68 COLOUR IN NATURE chap. 



contained in solid bodies, known as chromoplasts or 

 chromoleucites. 



Anthocyan pigment colours the petals of hyacinths, 

 bluebells, roses, etc., and such fruits as grapes, blae- 

 berries, cranberries, and so on. The property of 

 colour-change which we have already seen it to 

 possess is of considerable importance in the pro- 

 duction of the colours of flowers and fruits, for when 

 anthocyan is present in, for example, the cells of 

 petals, its tint depends in part on the degree of 

 acidity of the cell-sap. 



Thus we are all familiar with the change in the 

 colour of the flowers as they develop, which is 

 frequently so conspicuous a feature in various 

 members of the natural order Boraginacese. The 

 forget-me-not is pink in bud and blue when full- 

 grown, the pink colour occasionally persisting as a 

 variation. The colour-change is associated with a 

 diminished acidity of the cell-sap of the cells of the 

 petals. The sap is at first strongly acid, but as the 

 flower develops the acid disappears. Most flowers 

 which in natural conditions are blue show as a varia- 

 tion, or under cultivation, a tendency to become pink, 

 e.g. pink hyacinths, pink Campanulas, etc. ; a fact 

 which seems to indicate that the amount of acid 

 present in plants tends to vary, or is in an unstable 

 condition. Such a variation though most common 

 in cultivation probably also occurs in natural con- 

 ditions. Thus the common milkwort, Polygala 

 vulgaris, may be found in the same locality under at 

 least three different varieties, with blue, pink, or 

 white flowers respectively. It is reasonable to 

 suppose that this variation is the result of variation 



