PDF - Wallace Online

396 TROPICAL NATUREthese differ greatly from each other in their chemical composition.inquiriesThese are at present in their infancy, butas the original term chlorophyll seems scarcely applicableunder the present aspect of the subject, it would perhapsbe better to introduce the analogous word chromophyll asa general term for the colouring matters of the vegetablekingdom.Light has a much more decided action on plants thanon animals. The green colour of leaves is almost whollydependent on it; and although some flowers will becomefully coloured in the dark, others are decidedly affected bythe absence of light, even when the foliage is fully exposed toit.Looking therefore at the numerous colouring matterswhich are developed in the tissues of plants, the sensitivenessof these pigments to light, the changes they undergo duringgrowth and development, and the facility with which newchemical combinations are effected by the physiological processesof plants as shown by the endless variety in thechemical constitution of vegetable products, we have nodifficulty in comprehending the general causes which aid inproducing the colours of the vegetable world, or the extremevariability of those colours. We may therefore here confineourselves to an inquiry into the various uses of colour in theeconomy of plants, and this will generally enable us to understandhow it has become fixed and specialised in the severalgenera and species of the vegetable kingdom.Protective Coloration and Mimicry in PlantsIn animals, as we have seen, colour is greatly influencedby the need of protection from, or of warning to, theirnumerous enemies, and by the necessity for identificationand easy recognition. Plants rarely need to be concealed,and obtain protection either by their spines, their hardness,their hairy covering, or their poisonous A secretions. veryfew cases of what seem to be true protective colouring do,however, exist, the most remarkable being that of the " stonemesembryanthemum " of the Cape of Good Hope, which, inform and colour, closely resembles the stones among which itgrows; and Dr. Burchell, who first discovered it, believesthat the juicy little plant thus generally escapes the notice