168 THE GEOGRAPHY OF PLANTS. 



time was allowed it to shed its leaves, or recruit 

 its powers by hybernation. 



We have not space now to enter into details 

 respecting our other forest trees ; we may just 

 state, that the birch ceases on the coast of Nor- 

 way at 71; the beech at 58; the sycamore and 

 the ash at 63; the lime occurs in Kussia at 60; 

 the elm and maple at 57. This, too, is the 

 region of our corn plants. Wheat does not 

 succeed beyond 63 N. lat., and in Russia not 

 beyond 60; in Kamschatka the limit is 51. 

 On the east of America, corn does not extend 

 beyond 52. Barley sometimes ripens in fa- 

 vourable summers as high as 70 in Norway. 

 Nor does the cultivation of our corn plants 

 succeed better in the torrid zone than in polar 

 regions, except in elevated spots, where the heat 

 is moderated by the altitude. We know nothing 

 respecting the original place of growth of the 

 cerealia ; though there is every reason to 

 believe that their cultivation is coeval with the 

 existence of man upon the earth. Within the 

 temperate zones their cultivation is all but 

 universal, and in the more favourable situa- 

 tions, such as Egypt, etc., its produce is very 

 great. Humboldt mentions wheat plants m 

 Mexico which sent up forty, sixty, or seventy 

 stalks, the ears of which were almost equally 

 well filled, and contained one hundred to one 

 hundred and twenty grains each. 



Some of the distinguishing plants of this 

 zone, in Europe, are the cruciform plants ; such 

 as the mustard, cress, wall-flower, stock, cab- 



