276 ON THE GEOGRAPHICAL. 



thern provinces of Mexico. Magellanic plants likewise exist 

 on the Andes of Chili. In the southern parts of the United 

 States, the low ridge of the Alleghanies (which rarely exceeds 

 three thousand feet Of elevation) affords a multitude of plants 

 which avoid the low country, and are otherwise confined to 

 the northern states. 



Mountains thus introduce seeming confusion into our flo- 

 ras, while there exists in reality the most perfect order. 



A lofty and unbroken chain likewise presents a barrier in- 

 surmountable to many plants. There is much difference in 

 the vegetation between the northern and southern sides of the 

 European Alps — the flora of Chili differs essentially from that 

 of the country on the opposite side of the Andes. 



A great river is also an obstacle to the diffusion of plants, 

 apparently less easily overcome than a much more considera- 

 ble extent of ocean. Many plants on either side of the Mis- 

 sissippi do not cross it. The great rivers of Siberia are known 

 to exert a like influence on vegetation. 



Water, with those substances it dissolves in the soil, is the 

 the food of plants, and the quantity they require for the per- 

 formance of their functions varies in different species. A 

 plant can vegetate only in a soil containing a certain propor- 

 tion of moisture. The seeds of aquatics will not germinate 

 unless beneath the surface of water, while some plants flourish 

 only in the most arid sands. 



Difference in soil, so far as vegetation is concerned, is known 

 to consist mainly in the quantity of water it is capable of ab- 

 sorbing, and its power of resisting evaporation, two qualities 

 dependant on a variety of circumstances : — on the character 

 of the rocks from which the mineral part is derived, whether 

 such as resist decomposition, or yield to it, forming clay ; or 

 such as break down into gravel and sand, &c. — on locality, 

 whether on plains and the summits of hills, or on declivities 

 moistened by the filtering of water from higher places, or in 

 low grounds perpetually saturated with it, thus forming 

 marshes and bogs ; — whether in the vicinity of, or at a dis- 



