300 PHYSIOLOGICAL BOTANY. PART II. 



proportionably fertile. Sandy districts support only 

 such low or trailing plants as the wind cannot readily 

 root out, or those which have very deep and branching 

 roots ; whilst very tenacious clays are adapted only to 

 such species as have small roots, and which do not 

 require any great depth of earth. 



(307.) Influence of the Atinnx/ilicrr. Although the 

 atmosphere is every where of the same chemical com- 

 position, its effects may vary in proportion to the density 

 which it possesses at different elevations, or according 

 to the materials (as moisture, gases, &c.) which may 

 be suspended in it; or lastly according to its mecha- 

 nical action, in the greater or less degree of violence 

 with which it is moved in different regions. It is pro- 

 bable that the difference in density which the atmosphere 

 possesses at different elevations above the surface of the 

 earth, produces little or no effect in comparison with 

 those which result from the modifications which the 

 temperature, light, humidity of the air, &c. undergo. 

 Since the mean temperature diminishes in receding from 

 the equator much in the same proportion as in ascending 

 a mountain, many plants peculiar to the plains of higher 

 latitudes are found on the tops of mountains in w armer 

 climates. Hence a very, extensive range may be given 

 artificially to some plants, by cultivating them at 

 different altitudes in different latitudes. Humboldt 

 has likened the earth to two great mountains whose 

 bases meet at the equator, and whose summits are the 

 poles ; and, ceterig parihus, we may say that the 

 latitude at which a plant thrives best will vary as the 

 altitude above the sea at which it also flourishes under 

 the tropics. The potato offers an interesting illustra- 

 tion of this fact growing in Chili, at an altitude of 

 eleven or twelve thousand feet above the level of the 

 sea, and being well adapted to summer culture in the 

 plains of the temperate zone as far north as Scotland. 

 The olive has a much less extended range, and can only 

 be cultivated as far north as 24, and at an altitude of 

 twelve hundred feet in tropical climates. 



