302 
DARWINISM 
CHAP. 
The General Colour Relations of Plants. 
The green colour of the foliage of leafy plants is due to 
the existence of a substance called chlorophyll, which is 
almost universally developed in the leaves under the action 
of light. It is subject to definite chemical changes during 
the processes of growth and of decay, and it is owing to 
these changes that we have the delicate tints of spring 
foliage, and the more varied, intense, and gorgeous hues of 
autumn. But these all belong to the class of intrinsic or 
normal colours, due to the chemical constitution of the 
organism; as colours they are unadaptive, and appear to 
have no more relation to the wellbeing of the plants them¬ 
selves than do the colours of gems and minerals. We may 
also include in the same category those algae and fungi 
which have bright colours—the “red snow” of the arctic 
regions, the red, green, or purple seaweeds, the brilliant 
scarlet, yellow, white, or black agarics, and other fungi. 
All these colours are probably the direct results of chemical 
composition or molecular structure, and, being thus normal 
products of the vegetable organism, need no special explana¬ 
tion from our present point of view; and the same remark 
will apply to the varied tints of the bark of trunks, branches, 
and twigs, Avhich are often of various shades of brown and 
green, or even vivid reds or yellows. 
There are, however, a few cases in which the need of 
protection, which we have found to be so important an 
agency in modifying the colours of animals, has also deter¬ 
mined those of some of the smaller members of the vegetable 
kingdom. Dr. Burchell found a mosembryanthemum in 
South Africa like a curiously shaped pebble, closely resem¬ 
bling the stones among which it grew and Mr. J. P. Mansel 
Weale states that in the same country one of the Asclepi- 
adeae has tubers growing above ground among stones which 
they exactly resemble, and that, when not in leaf, they 
are for this reason quite invisible . 2 It is clear that such 
resemblances must be highly useful to these plants, inhabiting 
an arid country abounding in herbivorous mammalia, which, 
1 Burchell’s Travels, vol. i. p. 10. 
2 Nature, vol. iii. p. 507. 
