[335] SCIENTIFIC MANUAL. 49 



tial to the perfect discharge of its functions as an organ 

 of vegetation. Under the influence of sun-light, they ab- 

 sorb carbonic acid from the air, which, under the same in- 

 fluence, is decomposed, the oxygen exhaled, and the carbon 

 retained, and combining with the oxygen and hydrogen, 

 forms cellulose. 



Besides oxygen, leaves exhale a large quantity of watery 

 vapor into the air, when the condition of the latter is fa- 

 vorable for such exhalation. 



" In one of the well known experiments of Hale, a sun- 

 flower, three and a half feet high, with a surface of 5,612 

 square inches exposed to the air, was found to perspire at 

 ' the rate of twenty to thirty ounces avoirdupoise every 

 twelve hours, or seventeen times more than a man. 



A seedling apple tree with eleven square feet of foliage, 

 lost nine ounces a day." (Gray.) 



The leaves of plants, therefore, not only supply the air 

 with pure oxygen, so necessary to animal life, but afford 

 a vast amount of watery vapor when the air is dryest, and 

 most in need of moisture. The meliorating influence of 

 extended forests upon climate may be readily understood, 

 when we consider the vast aggregate of evaporating sur- 

 face presented by their foliage. 



The baleful effects of a promiscuous destruction of the 

 natural forests in the interior of continents, remote from 

 large bodies of water, are probably largely due to the in- 

 terruption of this beneficial exhalation. 



Since all of the water contained in plants, and that ex- 

 haled from their leaves, is absorbed by the roots, the im- 

 portance of an abundant supply of moisture to the latter 

 is apparent. If the evaporation from the leaves is less 

 than the absorption of water by the roots, the plant re- 

 mains fresh, and if other circumstances are favorable, 

 grows rapidly ; if, however, as often happens in periods of 

 continued drouth, the exhalation of watery vapor from the 



