G3 



^. AliitTuis.. 



(a) AljsolTJte altitude is lieig-ht a1:)0ve the se^. level. 



(1)) Relative altilidfteis lieiglit alcove the sn.rroundine ooxjritry. 

 the first has a strong effect and c-i-ves rise to distinct zones. 

 It shows especially over ereat heights while relative height effects 

 smaller areas which differ slitihtly in altit-ade.& In three hills 

 of ZOO, 400 and 800, the vegetation will be ahout the same, being 

 exposed to about the same conditions unless protected by otlier hills. 



In a hill in the latitude of N.Y. the most xerophytlc plants 

 will generally grow on the upper parts of the north and south slopes. 

 The north slope is xerophytic because there is less heat and an 

 increased ex]Jos\ire to colder winds, while on the south the sane 

 condition^ prevail, the soil being drier on account of the greater 

 amount of heat. J^^ast and west slopes will be more mesophytic. 



In the latitude of Georgia tlie south slope will be the hottest 

 and driest making; it most xeroph^rtic ; while the lower north slope 

 will be most mesophytic owing to the greater amount of moist-ure, 

 the latitude being such that the least supply on tlie north side 

 will be sufficient to prodiice a mesophytic flora. 



In the forests of Canada, a xerophytic forest is us\ially due 



to the absence of heat and not to the absence of moistu^-e. Honce 



the/most portion of a 



in this case ?i xerophytic hill will be the upper part of the north 



slope, since that region is most exposed to the cold. The lower 



part of the south slope will probably be mesophytic, due to the 



increased heat. 



