaching to black or to reds. Healthy unchanged 

 leaves also contain various substances belonging to the chrysotannic 

 group; but in many cases when these belong to the more typical kinds 

 of tannic acid, their color is so faint that they have little or no infiuence 

 on the general appearance of the leaves. 



On the approach of autumn, before the leaves have withered, the 

 foliage of difi^erent plants presents an exceedingly variable mixture of 

 chlorophyl, xanthophyl, and erythrophyl, with the diliereut members 

 of the chrysotauniu group, and it is to the changes which occur in some 

 or all of these substances that the very variable tints of autumn are 

 due. The most striking of these depend on the alteration of the chlo- 

 rophyl. So long as it remains green the production of the bright reds 

 and yellows is impossible ; but when it disappears, the yellow color of 

 the xanthophyl is made apparent, and if much erythrophyl be present 

 or contemporaneously developed, its color, combined with this yellow, 

 gives rise to scarlet or red. In many cases, however, the chlorophyl 

 does not disappear, but is changed into the dark olive modification, 

 easily prepared artificially by the action of acids on the more green 

 shades ; and when this is present, only dull and unattractive tints can be 

 produced. We may thus easily understand why the special tints of 

 early autumn are yellows and reds, or dull and dark greens. In these 

 changes the various pale-yellow substances of the chrysotannin group 

 remain comparatively unaltered, and even sometimes increase in quan- 

 tity, but they soon pass into the much darker red-browns of the piiaio- 

 phyl group, while the erythrophyl fades, and thus later in the autumn 

 the most striking tints are the brighter or the duller browns, character- 

 istic of the difierent kinds of plants or trees. 



As far as we are able to judge from the various facts described above, 

 we must look, according to Mr. Sorby, upon the more characteristic 

 tints of the foliage of e<\rly spring as evidence of the not yet matured 

 vital powers of the plant. In summer the deeper and clearer greens 

 are evidence of full vigor and high vitality, which not only resist but 

 also actually overcome the j^owerful affinity of oxygen. Later in the 

 season the vital powers are diminished, and i^artial changes occur ; but 



