128 INFLUENCE OF THE ATMOSPHERE. 



extract their chief support. The carbonic acid from which they derive 

 so much of their solid carbon is obtained principally from the atmos- 

 phere, as well as their nitrogen and ammonia. Hence their distribu- 

 tion is more or less affected by the materials within the atmosphere. 

 Saline particles abound in certain parts, especially in the vicinity of sea 

 water. From these is derived the soda necessary for the support of 

 certain kinds of plants. Many indeed can exist nowhere else, nor with- 

 out this alkaline salt. An important general influence is likewise ex- 

 erted by the air on plants by its hygrometic action, or its property of 

 retaining and imparting moisture. The motion of the air and conse- 

 quently that of plants, doubtless exerts a salutary influence on the health 

 and vigor of plants by the wholesome exercise thus given. Its rarity or 

 density, has also a very important effect on plants. Rare air has a 

 greater capacity for heat than dense air. Hence the condition of the 

 atmosphere at different elevations, in its effects upon plants whicn are 

 less affected by heat m temperate climates, is important. The potato, 

 ;vhich flourishes in plains with us, grows in Peru at an elevation of 

 10,000 feet above the sea, but the ol ive, growing at 44 N. will not grow 

 above 12.50 feet. A less degree of moisture exists in proportion to the 

 greater degree of height. The general effect of elevation on the growth 

 of plants is however in the diminution of temperature. But the most 

 general influence in the distribution of plants by the atmosphere is in 

 its conveyance of seeds. These are, it is well known, widely diffused 

 by it, and very many plants have thereby been tested as to their 

 capability of growing in various climates and parts of the world. 

 That it has been thus instrumental during high winds in distributing 

 plants and inducing their growth, where otherwise they would not have 

 grown, must be apparent. It likewise affords the means by which 

 birds become agents in their diffusion. 



General Remarks. 



The adaptation of vegetable productions to the peculiar constitution 

 of plants is not more remarkable than it is to the necessities of man 

 and other animals. The luxurious profusion of vegetable nature in 

 the tropical regions ; the less vigorous and prolific powers of plants 

 in the temperate zone and the scanty and stinted productions ol the 

 polar circles, prove the same wise adaptation of nature to the re- 

 quirements of human society and the necessities of animals and of 

 plants themselves. But it is natural for men to conceive their own 

 latitudes best supplied with plants. The ancients of the temperate 

 climates thus conceived the torrid and frigid zones to be uninhabitable ; 

 and the people of the tropics may readily imasine that, on departing 

 from the track of the sun, the great source of vegetable life, they 

 must meet with nothing but sterility and death. But, though most 



