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constants and where migration was unnecessary ; while whatever direct effect may be produced by light or heat, will necessarily have acted more powerfully within the tropics. And lastly, all these causes have been in action over an actually greater area in tropical than in temperate zones; while estimated potentially, in proportion to its life-sustaining power, the lands which enjoy a practically tropical climate (extending as they do considerably beyond the geographical tropics) are very much larger than the temperate regions of the earth.
Combining the effects of all these various causes we are quite able to understand the superiority of the tropical parts of the globe, not only in the abundance and variety of their forms of life, but also as regards the ornamental appendages and vivid coloration which these forms present.
THE COLOURS OF PLANTS AND THE ORIGIN
OF THE COLOUR-SENSE.
Source of Colouring matter in Plants-Protective Coloration and Mimicry
among Plants--Attractive Colours of Fruits-Protective Colours of Fruits-Attractive Colours of Flowers—Attractive Odours in FlowersAttractive grouping of Flowers—Why Alpine Flowers are so BeautifulWhy allied species of Flowers differ in Size and Beauty--Absence of Colours in Wind-fertilized Flowers—The same Theory of Colour applicable to Animals and Plants-Relation of the Colours of Flowers and their Geographical Distribution-Recent Views as to the Direct Action of Light on the Colours of Flowers and Fruits-On the Origin of the Coloursense-Supposed increase of Colour-perception within the Historical Period—Concluding Remarks on the Colour-sense.
The colouring of plants is neither so varied nor so complex as that of animals, and its explanation accordingly offers fewer difficulties. The colours of foliage are, comparatively, little varied, and can be traced in almost all cases to a special pigment termed chlorophyll, to which is due the general green colour of leaves; but the recent investigations of Mr. Sorby and others have shown that chlorophyll is not a simple green pigment, but that it really consists of at least seven distinct substances, varying in colour from blue to yellow and orange. These differ in their proportions in the chlorophyll of different plants ; they have different chemical reactions; they are
differently affected by light; and they give distinct spectra. Mr. Sorby further states that scores of different colouring matters are found in the leaves and flowers of plants, to some of which appropriate names have been given, as erythrophyll which is red, and phaiophyll which is brown; and many of these differ greatly from each other in their chemical composition. 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
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 processes 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 understand 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 of cattle and wild herbivorous animals. Mr. J. P. Mansel Weale also noticed that many plants growing in the stony Karoo have their tuberous roots above the soil; and these so perfectly resemble the stones among which they grow that, when not in leaf, it is almost impossible to distinguish them (Nature, vol. iii. p. 507). A few cases of what seems to be protective mimicry have also been noted; the most curious being that of three very rare British fungi, found by Mr. Worthington Smith, each in company with common species which they so closely resembled that only a minute examination could detect the difference. One of the common species is stated in botanical works to be “bitter and nauseous,” so that it is not improbable that the rare kind may escape being eaten by being mistaken for an uneatable species, though itself palatable. Mr. Mansel Weale also mentions a labiate plant, the Ajuga
ophrydis, of South Africa, as strikingly resembling an orchid. This may be a means of attracting insects to fertilize the flower in the absence of sufficient nectar or other attraction in the flower itself; and the supposition is rendered more probable by this being the only species of the genus Ajuga in South Africa. Many other cases of resemblances between very distinct plants have been noticed—as that of some Euphorbias to Cacti; but these very rarely inhabit the same country or locality, and it has not been proved that there is in any of these cases the amount of inter-relation between the species which is the essential feature of the protective “mimicry” that occurs in the animal world.
The different colours exhibited by the foliage of plants and the changes it undergoes during growth and decay, appear to be due to the general laws already sketched out, and to have little if any relation to the special requirements of each species. But flowers and fruits exhibit definite and well-pronounced tints, often varying from species to species, and more or less clearly related to the habits and functions of the plant. With the few exceptions already pointed out, these may be generally classed as attractive colours.
Attractive Colours of Fruits.—The seeds of plants require to be dispersed, so as to reach places favourable for germination and growth. Some are very minute, and are carried abroad by the wind; or they are violently expelled and scattered by the bursting of the containing capsules. Others are downy or winged, and are carried long distances by the gentlest breeze; or they are hooked and stick to the fur of animals. But there is a large class of seeds which cannot be dispersed in either of these