428 
PHYSIOLOGY. 
BOOK II. 
in plants each particular cell of the parenchyma has its own 
colour, that there is no intermixture of lints, but that 
whatever the hues may be, each has its own cluster of cells to 
represent it : and even in the midst of a large mass of uniform 
colouring, a few cells, or even a single one, will secrete a 
colouring matter which forms the strongest contrast with what 
surrounds it; as in the blood- red orange, and similar cases. 
“ We are so accustomed, ’’ says De Candolle, ‘‘ to see plants 
decorated with the most brilliant colours, or invested with the 
green hue which characterises every scene, that we cannot 
without diflBculty accustom ourselves to the idea that such 
colours do not exist in a primitive state, but are commu- 
nicated to vegetation by its own act, as it were ; and yet this 
is the exact truth. The tissue of plants is for the most part 
colourless, of a silvery white or of an exceedingly pale yellow ; 
the matter originally contained in the tissue is, with a few ex- 
ceptions, of the same hue ; but all is changed when plants are 
once exposed to solar light. 
“ We are accustor led to say that green plants become white 
in total darkness, I ;cause the phenomenon, inaccurately ob- 
served, is usually preserved 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 b}'^ 
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 the parts 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 the blotches 
