141 
lar portions of leaf have the green hue of their paren- 
chyme improved, and even the red epidermis finally 
assumes a green colour. This alternate change and 
exaltation of colour, according as acid ot alkaline 
matter is made to predominate, authorise us to as* 
cribe the production of colour, in the epidermis of 
these leaves, to the same excess of acid which imparts 
the red tinge to autumnal leaves (387. ), and to the 
young leaves of certain other plants (389.), although 
the seat of colour be placed in a different part of the 
vegetable structure. 
395. In the same manner, the colour of certain 
roots, as of the radish, of some varieties of potatoe, 
and of the turnip, seems to reside in the outer coat, 
while, in the carrot and beet, it is diffused through 
the substance of the root. In the radish, the colour 
extends over almost the whole root ; in the turnip, 
it is confined to that part which is exposed to light ; 
and even the radish, where it comes into contact 
with light (286.), assumes a purple hue. Now, if 
light operate in the way that has been stated, it will 
variously affect the colours of these parts, according 
to the degree of its action. It is, as we have seen, 
in the green parts alone that oxygen is largely ex- 
pelled ; in other words, that carbonic acid is largely 
decomposed ; and, therefore, it is in those parts only 
that the alkali is enabled to exert its full effect. But if 
the light act in a less degree, and decompose less 
acid, the alkali will be less predominant, and im- 
part not a full green, but some modification of pur- 
ple. 
396. In illustration of these views, we may re- 
