CHAP. XIV. 
COLOUR AND SMELL. 
355 
and similar cases. To what causes all these extraordinary cir- 
cumstances are to be ascribed no one knows. As yet our 
information concerning vegetable colours is of the vaguest 
nature. But as it is one of very great interest, and particu- 
larly deserving the notice of the philosophical botanist, we 
make no scruple of borrowing from De Candolle an abstract 
of what is at present known or conjectured of the subject. 
" We are so accustomed, he remarks, to see plants decorated 
with the most brilliant colours, or at least invested with the 
green hue which characterizes every scene, that it is not with- 
out difficulty that we accustom ourselves to the idea that such 
colours do not exist in their primitive state, but are commu- 
nicated, as it were, to vegetation by its own act ; and yet this 
is the exact truth. The tissue of plants is in itself completely 
colourless, of a silvery white, or of an exceedingly pale yellow; 
the matters contained in the cellules are, with a few excep- 
tions, of the same hue ; but all is changed when they are once 
exposed to solar light. 
" We are accustomed to say that green plants become white 
in total darkness, because the phenomenon, inaccurately ob- 
served, is usually presented to us under that form: but the 
truth is, that although the parts of plants which originally are 
white or black become more or less coloured when exposed 
to the action of light, yet organs, once coloured, do not in 
reality lose their colour when kept in darkness ; if they some- 
times seem to do so it is owing to this, — that if half-deve- 
loped leaves are placed in the dark, they grow larger, and the 
green matter which coloured them, being diluted by water 
and spread over a greater space, appears to be paler without 
being itself less coloured. That the action of solar light is, 
in reality, the grand cause of colour in plants is proved by 
leaves half covered from light and half exposed, of which the 
latter become green and the former remain colourless; all 
gradations of intensity being produced in proportion to the 
intensity of light to which the parts are exposed. 
" There are plants which, in those parts that are destined 
to become green, have spaces that preserve their original 
whiteness : such plants we call variegated, and find through 
almost all the divisions of the vegetable kingdom. In Exogens 
A A 2 
