106 
PHYSIOLOGICAL VIEWS. 
kno^v neither the cause of this irritability, nor in what this vital power consists, 
The ascent of the sap is in a degree owing to capillary attraction, assisted by heat ; 
the vessels containing this fluid are very small tubes, no larger than a h;iir, and in 
most cases much smaller, since few are visible to the naked eye. Capillary tubes 
have the property of raising liquids against the laws of gravitation, and with a force 
proportional to their smallness of diameter ; — this law explains in some degree the 
phenomenon we are considering. We will trace the progress of the sap after it has 
ascended to the leaves and extremities of the plant. A considerable portion of it 
is, by pores in the leaf, exhaled in the form of almost pure water, while the par- 
ticles of various kinds, which the sap held in solution, are deposited within the sub- 
stance of the lejif. This process is sometimes termed the perspiration or transpira- 
tion of plants. It is visible in some grass-like plants, particularly upon the leaves 
of Indian corn. If these are examined before sunrise, the perspiration appears in 
the form of a drop at the extremity of the leaf; the ribs of the leaf unite at this 
point, and a minute aperture furnished for the passage of the fluid may be discov- 
ered. The sap which remains, after the exhalation by means of the leaves, is sup- 
posed to consist of about one-third of that originally absorbed by the root ; this re- 
mainder possesses all the nutritive particles which had before been divided through 
the whole of the sap. At this period, an important change in its nature takes 
place, and one which has its analogy in the animal economy. 
122. The elaborated sap is sometimes transparent, at others 
milky or opake ; it has been called latex^ and the vessels trans- 
mitting it lactiferous. Although the sap has been compared to 
the blood of animals, it is, in reality, more like the animal sub- 
stance, chyle. 
A considerable part of this chyle is converted into blood, which, passing first into 
the arteries and then into the veins, is by the latter conveyed to the heart, from 
whence it passes to the lungs. At each inspiration of the breath, oxygen from the 
atmospheric air is absorbed by the lungs ; here uniting with the carbon of the blood, 
it forms carbonic gas, which is thrown off at every expiration of the breath. Thus 
the carbon, which in the animal system is accumulated by feeding on vegetables, 
and which requires to be diminished, is carried off; it is said that a person in 
breathing twenty- four hours, expires almost one pound of carbon, or the basis ol 
charcoal ! Let us return to the sap in the leaves of plants, and see whether a 
change takes place, analogous to that in the animal system. We will consider the 
sap 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 
gases ; but here our comparison fails, since, instead of ox3'gen, the plant inhale.s 
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, however, is necessary for this process of res- 
piration in the plant ; deprived of this agent, vegetables absorb instead of giving off 
oxygen. The carbon which is deposited in the sap, in order to be fitted for the 
nourishment of the plant seems to require the further agency of oxygen to convert 
it into carbonic acid ; this is effected by means of the oxygen, which during the 
night is absorbed by the leaves. At the appearance of light, carbonic acid is again 
decomposed and oxygen evolved. Besides the oxygen which the plant separates 
from the carbonic acid inhaled by its leaves, it is undoubtedly furnished witli this 
gas by the decomposition of water, and other substances which are absorbed by the 
root. 
123. The Camhium is the sap elaborated by the chemical 
process carried on in the leaves, and rendered fit for the nour- 
ishment of the plant. The descending or elaborated sap having 
Exhalation — Nature of the sap which remains after exhalation. — 122. Latex — Sap compared to ani- 
fftal chyle — Analogy in the animal and vegetable system — In what respect does the c(»mparison fail ?-- 
What IS needed in order to fit the carbon for the nourishment of the plant ?— 12^. Cambium-— Ho'v 
conveyed I 
