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Show 302 DARWINISM OHAl'. The Geneml Colou1· 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 ~t is owing. to these chan(~es that we have the dehcate tmts of sprmg foliao-e anl the more varied, intense, and gorgeous hue. of ttutu~~. But these all belong to tho class of intrin. ic or· normal colours, dne to the chemical constitution of tbe organism; as colours they are um1daptive, and appear to have no more rela.tion to the wellbeing of the plants themselves than do the colours of gems and minerals. We m<ty also include in the same category those algro and fun gi 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, aud other fun gi. All these colours arc probably the direct results of chemical composition or molecular structure, and, being thus normal products of the vegetable organism, need no special explanation from our present point of view ; and the same remark will apply to the varied tints of the bark of trunks, bran ches, and twigs, which arc 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 :dso eldermined those of some of the smaller members of the vegeta1)lc kingdom. Dr. Burchell found a mescmbryrwthemmn i11 South Africa like a curiously shaped pchhlc, closely res<'lllhling the stones among which it grew; 1 and Mr. J.P. 1\lansel \Veale states that in the same country one of the Ascl<·piadere has tubers growing above gronncl among ston eR \\'lrid1 they exactly resemble, and that, when not in loaf, they are for this reason quite invisible.2 It is clenx that sneh resemblances must be highly useful to these pl:urts, inl rahit.ing an arid country abounding in herbivorous mammc.tl in, ·wh ich, 1 Burchell 's Tm1:els, vol. i. p. 10. 2 Nctt~we, vol. iii. p. 507. XI THE SPECIAL COLOURS OF PLANTS 303 in times of drought or scarcity, will devour everything in the shape of a fleshy stem or tuber. . True J?~micry is very rare in plants, though adaptation to hkc ~oncht1?r~s often produces in foliage and habit a similarity that IS decmvu~g. Enp~orbias growing in deserts often closely r~s?mble cacti. Seaside plants and high alpine plants of different orders arc often much alike; and innumerable resemblances of this kind arc recorded in the names of pl.ants, as Veronica cpacricl.ca (the vcr?nica like an epacris ), Limnanthcmum nymphromclcs (the hmnanthcmum like a nymphroa), the resembling species in each case bclonainr:r to totally distinct families. But in these cases, and in most others that have been observed, the essential features of true mimicry are absent, inasmuch as the one plant cannot be suppo.-ed to derive any benefit from its close rescm blanco to the other and this is still more certain from the fn.ct that the tw~ species usnally inhabit differoJJt localities. A few cases exist however, in whic·h· there docs seem to be the nccessar; accordance and utility. Mr. Mansel W calc mentions a labiate plant (Aju~a oph:ydi.s), th.c ?nly SJ~ocies of the genus Ajuga in South Afnca, whiCh Is str1lnngly hke an orchid of the same country; while a balsam (Impatiens capcnsis), also a solitary species of the genus in that country, is equally like an orchid growing in the same locality and vi itcd by the same insects: As both these genera. of plants arc specialised for insect fertilisation, and both of the plants in question arc isolated species of their respective genera, we may suppose that, when they first reached South Africa they were neglected by t?e i~sects of the country; hut, being both remotely like orchids m form of flower, those varieties that approached nearest to the familiar species of the country were visited hy insects and cross-fertilised, and thus a closer resemblance wonld at length be brought about. Another case of close general resembla.nce, is that of our common white deadnettle ~Lamium album) to the stinging-nettle (Urtica dioica); and Su· John Lubbock thinks that this is a case of true mimicry, the dead-nettle being benefited by beiuo· mistaken by grazing animals for the stinging-nettle.I 0 1 Flowers, Fruits, and Leaves, p. 128 (Fig. 79). |