302 DARWINISM chap. 



The General Colour Belations 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, which 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 membei's of the vegetable 

 kingdom. Dr. Burchell found a mesembryanthemum in 

 South Africa like a curiously shaped pebble, closely resem- 

 bling the stones among which it grew ;^ and Mr. J. P. IVIansel 

 Weale states that in the same country one of the Asclepi- 

 ade?e has tubers growing above ground among stones which 

 they exactly resemble, and that, when not in leaf, they 

 are for this reason quite invisilJe.^ It is clear that such 

 resemblances must be highly useful to these plants, inhabiting 

 an arid country abounding in herbivorous mammalia, which, 



^ Burchell's Travels, vol. i. \\ 10. 

 2 Kature, vol. iii. p. 507. 



