FLUID PAHTS OF VEGETABLES 
Hi 
of raising liquids against the laws of gravitation, and with a force 
proportional to their smailness of diameter; — this lav/ seems to ex- 
plain, in some degree, the phenomenon we are considering. 
But it is necessary for us now to 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 particles of various kinds, which the 
sap held in solution, are deposited within the substance of the leaf 
This process is sometimes termed the jperspiration of plants: it is 
visible in some grass-like plants, particularly upon the leaves of In- 
dian corn. If these are examined before sunrise, the perspiration 
appears in the form of a drop at the extremity of the leiif ; the ribs 
ofthe leaf ui\ite at this point, and a minute aperture furnished for the 
passage of the fluid, may be discovered. 
The sap which remains, after the exhalation by means, of the 
leaves, is supposed to consist of about one third of that originally 
absorbed by the root ; this remainder 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. 
We have compared the sap to the blood of animals, but it is, in 
reality, more like the animal substance, chyle^ which is a milk-like 
liquor, separated by digestion, from the food taken into the stomach. 
A, considerable part of this chyle is converted into blood, which 
passing first into the arteries and then into the veins, are by the lat- 
ter conveyed to the heart; the heart, by its contractions, sends the 
blood to the lungs. At each inspiration of the breath, oxygen from 
the atmospheric air is absorbed by the lungs ; iiere uniting with the 
carbon oC the blood, it forms carbonic gas, which is thrown off at 
every expiration of the breath. Thus the carbon, which, in the an- 
imal system, is accumulated by feeding on vegetables, and which 
requires to be diminished, is carried off ; it is said that a person in 
breathing tv/enty-four hours, expires almost one pound of carbon, 
or the basis of charcoal ! 
We v/ill now return to the sap in the leaves of plants, and see 
whether a change takes place, analogous to that in the animal sys- 
tem. 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 oxygen, the plant 
inhales carbonic acid this it decomposes, and converting to its own 
3^se the carbon, which is an important element of vegetable com- 
pounds, it exhales the oxygen necessary for the support of animal 
life. Light, however, is necessary for this process of respiration in 
the plant; deprived of this agent, vegetables absorb instead of giv- 
ing 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 fur- 
Exhalation of sap — Per>;piration of plants— Y\'"!i at is the nature of the sap wliich re- 
mains after exhalation ?— Sap compared to animal chyle— Forrnadon of carbonic gns 
—In what respect does the comparison between the respiration of plants and anim'^als 
fail ?— What ie needed in order to fit the carbon fyr the nourishment of the plant? 
