CHAP. XVI. 
COLOUR. 
435 
Macquart supposes that the red colour so common in plants 
in the autumn is caused by the developement of anthocyane, 
which becomes red when acted upon by an acid ; and not by 
any alteration in the chlorophyll itself. Mold, however, doubts 
if this is always the case (Ann. Sc., n. s., ix. 218.) ; he considers 
that, in numerous instances, the red tint gained by evergreen 
leaves in winter is owing to a change in their physical func- 
tions at that season, when the leaves attract little or no 
ascending sap, but exist at the expense of the matter already 
stored up in them. This is, however, a chemical, rather than 
a botanical question, and must not be treated of here at 
greater length. 
With regard to white colour, it is by no means caused by 
the emptiness of tissue, as has been represented, but is owing 
to the colourless quality of the fluid contained in tissue. It 
may be doubted, indeed, whether it really exists in a state of 
purity in flowers, and it seems to be rather some other colour 
reduced to an exceedingly light tint. Redoute, the French 
flower-painter, is said to have availed himself with great ad- 
vantage of this fact. He always placed the flower he wished 
to represent before a sheet of paper like that on which he had 
made his drawing, and he uniformly found that the flower 
would differ from the paper in being more yellow, or more 
pink, or more blue, or in some other way. White Campa- 
nulas become blue when they are dried ; infusions of white 
flowers in alcohol have always a perceptible tinge. Flowers 
which are white, verging upon yellow, yield infusions which 
alkalies bring to a more decided yellow or a more positive 
brown ; infusions of those which are white, tending to blue 
or red, become light red by the action of acids, and greenish 
by the action of alkalies. 
If the white petals of Convolvulus tricolor are examined, 
their cells will appear quite empty. If weak sulphuric acid is 
applied to them, a slight cloudiness makes its appearance, but 
still there is nothing distinct enough to satisfy the observer 
that a colourless fluid is present : but, if iodine is then ap- 
plied, the interior of the before colourless, and apparently 
empty, cells is filled with an olive-brown coagulum, among 
which round granules become distinctly visible. The rapid 
F F 2 
