146 LEAF, COLOURATION AND FALL OF 



seems to be shown by the nature of Evergreens, which 

 we know are hardy plants, in one sense of the word, 

 and are able to keep up a degree of vegetative vigour, 

 when others do not, or cannot, although they are often 

 unable to bear the same degree of cold which these plants 

 may, in consequence of their dormant, or as it were, 

 hybernating condition. In Evergreens we often find a 

 discolouration taking place during winter, and the leaves 

 becoming green again on the return of activity in the 

 spring. 



6. According to Berzelius, the yellow colour in the au- 

 tumnal tint of leaves has no connection with the yellow 

 colour, or anthoxanthin, of the coloured parts at this or 

 other periods, but is a peculiar matter between fatty oil 

 and resin, which he calls xanthophyll ; and says, in regard 

 to its origin, " We have every reason for believing, since 

 the green colour disappears and a yellow takes its place, 

 that the xanthophyll has its origin in the green by 

 means of a change in the organization of the leaf, pro- 

 duced by the cold, and thus giving rise to altered or- 

 ganic processes. The brown colour has likewise no 

 connection with the yellow, and the different propor- 

 tions in which this brown occurs with the yellow, give 

 rise to the diversity of tints in autumnal colours." 



FALL OF THE LEAF. 



7. Various opinions have been given upon the sub- 

 ject, but the causes of this phenomenon have been con- 

 sidered as too much dependent upon individual circum- 

 stances. 



8. It does not appear to me that it takes place from 

 the derangement of any particular structure or function 

 producing deatli of the part, and then its separation 

 from the healthy ones, but that leaves possess, as Vro- 

 lick believed, a peculiar life, in which various periods 

 may be distinctly marked ; and that when they fall 

 they have attained their greatest age, the whole func- 



