ON THE GEOGRAPHICAL DISTRIBUTION OF PLANTS. 275 



station meets a climate fatal to it, from a deficiency or excess 

 of heat, marking its northern ami southern limits. 



A plant can thus exist only within two lulls encircling the 

 globe, one in the northern, the other in the southern hemi- 

 sphere, in general coinciding with the parallels of latitude, 

 hut rendered irregular by the variation of climate at different 

 meridians, elevation <>f the eartl-.'s surface, 8tc 



The breadth of these belts varies greatly in differenl spe- 

 cies, and though experience has sufficiently demonstrated that 

 it may he determined to within one degree of latitude, this 

 has not yet been fully accomplished in a single instance. A 

 careful examination of North American plants, and a compa- 

 rison of authorities have yielded the following result. In a 

 great part, perhaps one third, of the North American species, 

 it does not exceed five degrees of latitude, and rarely attains 

 twenty. 



The range of plants is. however, far from being as exten- 

 sive as clhiuih would permit. The plants of the northern 

 and southern hemispheres are different : the same may he said 

 with respect to the plants of the eastern and western conti- 

 nents, except in the extreme north. Hence we must seek 

 for some other cause restricting the diffusion of plant-, and 

 this will be found in the ocean, a great exit nt of which at 

 once prevents farther progress. 



\_ain. few plants stretch aero— (he whole extent of the 

 continents, in their wider parts, their range is usually still more 

 limited. This leads to the examination of oth< r can lea which 

 impede their diffusion. 



My ascending above the level of the ocean, the temperature 

 is found to he reduced in the same manner ;«- on advancing 



towards the pole-, ami at the heighl of perpetual mih« we 



find a polar climate. The relative mean proportion has been 

 estimated at about six hundred feet of elevation to a degree 



of latitude. Accordingly, if a chain of mountains extends in 



the direction of the meridians, plants of cold regions, meeting 



a parity of climate, descend on their summit- into Lowef lati- 

 tudes. On the Rocky mountains, arctic plants reach the nor- 



