[370] The reader will find an abstract of Mr. Darwin's views in his work on the 'Variation of Animals and Plants,' vol. ii, p. 181. [371] Loc. cit., i, 336. CHAPTER III. ALTERATIONS OF COLOUR.[372] Changes in the colour of the several organs of plants are more often either pathological or the result of variation than of malformation properly so called. Alterations in colour arise from a diminished or an increased amount of colouring matter, or from an unusual distribution of the solid or fluid matters on which the colour depends. The superposition of cells containing colouring material of different tints produces naturally a very different set of hues from those which are manifested when the colours are not blended. Referring the reader to the ordinary text-books on vegetable physiology and chemistry for details as to the nature and disposition of colouring materials in plants under natural circumstances, it will only be necessary to cite a few instances of deviation from the general colour of plants or their organs. =Albinism.=--This change is due to the deficient formation of green colouring matter or chlorophyll, and is more a pathological condition than a deformity. It seems necessary to draw a distinction between this state and ordinary blanching or etiolation. In the former case chlorophyll seems never to be formed in the affected parts, even if they be exposed to light, while an etiolated organ, when placed under favorable circumstances, speedily assumes a green colour. In _Richardia aethiopica_ one or more leaves become occasionally as white as the spathe is usually. =Virescence.=--Engelmann[373] pointed out that, so far as flowers were concerned, there are two ways in which they assume a green colour, either by a simple development of chlorophyll in place of the colouring matter proper to the flower, or by an actual development of leaf-like organs in the room of the petals--frondescence. Morren[374] judiciously proposed to keep these two conditions separate, calling the one virescence, the other frondescence (see p. 241). Many of the cases recorded as reversions of the parts of the flower to leaves are simply instances of virescence; indeed, it is not in all cases easy to distinguish between the two states. The examination of the arrangement of the veins is often of assistance in determining this point; for instance, if, under ordinary circumstances, the venation of the petal be such as is characteristic of the sheath of the leaf, while in the green-coloured flower of the same species the venation is more like that which belongs to the blade of the leaf, the inference would, of course, be that the green colour was due to frondescence or phyllody. The persistence or duration of petals is often increased when they are subject to this change; instead of falling off speedily they become persistent when so affected.