1 6 Minnesota Plant Life. 



with one another amid favorable conditions of temperature, 

 moisture and illumination. The equatorial region of the world 

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

 mendous struggle for existence many of the modern improve- 

 ments and refinements in plant structure began to originate. 

 During the centuries, 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 be expected not between the plants of 

 North America and Europe, both of them tenanted by north- 

 bound immigrants from the equatorial 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 quite flat, without differences in elevation above 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 level plain, for two great mountain 

 ranges run from north to south and the continent is connected 

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

 which prevent an even division of forest and prairie. Mountain 

 ranges extending from north to south are not, as mountain 

 ranges extending from east to west would be, barriers against 

 plant distribution from the tropics toward the poles. This is 

 the reason why North America has what the botanists call a 

 "richer flora" than Europe and Asia. In the Old World the 

 principal mountain ranges, such as the Pyrenees, the Alps, the 

 Appenines, the Carpathians, the Caucasus and the Himalayas 

 are transverse, extending in a generally east-and-west direction. 

 For this reason when the glacial period came on, unfortunate 

 European or Asiatic plants as they migrated south, found them- 

 selves compelled to climb some mountain range in order to 



