132 
FLUID PARTS OF VEGETABLES. 
the naked eye. You have in the study of Natural Philosophy, 
learned that 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 seems to explain, in some 
degree, the phenomonon of the ascent of the sap. Yet still, 
this is but a secondary cause, and we must realise that our re- 
searches, here, as in every other case, terminate in mysteries 
impenetrable by our limited faculties. 
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, exhal- 
ed in the form of almost pure water, while the particles of va- 
rious kinds which the sap held in solution are deposited within 
the substance of the leaf. This process is sometimes termed the 
perspiration of plants ; it is visible in some grass-like plants 
particularly upon the leaves of Indian corn ; if these are examin- 
ed before sunrise, the perspiration appears m 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 this purpose, may 
then be observed. 
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, a change 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 latter conveyed to the heart ; the heart by its 
contractions sends the blood to the lungs. At each inspiration 
of the breath, the oxygen from the atmospheric air is absorbed 
by the lungs ; here uniting to 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 accu- 
mulated, by feeding on vegetables, and which requires to be di- 
minished, is carried off ; it has been said that a person exhales, 
in breathing twenty four hours, almost one pound of carbon, 
which you know is the basis of charcoal ! 
We will now return to the sap in the leaves of plants and see 
whether a change takes place analogous to that just described as 
taking place in the animal system. We will consider the sap 
Researches often terminate in mysteries — Exhalation of sap — Perspira- 
tion — Sap compared to chyle — Animal chyle — Formation of carbonic gas. 
