106 PHYSIOLOGICAL VIEWS. 



kno-w neither the cause of this irritability, nor in what tliis vital power consists. 

 The ascent of the sap is in a degree owing to capillart/ attraction, assisted by heat ; 

 the vessels containing this fluid are very small tul)es, 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 tlic loaves 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 leaf. Tliis process is sometimes termed i\\Q peri^piration 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 hi 

 the form of a drop at the extremity of the leaf; the ribs of the leaf unite at this 

 pohit, 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 oft" 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 duninished, is carried off; it is said that a person in 

 breathing twenty- four hours, expires almost one pound of carbon, or the basis of 

 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 inhales 

 carbonic acid ; tliis 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 with this 

 gas by the decomposition of water, and other substances which are absorbed by the 

 root. 



123. Tlie Camliiim 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 — Natnre of the sap which remains after exhalation. — 122. Latex — Sap compared to ani- 

 mal chyle — Analogy in the animal and vegetable system — In what respect does the comparison fail ? — 

 What is needed in order to fit the carbon for the nourishment of the plant? — 123. Cambium — IIow 

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