302 PHYSIOLOGICAL BOTANY. PART II. 



11. Forests. These districts may be 'considered 

 with respect to the trees which compose the forests, and 

 also with reference to the humbler species which seek 

 their shade. 



12. Copses and Hedyrx. 



13. Subterranean G/IV.V. 



14. Alpine. 



15. PARASITIC. (See art. 234.) 



16. PSEUDO-PARASITIC. (See art. 234.) 



(SOQ.) Jiot'iiiii-al Habitations. Greater uncertainty 

 prevails respecting the different habitations of plants 

 than their stations. If indeed the extent of their ha- 

 bitations were entirely dependent upon their range in 

 latitude, the difficulty of determining them would not 

 be so great ; but it is a remarkable circumstance, that 

 the vast majority of species grow naturally within cer- 

 tain limits restricted in longitude as well as in latitude ; 

 that is to say, the limits within which they naturally 

 occur, are much more restricted than the regions through- 

 out which they might readily grow, so far as climate is 

 concerned in this question. There are indeed some 

 species which have a very extensive range in longitude 

 as well as in latitude, and are even found in both 

 hemispheres, but several of these have undoubtedly 

 become thus generally dispersed by the agency of man. 

 Others we may equally conclude to have been trans- 

 ported by natural causes, from the habitations to which 

 they were first restricted. But when we have made all 

 such allowances, we find the great majority of species 

 so far restricted in their range, as to lead us to the very 

 probable supposition that each was originally assigned 

 by the Creator to some definite spot upon the surface of 

 the earth, from whence it has wandered to a greater or 

 less extent in all directions, until it happened to meet 

 with such obstacles as were sufficient to check its fur- 

 ther progress. It may be worth while to consider the 

 nature of those obstacles which afford the most effectual 

 barrier to the migration of species from one part of the 



