NATURAL HISTORY OF PLANTS. frQ 



It would be superfluous here to take notice of other alterations 

 which do not tend to the improvement of the science. 



[JVildenozc. 



CHAP. Ill: 



NATURAL HISTORY OF PLANTS. 



X>v the Natural History of Plants, we mean a comprehensive view 

 of the effects of Climate on Vegetation ; the changes which it is 

 probable plants undergo from the revolutions of our globe ; their 

 dispersion over ils surface ; their migration ; and, lastly, the means 

 pursued by Nature for their preservation. 



Geographers have imagined the globe to be surrounded by cer- 

 tain Zones, and they have divided these into degrees and circles. 

 They suppose the hottest climate to be under the line, or at the 

 equator; a hot climate under the tropics; between these and the 

 polar circles, two different climates, a temperate and a cold ; and, 

 lastly, they consider, under the polar circle, a very cold climate to 

 prevail. 



Upon the whole, these divisions sufficiently coincide; but great 

 differences are produced by mountains, vallies, rivers, marshes, 

 woods, seas, and inequalities of surface ; so that there are places 

 which, according to the above divisions, ought to be hot, which 

 are however temperate, or even cold, and rice versa. We must 

 therefore distinguish between a physical and geographical climate. 

 America and Asia are much colder in the same northern geogra- 

 phical latitude than our part of the world. Plants which in Ame- 

 rica grow under the 42d degree of northern latitude, bear in our 

 climate the cold of 52. The reason of this great difference seems 

 to be the enormous swamps and woods of America, and the im- 

 mense elevation of the land in Asia. Africa, under the tropics, is 

 incomparably warmer than Asia or America. The chains of moun- 

 tains in these last countries, and the humidity of the vallies, mode- 

 rate the great heats, while the burning sands, of which almost the 



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