Functions of Vegetables. 
205 
branches and leaves and in the leaves it undergoes 
peculiar changes, in consequence of part being exhaled, 
and of the absorption of different principles, which, com- 
bining with it, contribute to the changes that are ef- 
fected. During these changes the peculiar juice of the 
plant is prepared, which, in its progress from the leaves 
towards the roots, deposits those ingredients which pro- 
duce all the variety of substances which are detected 
by the remarkable diversity of odour, taste, and consis- 
tence. The exhalation of a portion of the moisture 
taken in by the roots, and the absorption and decom- 
position of carbonic acid gas, by which the carbone is 
retained in the plant and the oxygen is given out, con- 
stitute one part of the functions of leaves. These pro- 
cesses are most vigorous during the day, and particu- 
larly during bright sun-shine ; but in the night they are 
reversed. Carbonic acid gas is given out, and moisture 
and oxygen gas are absorbed ; and this absorption and 
moisture are chiefly effected in many plants by the under 
surface of the leaves. 
The effects of light on vegetables are very remark- 
able. When it is entirely excluded, although they en- 
joy the influence of air, heat, and moisture, they never 
acquire their rich green colour, but remain of a pale, 
sickly white. The familiar practice of blanching or 
whitening celery, affords a good illustration of this fact; 
and not only the colour, but the smell and taste un- 
dergo equal changes. 
Sleep of plants * — When the influence of those agents 
which have a powerful effect on vegetables is withdrawn, 
many plants exhibit a very remarkable change in their 
aspect. This is particularly the case with plants which 
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