270 The Absence of Trees from Prairies. [June, 



a long time, but when permitted to occur, they are always very violent, 

 and the land is inundated with a deluge of water. In mountainous 

 regions, rains are much more frequent but not so excessive ; indeed, the 

 districts occupied by the Andes receive their supplies of water, not in 

 heavy showers, but in mists of several weeks continuance. 



The green vesture of vegetable forms which adorn the surface of our 

 planet, assumes peculiarities dependent on the meteoric influences which 

 operate upon them, and every climate has the kind of plants adapted to 

 the physical conditions it presents for their support. As the total 

 absence of rain excludes all plants from deserts, it would be reasonable to 

 expect that the vegetation on extensive plains should acquire a peculiar 

 character, in consequence of the occasional prevalence of long drouths 

 and excessive rains. Plants, like animals, are often the victims of dis- 

 ease ; and my present object is to shew, that the uufivorable conditions 

 to which trees are exposed on vast plains will be productive of so much 

 disease and degeneracy, as frequently to cause them to disappear entirely 

 from the soil. 



Though much obscurity still hangs over several vital operations, the 

 chief points connected with the growth of trees can be determined with 

 considerable certainty. The vital fluid essential to vegetation, and cor- 

 responding to the blood of animals, is called the sap, and this rises along 

 the woody part of the tree from the roots to the leaves, and returns along 

 the bark and the space which separates it from the wood. By this cir- 

 culation, nutritive matter is conveyed from the soil to every portion of 

 the vegetable fabric ; and it is gradually converted into wood, forming a 

 new layer every year, and increasing the density of the wood of an earlier 

 origin. We may accordingly, ascertain the age of a tree by counting 

 the number of annual circles, each of which represents the growth of a 

 single year. These remarks are only intended to apply to the trees of 

 temperate climates ; since, of those growing between the tropics, many 

 have endogenous stems, in the interior of which, the woody tissue is 

 deposited, and on tliis account, their age can not be determined with the 

 same facility. 



The causes which contribute to change the soluble organic matter 

 which the soil supplies into wood, deserve special consideration. Though 

 the leaves are concerned in producing this change, it is evident that the 

 woody matter must be formed where it is deposited ; and in this place, 

 access is denied to the solar light, which is capable of exerting a con- 

 siderable influence on several kinds of chemical and vital operations. 

 But it appears, from the discoveries of modern times, that electricity 

 serves as a very important instrument for performing chemical action. 



