396 TROPICAL NATURE 



these differ greatly from each other in their chemical com- 

 position. These inquiries are at present in their infancy, but 

 as the original term chlorophyll seems scarcely applicable 

 under the present aspect of the subject, it would perhaps 

 be better to introduce the analogous word chromophyll as 

 a general term for the colouring matters of the vegetable 

 kingdom. 



Light has a much more decided action on plants than 

 on animals. The green colour of leaves is almost wholly 

 dependent on it; and although some flowers will become 

 fully coloured in the dark, others are decidedly affected by 

 the absence of light, even when the foliage is fully exposed to 

 it. Looking therefore at the numerous colouring matters 

 which are developed in the tissues of plants, the sensitiveness 

 of these pigments to light, the changes they undergo during 

 growth and development, and the facility with which new 

 chemical combinations are effected by the physiological pro- 

 cesses of plants as shown by the endless variety in the 

 chemical constitution of vegetable products, we have no 

 difficulty in comprehending the general causes which aid in 

 producing the colours of the vegetable world, or the extreme 

 variability of those colours. We may therefore here confine 

 ourselves to an inquiry into the various uses of colour in the 

 economy of plants, and this will generally enable us to under- 

 stand how it has become fixed and specialised in the several 

 genera and species of the vegetable kingdom. 



Protective Coloration and Mimicry in Plants 

 In animals, as we have seen, colour is greatly influenced 

 by the need of protection from, or of warning to, their 

 numerous enemies, and by the necessity for identification 

 and easy recognition. Plants rarely need to be concealed, 

 and obtain protection either by their spines, their hardness, 

 their hairy covering, or their poisonous secretions. A very 

 few cases of what seem to be true protective colouring do, 

 however, exist, the most remarkable being that of the " stone 

 mesembryanthemum " of the Cape of Good Hope, which, in 

 form and colour, closely resembles the stones among which it 

 grows; and Dr. Burchell, who first discovered it, believes 

 that the juicy little plant thus generally escapes the notice 



