34 STAGNANT AIR. 



tain proportion of carbonic acid, which is the chief 

 source of the carbon of plants, constituting the 

 greater part of their wood. This is received by the 

 plant not only through the roots, but by means of 

 all the green portions of the tree. If the air was 

 not in circulation, it can readily be imagined that 

 this gas would be more or less exhausted in that 

 part of the atmosphere which immediately surrounds 

 the plant, and that the tissue would consequently 

 be pithy and soft. Therefore we can discern the 

 use, in the great system of Nature, of those gentle 

 breezes which so lightly rustle the foliage, and sup- 

 ply to the most minute leaf its proper share of this 

 great element of life. The stagnation of the air, 

 together with electrical influences, we shall discover, 

 when we investigate the diseases of fruit-bearing 

 plants, to be the probable causes of the American 

 pear-blight. 



4. Aqueous vapor. Not only does the air contain 

 carbonic acid with hydrogen and oxygen, but aqueous 

 vapor, which affects plants powerfully. The first 

 three constituents form the primary elements of the 

 bodies of both animals and plants ; these, by their 

 death and decay, restore the gases to their original 

 condition ; but the rapidity of these phenomena is 

 regulated, in a great measure, by the presence of 

 watery vapor. Its quantity varies with the locality 

 and the season of the year. Its absence would cause 



