FLUID PARTS OF VEGETABLES. 
133 
as bearing a resemblance to the animal chyle, and the leaves to 
the animal lungs. These vegetable lungs are furnished with 
pores, by which they too, inhale gasses ; but here our comparison 
fails, since instead of oxygen, the. plant inhales carbonic acid; 
this it decomposes ; and converting to its own use the carbon, 
which is an important element of vegetable compounds, it exhales 
the oxygen necessary for the support of animal life. Light how- 
ever is necessary for this process of respiration in the plant ; 
deprived of this agent, vegetables absorb instead of giving off 
oxygen. The carbon, which is deposited in the sap, seems, in 
order to be fitted for the nourishment of the plant, to require 
the farther agency of oxygen, to convert it into carbon- 
ic acid ; this is done by means of the oxygen, which, during the 
light, is absorbed by the leaves. At the appearance of light, car- 
bonic acid is again decomposed and oxygen evolved. Besides 
the oxygen which the plant separates from the carbonic acid in- 
haled by its leaves, it is undoubtedly furnished with this gas by the 
decomposition of water and other substances which are absorbed 
by the root. The cambium is the sap elaborated by the chem- 
ical process carried on in the leaves, and rendered fit for the 
nourishment of the plant. In tracing the descent of this return- 
ing sap, we shall not find it passing through the same vessels 
by which it ascended ; it is chiefly conveyed by a system of ves- 
sels between the liber or inner layer of bark, and the alburnum 
or young wood ; here it contributes both to the formation of 
new wood and new bark, and extending from the extremity of 
the roots, to the upper extremity of the plant, it furnishes ma- 
terials for the formation of new buds and radicles. If a ring is 
cut through the bark of a tree, the cambium will be arrested in 
its course and accumulating around the upper edge of the bark, 
will cause a ridge or an annular* protuberance. This vegetable 
blood being thus prevented from having access to the lower part 
of the plant, the roots cease to grow, the sap ascends but feebly, and 
the tree dies in two or three years. If the incision is not made 
too deep, the wound will soon heal by the union of the discon- 
nected bark, and the circulation of the cambium proceeds as be- 
fore. This experiment proves the importance of this fluid to 
the existence of the plant. 
The Proper Juices of vegetables. This division comprehends 
all the fluids furnished by the plant except the sap and cambium ; 
as oils, gums, &c. These are the product of the cambium, as in 
the animal, tears are secreted from blood. The secretions, car- 
*From the Latin word annulus a ring. 
Cambium or descending sap — Conveyed by a system of vessels — impor- 
tance of this fluid — The proper juices of vegetables — Secretionsof two kinds. 
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