PLANTS DECOMPOSE WATER. 35 



SECTION V. SOURCE OF THE OXYGEN AND HYDROGEN OF 

 PLANTS. 



Beside carbonic acid, the leaves of plants absorb 

 through their pores a large quantity of water. During 

 the day when the hot sun is upon them, the evapora- 

 tion is of course far more than the absorption, and in 

 a dry time the leaves may be seen to droop in the 

 afternoon; but let the sun be obscured and the atmo- 

 sphere become misty and damp, and they soon absorb 

 enough moisture to strengthen their failing stems. 

 Every farmer knows that a light shower, which only 

 moistens the leaves without wetting the ground at all, 

 will revive his crops for many hours. Nothing in this 

 case can have been taken in through the roots. 



Water, as has been said, is composed of oxygen and 

 hydrogen. These two bodies are needed by the plant, 

 and water is consequently not only of service in mois- 

 tening its various parts and furnishing a circulating 

 fluid, but gives its oxygen or its hydrogen or both, as 

 the plant may happen to require. Water has a pe- 

 culiar adaptation to this purpose, and to others equally 

 useful in the interior of the plant, in the facility with 

 which it is decomposed. Carbonic acid and other 

 chemical substances only decompose with great dif- 

 ficulty; but the elements of water, a substance so 

 universally diffused and so indispensable, separate 

 easily, affording hydrogen here, oxygen there, to the 

 necessities of the plant. 



SECTION VI. SOURCES OF THE NITROGEN OF PLANTS. 



We have now seen how the plant gets carbon, 

 hydrogen and oxygen in abundance; but there is yet 

 one more of the organic bodies, which are so necessary 

 *o them: this is nitrogen; it remains for us to consider 

 'he most probable source of this gas. a. As it has 



