FLUID PARTS OF VEGETABLES. 145 



cases, much smaller, since few are visible to the naked eye. 

 You have, in the study of Natural Philosophy, learned that ca- 

 pillary tubes have the property of raising liquids, against the 

 laws of gravitation, and with a force proportional to their small- 

 ness of diameter : this law seems to explain, in some degree, 

 the phenomenon we are considering. Yet we must realize that 

 our researches here, as in every other case, terminate in mys- 

 teries, 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, exha- 

 led in the form of almost pure water, while the particles of va- 

 rious kinds, which the sap held in solution, are deposited with- 

 in 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 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 discovered. 



The sap which remains after the exhalation by means of the 

 leaves, is supposed to consist of about one third of that origin- 

 ally absorbed by the root ; this remainder possesses all the nu- 

 tritive particles, which had before been divided through the 

 whole of the sap. At this period, an important change in its na- 

 ture 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 inspira- 

 tion of the breath, the oxygen from the atmospheric air, is ab- 

 sorbed 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 

 accumulated, by feeding on vegetables, and which requires to 

 be diminished, is carried off; it has been said that a person ex- 

 hales, inbreathing twenty. four hours, almost one pound of car- 

 bon, or the bais of charcoal ! 



We will now return to the sap in the leaves of plants, and see 

 whether a change takes place, analagous to that in the animal 



Exhalation of sap Perspiration Sap which remains after exhalation by 

 means of the leaves Sap compared to chyle Formation of carbonic gas, 



13 



