[423] SCIENTIFIC MANUAL. 137 



CHAPTER XII. 



METEOROLOGY IN ITS RELATIONS TO AGRICULTURE. 



Although we have no control over either the composi- 

 tion of the atmosphere, or the amount or distribution of 

 rainfall, still it is a matter of some importance that we un- 

 dersVnd the influence exerted by both upon vegetation. 



The atmosphere supplies to plants nearly the whole of 

 their organic constituents, and affords an inexhaustible 

 source of supply, which can be neither increased nor di- 

 minished by artificial means. It is composed of oxygen 

 and nitrogen, with small quantities of watery vapor, car- 

 bonic acid and ammonia. 



Notwithstanding the very small per cent, of carbonic 

 acid l-1600th by weight contained in the atmosphere, 

 l t is supposed to furnish, through the medium of the, leaves 

 and other growing parts of plants, the bulk of the carbon 

 which makes up their cellular structure. Indeed, more 

 than ninety per cent, of the substance of our agricultural 

 plants is derived from the atmosphere. 



Not only is carbonic acid in large quantity, and ammo- 

 nia in small qnantity, absorbed through the stomata or 

 leaf-pores, but the presence of oxygen is essential to the 

 germination of the seed, and the growth, both of the stem 

 and roots of plants. 



Again, the fact that air brought into contact with sub- 

 stances colder than itself, is deprived of some of its moist- 

 ure, which is condensed and deposited in the form of dew, 

 renders the atmosphere an important source of moisture 

 to plants, both directly, and indirectly through the medium 

 of the soil. 



Although plants probably do not absorb moisture direct- 

 ly from the atmosphere, the deposition of dew upon their 

 leaves arrests evaporation, and thus diminishes the drain 

 upon the moisture stored in the soil. 



