1 6 Minnesota Plant Life. 



with one another amid faxuraljle conclititjus of temperature, 

 moistitre and ihumination. The ef|iiatorial reji"ion of the world 

 is at once the crache and the crucible of plant-life. In that tre- 

 mendous stru£i'^ie for existence many of the modern improve- 

 ments and refinements in plant structure beg'an to originate. 

 During" the centmies. forms unfavorable were eliminated and 

 destroyed, leaving the stronger in a condition to migrate north 

 or south as rapidly as they accommodated themselves to the 

 increasing obliqueness of the sun. Evidently, then, the great- 

 est differences should l)e expected not between the plants of 

 North America and Europe, both of them tenanted l)y north- 

 bound immigrants from the ecjuatorial region, nor even between 

 the north temperate regions and the tropics, since the plants in 

 the former are but the traveled relatives of those at home in the 

 latter region. But the greatest difference should be expected 

 to exist, as it does, between those plants which have left the 

 tropics and have slowly made their way, changing their form 

 and habits as they wandered, some to the far north and others 

 to the south. 



North American flora. If the North American continent 

 were (juite flat, without differences in ele\ation al)o\e the sea, 

 and were connected with the tropics by a continuous stretch of 

 land, it could be imagined that the forest region might have 

 extended directly across the northern half of the continent. 

 It is, however, not such a le\el plain, for two great uKnuitain 

 ranges run from north to south and the continent is cormected 

 with the tropics b\ a narrow isthmus, so that there are factors 

 which pre\-ent an even di\ision of forest and prairie. r\I(UuUain 

 ranges extending from north to south arc not, as mountain 

 ranges extending from east to west would be, barriers against 

 plant distribution from the tropics toward tiie poles. This is 

 tin- reason why North America has what the botanists call a 

 "ricb.er tlora" than lun-ope and Asia. In the ( )ld World tiie 

 princi])al nionniain ranges, such as the I'xrenees, the A1])S. the 

 Ai)penines, the Carpathians, the C'aucasus and llie llimalayas 

 are transverse, extending in a generally east-and-w est direction, 

 i'or this reason when the glacial ])erio(l came on. unfortunate 

 European or .\siatic plants as they migrated >oulli, fouml them- 

 seK'CS compelled to (.-limb some nionnt;iin range in order to 



