66 MODEKlSr GEOGEAPHT EMBEACES ORGAlSriC LIFE. 



torrent had lowered. So important an element of reconstruction 

 is this, that it has been seriously questioned whether, upon the 

 whole, vegetation does not contribute as much to elevate, as the 

 - waters to depress, the level of the surface. 



Whenever man has transported a plant from its native habitat 

 to a new soil, he has introduced a new geographical force to act 

 upon it, and this generally at the expense of some indigenous 

 growth which the foreign vegetable supplants. The new and 

 the old plants are rarely the equivalents of each other, and the 

 substitution of an exotic for a native tree, shi-ub, or grass, in- 

 creases or diminishes the relative importance of the vegetable 

 element in the geography of the country to which it is removed. 

 Further, man sows that he may reap. The products of agricul- 

 tural industry are not suffered to rot upon the ground, and thus 

 raise it by an annual stratum of new mould. They are gathered, 

 transported to greater or less distances, and after they have served 

 their uses in human economy, they enter, on the final decompo- 

 sition of their elements, into new combinations, and are only in 

 small proportion returned to the soil on which they grew. The 

 roots of the grasses and of many other cultivated plants, however, 

 usually remain and decay in the earth, and contribute to raise its 

 surface, though certainly not in the same degree as does the 

 forest. 



The smaller vegetables which have taken the place of trees un- 

 questionably perform many of the same functions. They radiate 

 heat, they absorb gases, they exhale uncombined gases and watery 

 vapor, and consequently act upon the chemical constitution and 

 hygrometrical condition of the air ; their roots penetrate the earth 

 to greater depths than is commonly supposed, and form an inex- 

 tricable labyrinth of filaments which bind the soil together and 

 prevent its erosion by water. The broad-leaved annuals and per- 

 ennials, too, shade the ground, and prevent the evaporation of 

 moisture from its surface by wind and sun.* At a certain stage 



* It is impossible to say liow far the abstraction of water from the earth by 

 broad-leaved field and garden plants — such as maize, the gourd family, the 

 cabbage, etc. — is compensated by the condensation of dew, which sometimes 

 pours from them in a stream, by the exhalation of aqueous vapor from their 

 leaves, which is directly absorbed by the ground, and by the shelter they 

 afEord the soil from sun and wind, thus prevejiting evaporation. American 



