2 Minnesota Plant Life. 



would have immigrated equally from all directions. Such, how- 

 ever, is bv no means the fact and it is needful to inquire further 

 into the conditions which regulate plant distribution before 

 the true situation can be understood. 



Minnesota climate. Connected with the geographical posi- 

 tion of the state, and to a very great degree dependent upon 

 it, is that combination of average winds, average temperature, 

 average precipitation of moisture, and average illumination 

 by the sun to which is given the general name of climate. Min- 

 nesota enjoys what is known as a mid-continental climate, 

 characterized by warmth in the summer and cold in the winter. 

 There are no prevailing winds from years end to year's end 

 as there are at some places by the sea. The sun never shines 

 with equatorial directness, nor are there ever weeks or months 

 of twilight, or of darkness, as in the regions of the poles. There 

 are no great mountain ranges to cool the clouds as they move 

 across the sky and to force them to yield their moisture in the 

 eternal snows; and during the year there may always be ex- 

 pected an average rain-fall of about twenty-five inches. Through 

 the spring and summer there is always a rise in temperature to 

 stimulate growth, but there is never that fervent, damp heat 

 which favors the rank and luxuriant vegetation of the tropics. 

 Consequently there are to be found in Minnesota, plants adapted 

 to the rhythm of the seasons, to the oblique illumination of the 

 sun, to the average moisture of the air and of the soil, and to 

 the winds which sometimes sweep over the prairies with an 

 almost resistless force. 



As an illustration of the adaptation of plants to seasonal 

 rhythm may be mentioned the autumnal habit of most trees in 

 Minnesota of shedding their leaves. Indeed, this is so common 

 a fact of experience that it is scarcely realized to be a definite 

 reaction of the plant to its environment. Yet leaves do not fall 

 merely because the nights are growing cold, but because there 

 is formed at the base of each leaf-stalk a little layer of cork 

 which, when complete, cuts the leaf from the twig as if by a pair 

 of shears. Certainly such is not everywhere a necessary habit, 

 for it is known that in the tropics many trees do not lose their 

 leaves each year, but retain them for varying periods of time 

 until their usefulness is past a character shared also by some 



