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PHYSIOLOGY. 
BOOK II. 
CHAPTER VI. 
OF THE LEAVES. 
Leaves are at once organs of respiration, digestion, and 
nutrition. They elaborate the crude sap impelled into them 
from the stem, decomposing its water, adding to it carbon, and 
exposing the whole to the action of air ; and while they supply 
the necessary food to the young tissue that passes downwards 
from them and from the buds, in the form of alburnum and 
liber, they also furnish nutriment to all the parts immediately 
above and beneath them. There are many experiments to 
show that such is the purpose of the leaves. If a number of 
rings of bark are separated by spaces without bark, those 
which have leaves upon them will live much longer than those i 
which are destitute of leaves. If leaves are stripped from a i 
plant before the fruit has commenced ripening, the fruit will | 
fall off and not ripen. If a branch is deprived of leaves for a i 
whole summer, it will either die or not increase in size per- 
ceptibly. The presence of cotyledons, or seminal leaves, at a 
time when no other leaves have been formed for nourishing 
the young plant, is considered a further proof of the nutritive 
purposes of leaves : if the cotyledons are cut off, the seed will 
either not vegetate at all, or slowly and with great difficulty ; 
and if they are injured by old age, or any other circumstance, 
this produces a languor of habit which only ceases with the 
life of the plant, if it be an annual. This is the reason why 
gardeners prefer old melon and cucumber seeds to new ones : 
in the former the nutritive power of the seed-leaves is im- 
paired, the young plant grows slowly, a languid circulation is 
induced from the beginning; by which excessive luxuriance 
is checked, and fruit formed rather than leaves or branches. 
Nothing can be more admirable than the adaptation of 
leaves to such purposes as those just mentioned. It has been 
already shown, in speaking of the anatomy of a leaf, that in 
