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vi COLOURS OF PLANTS 405and hazel, our nettles, grasses, sedges, and many others. Insome of these the male flowers are very conspicuous, as thecatkins of the willows, and these secrete honey and attractnumerous insects at a season when there are few other flowers,and thus secure cross -fertilisation. Sedges and grasses arealso occasionally visited by insects.The same Theory of Colour applicable to Animals and PlantsItmay be thought that this absence of colour where it isnot wanted isopposed to the view maintained in the earlierpart of the preceding chapter, that colour is normal and isconstantly tending to appear in natural objects. It must beremembered, however, that the green colour of foliage, due tochlorophyll, prevails throughout the greater part of the vegetablekingdom, and has, almost certainly, persisted throughlong geological periods. It has thus acquired a fixity ofcharacter which cannot be readily disturbed ; and, as a matterof fact, we find that colour rarely appears in plants except inassociation with a considerable modification of leaf -texture,such as occurs in the petals and coloured sepals of flowers.Wind -fertilised plants never have such specially organisedfloral envelopes, and, in most cases, are entirely without acalyx or corolla. The connection between modification ofleaf-structure and colour is further seen in the greater amountand variety of colour in irregular than in regular flowers.The latter, which are least modified, have generally uniformor but slightly varied colours, while the former, which haveundergone great modification, present an immense range ofcolour and marking, culminating in the spotted and variegatedflowers of such groups as the Scrophularinese andOrchideae. The same laws as to the conditions of a maximumproduction of colour are thus found to obtain both in plantsand animals.Relation of the Colours of Flowers and their GeographicalDistributionThe adaptation of flowers to be fertilised by insectsoften to such an extent that the very existence of the speciesdepends upon it has had an important influence on the distributionof plants and the general aspects of vegetation.