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 Boraginaceae. 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, 
eg. 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 
