70 COLOUR IN NATURE chap. 



We have thus considered the common colouring- 

 matters of flowers and fruits, and seen that in general 

 terms they fall into two groups, the fixed or lipo- 

 chrome pigments, occurring in the form of solid 

 particles in the cell, and the free or anthocyan pig- 

 ments occurring in solution in the cell-sap, and 

 varying in tint according to its reaction. In flowers, 

 further, we frequently find a pure white colour, which 

 we have already seen to be an optical colour caused 

 by the presence of air-spaces between colourless cells. 

 Colours which are not primary colours are caused by 

 a superposition of differently coloured elements, or of 

 colourless elements on coloured ones. 



Besides the actual bright colour, flowers are often 

 conspicuous by the beauty and regularity of their 

 markings. Sometimes this is of exceedingly simple 

 nature, and bears an obvious relation to the nature 

 of the parts. Thus in the snowdrop the petals are 

 delicately veined with green, and in the wood-sorrel 

 with violet. A veining with green recalls the appear- 

 ance of some autumnal leaves when the chlorophyll 

 seems to linger longest at the sides of the veins of 

 the leaf. In the same way the purple veins of wood- 

 sorrel {Oxalis) are quite similar to the reddened veins 

 of many leaves in spring, and just as in leaves the 

 red colour may occasionally spread over the whole 

 leaf, so it is not very uncommon to find in the wood- 

 sorrel as a variation that the petals have become 

 completely purple. This occurs especially in shady 

 places. The markings of many petals are, however, 

 of far more complex nature, and bear no obvious 

 relation to the structure of the parts. Such are, for 

 example, the markings on the labellum in many 



