SOURCK OF THEIR ORGANIC CONSTITUENTS. 183 



bringing with it to the ground a portion of carbonic acid, and of 

 nitrogen or ammonia, &c., supplies the appropriate food of the plant 

 to the rootlets (sometimes in a liquid, but also much of it in a gaseous 

 form). Imbibed by these, it is conveyed through the stem and into 

 the leaves, where the superfluous water is restored to the atmosphere 

 by exhalation,* while the residue is converted into the proper nour- 

 ishment and substance of the vegetable. 



332. The atmosphere is therefore the great storehouse from 

 which vegetables derive their nourishment ; and it might be clearly 

 shown that all the constituents of plants, excepting the small earthy 

 portion that many can do Avithout, have at some period formed a 

 part of the atmosphere. The vegetable kingdom represents an 

 amount of matter, which plants have withdrawn from the air, organ- 

 ized, and confined for a time to the surface. 



333. Does it therefore follow, that the soil merely serves as a 

 foothold to plants, and that all vegetables obtain their whole nour- 

 ishment directly from the atmosphere ? This must have been the 

 case with the first plants that grew, when no vegetable or animal 

 matter existed in the soil ; and no less so with the first vegetation 

 that covers small volcanic islands raised in our own times from the 

 sea, or the surface of lava thrown from ordinary volcanoes. No 

 vegetable matter is brought to these perfectly sterile mineral soils, 

 except the minute portion contained in the seeds Avafted thither by 

 winds or waves. And yet in time a vast quantity is produced, which 

 is represented not only by the existing vegetation, but by the mould 

 that the decay of previous generations has imparted to the soil. We 

 arrive at the same result by the simple experiment of causing a 



* The water exhaled may be again absorbed by the roots, laden with a new 

 supply of the other elements from the air, again exhaled, and so on ; as is 

 beautifully illustrated by the cultivation of plants in closed Ward cases, where 

 plants are seen to flourish for a long time with a very limited supply of water, 

 every particle of which (except the small portion actually consumed by the 

 plants) must pass repeatedly through this circulation. This vegetable micro- 

 cosm well exhibits the actual relations of water, &c. to vegetation on a large 

 scale in nature ; where the water is alternately and repeatedly raised by evapo- 

 ration and recondensed to such extent that what actually falls in rain is esti- 

 mated to be re-evaporated and rained down (on an average throughout the 

 world) ten or fifteen times in the coui-se of a year. In this way the atmosphere 

 is repeatedly washed by the rain ; and those vapors icashed out which else by 

 their accumulation would prove injurious to men and animals, and conveyed to 

 the roots of plants, which they are especially adapted to nourish. 



