FLUID PARTS OF VEGETABLES 11] 



of raising liquids against the laws of gravitation, and with a forctj 

 propoi'tional to their smallness of diameter ;— this law 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 jjerspiration 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 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 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 itispiration 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 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 twenty-four hours, expires almost one pound of carbon, 

 or 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 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 

 use 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 ils leaves, it is undoubtedly fur- 

 Exhalation of sap— Perspiration of plants— What is the nature of the sap which re- 

 mains after exhalation?— Sap compared to animal chyle- Formation of carbonic gas 

 —In what respect does the comparison between the respiration of plants and animals 

 %il ?— What is needed in order to fit the carbon for tlie nourishment of the plant 1 



