396 TROPICAL NATURE vi 
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 
