Minnesota Plant Life. 3 



trees of temperate regions. Apparently, then, the habit of 

 rejecting leaves that would be killed by the winter's cold and 

 would become burdensome another summer may be directly 

 connected with the geographical position of the state. 



Again, trees with enormous, delicate leaves like those of many 

 palms or bananas, are not found upon the prairies of the Red 

 river, because, clearly, if trees with such thin, large leaves were 

 exposed to the wind they would be blown to pieces and their 

 life would be destroyed. Large delicate-leaved forms are more 

 characteristic of regions where the wind is slight or where it is 

 broken by masses of surrounding vegetation. 



Furthermore, in a state so well watered as Minnesota there 

 is no development of those curious desert types which are seen 

 in Arizona, in the Sahara, or in the arid regions of South Africa 

 and Australia ; for where it is arid those plants only can grow 

 that by structure and habits are fitted to utilize the relatively 

 small quantities of moisture. The strange columnar cacti of 

 the Gila, standing leafless and rigid — vegetable pillars of the 

 desert — would be out of place wherever the rain-fall permits the 

 production of ordinary leaves and branches. Thus in survey- 

 ing the vegetation of the world one is impressed with the influ- 

 ence of climate upon the plant population of every district. 



The physical history of Minnesota. A knowledge of its 

 geography and climate does not, however, afTord all the data 

 for comprehending the vegetation of any region, since it is not 

 alone the climate of to-day, but even more strongly the climate 

 and other conditions of the past, that are reflected in the forms 

 and structures of the plants. Therefore, a knowledge also of 

 the geological history of the state and of its various soils is 

 essential to an understanding of its vegetation. There is strong 

 reason to suppose that about ten thousand years ago much of 

 the surface of North America was covered by a thick sheet of 

 ice which advanced slowly from the north and later as slowly 

 retreated. The period of ice-advance is known to geologists as 

 the glacial period, and throughout Minnesota are to be found 

 the traces of glacial action. The clays, pebbles and bowlders so 

 abundant throughout the state are believed to have been depos- 

 ited either upon the front of a glacial mass, or underneath, or 

 from the waters caused by its melting. When such a move- 



