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mark, that the skin of the turnip, beneath the soil, 
where light cannot act, is entirely white, because the 
alkaline matter is fully saturated with acid ; in the 
part above ground, the colour is purple from a par- 
tial decomposition of acid ; and in some parts, it be- 
comes nearly green as more acid is decomposed, and 
the alkali exerts a fuller degree of action. We have, 
also, often observed different parts of the same leaf of 
this plant to exhibit green, yellow, red, and purple co- 
lours, from accidental circumstances, which deter- 
mined, as we suppose, in different degrees, the pre- 
dominance of acid or alkaline matter. Thus, too, in 
the root of the radish, where it is entirely secluded 
from light, the acid may be conceived to prevail ; 
but exposure to light effects a partial decomposition, 
which reduces the excess of acid matter, and permits 
the purple tint to appear. This series of changes is 
well shewn by an infusion of red cabbage, which has, 
at first, a bluish tint, but passes through various 
grades of purple to a confirmed green, by successive 
additions of alkaline matter : or, if an acid be em- 
ployed, the same infusion is gradually heightened to 
redness, and may be again brought back to purple 
by adding to it alkaline matter. And since the ac- 
tion of light, in the colouration of plants, has been 
shewn to be entirely local (355.), it follows, not only 
that a succession of colours may thus be produced in 
the roots or leaves, but that various colours may, at 
the same time, be present in different parts of the same 
leaf or root, according as it is more or less subjected 
to the influence of light, and, probably, according as 
the structure of the plant, and the qualities of its co- 
