FUNCTIONS OF THE LEAVES. 175 



absorption as great as those of inland plants, 

 could not furnish to the stem, the leaves, and the 

 flowers a supply of moisture equal to the great 

 demand arising from a situation not only ex- 

 tremely dry, but exposed to the greatest heat of 

 the sun. Two objects, therefore, require to be 

 effected, both of them apparently indispensable 

 to the very existence of the plant : one of these 

 is the power of obtaining moisture otherwise than 

 by the root ; the other is the means of preventing 

 the evaporation or loss of the moisture when so 

 obtained. To perceive how this object is accom- 

 plished, it is requisite to consider the structure 

 and functions of leaves in general, and those of 

 succulent plants, such as we now speak of, in 

 particular. 



The functions of the leaves of plants are pre- 

 cisely similar to those of the gills of fishes and 

 the lungs of land animals. The gills and lungs 

 of animals expose over a wide surface the venous 

 blood to the action of the oxygen contained in 

 water or in air ; by this means the blood, which 

 during the process of circulation had parted with 

 some of its most important constituents, is again 

 rendered fit for the purposes of imparting vigour 

 and health and continuing the growth and life of 

 the animal. Unless the vital fluid be thus con- 

 tinually supplied with these qualities, obtained by 

 respiration, the animal necessarily ceases to exist ; 

 if the supply be deficient it ceases to possess 

 health. This great rule is often exemplified by 

 members of the human family. The fresh breezes 



