424 



Minnesota Plant Life. 



dififerently conditioned organs of the same plant, will appear 

 altogether comprehensible. After snch facts have become 

 familiar it should not be difihcult to infer something of the his- 

 tory of a i)lant from its general architectural construction. The 

 droop in the branches of a spruce tree comes to have its sig- 

 nificance, and the prostrate bodies of the cranberry or par- 

 tridgeberry tell a similar story of the flattening ettect produced 

 by heavy falls of snow. 



Heat. Another natural force to which plants must necessa- 

 rily adapt themselves is that of heat. Therefore, under different 

 temperatures different forms and habits of vegetation may arise. 

 The range of temperature under which dormant life can be 

 maintained is apparently a prett}- wide one — between 400 and 

 500 degrees centigrade. The spores of some Ijacteria can en- 

 dure boiling for an hour or more and. for a short time, a dry 

 heat somewhat higher, while certain seeds have been exposed 

 for a season, without killing them, to the low temperature of 

 liquid aid. But the limits of plant growth, so far as regards tem- 

 perature, are considerably narrower. A few blue-green algae 

 are able to maintain themselves in hot springs, the waters of 

 which would bin-n the hand, while the little red-snow plant 

 grows upon snowdrifts on moinUain tops. Between these limits 

 — scarcely 100 degrees centigrade — are the tcm])eratures at 

 which active plant life and growth is possible. 



The form of plants is quite different under an average low 

 temperature from that which is developed in areas of greater 

 warmth. So, therefore, a very characteristic arctic vegetation 

 arises in contradistinction to that of the tropics. A comparison 

 of polar with tropical \egctation will scr\e to indicate what are 

 the inlhiences ui)on plant form and structure of rclatixel}" high 

 and relatixely low temperature. 1m )r the most i)art the \ege- 

 tation of the polar regions is dwarfed, consisting of low, tufted 

 plants, little shrubs, stimted herbs, mosses and lichens. In such 

 regions the great luxuriant trees and herbs of the tropics are 

 unknown. So, in the matter of size, the poles and the tropics 

 favor just the op])osite sorts of plants. In polar regions there 

 is only a short growing season and during the rest of the year 

 the temperatiu'e is so low, and the ilhuninalion so poor, that 

 ]ilants, like many of the animals, are compelled to pass into a 



