SOURCE OF THEIR ORGANIC CONSTITUENTS. 187 



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

 the plant to the rootlets. Imbibed by these, it is conveyed through 

 the stem and into the leaves, where the now superfluous water is 

 restored to the atmosphere by exhalation,* while the residue is con- 

 verted into the proper nourishment 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 without, have at some period formed a 

 part of the atmosphere. The vegetable kingdom represents an 

 amount of matter, which the force of organization has withdrawn 

 from the air, 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 wafted 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 seed of known weight to germinate on powdered 

 flints, watered by rain-water alone. When the young plant has 



* 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 

 evaporation and recondensed to such extent that what actually falls in rain is 

 estimated to be reevaporated and rained down (on an average throughout the 

 world) ten or fifteen times in the course of a year. In this way the atmos- 

 phere is repeatedly washed by the rain; and those vapors washed 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. 



