PHYSIOLOGY. 275 



good illustration of the proportion between the number ol 

 pores in the leaves, and the quantity of aqueous transpiration. 

 It is well known that this plant requires a large and constant 

 supply of water, and that if it is not supplied its leaves soon 

 wither and become crisp. The number of pores in the leaves, 

 being equal to 160,000 to each square inch, appear to account 

 for this phenomena. 



These facts prove that foliar pores are exhalants, while 

 others equally conclusive show that they are not absorb- 

 ents. Thus no cuticular pores are found on the under sur- 

 faces of leaves which float on the water, but the upper 

 surfaces are thickly covered with them, and yet it is known 

 that these leaves absorb largely by their under surfaces, and 

 exhale by their upper. It is most probable, therefore, that 

 the absorption of water, which it is well known most leaves 

 under certain circumstances have the power of perform- 

 ing, is dependent on other vessels; besides those above de- 

 scribed. 



Respiratory organs of Plants. It has been supposed by 

 several physiologists, that the respiratory organs of plants 

 were a different system of vessels, from the exhalants above 

 described ; but there are several reasons for believing that 

 these cuticular pores, not only serve the office of aqueous 

 transpiration, but also that of the aeration of the sap, or that 

 they are the lungs of plants. 



In \\iQfrst place, these apertures are found only on leaves 

 which are exposed to the air, and even leaves which are not 

 naturally under water, lose them, after a time, on being sub- 

 merged. 



Second, Thin membranous leaves belonging to terrestrial 

 plants are most liberally supplied with them, while those 

 which have thick immoveable leaves, are but sparingly 

 furnished with these pores. Thus we can perceive an analo- 

 gy between the respiratory organs of animals and plants ; 

 the more perfect of both requiring the fullest supply of oxygen, 

 while the cold blooded animals, which may be compared 

 with the thick, rigid leaves of plants, have small lungs, and 

 require but a limited portion of air. 



Effects of the Leaves on the Sap. The sap is carried 

 into the leaves by the ascending, or conducting vessels, and 

 is there exposed to the action of the atmosphere, where it 

 exhales a large proportion of its volume in the form of 

 water. Both the influence of the air, and the exhalation. 



