PAPERS OX BIOLOGY AND AGRICULTURE 261 



the ice front and the Ozark Hills in Illinois and the dis- 

 sected bluffs of the Missouri river in Missouri. In the 

 Plains Region the tundra probably was bordered by prai- 

 rie. The dessicating action of the strong winds would 

 not permit the growth of trees except along protected 

 stream slopes and possibly in some depressions. 



Each glacial advance probably represents a long period 

 of years during which the annual snowfall was greater 

 than the annual loss of ice by melting. On the other hand 

 the glacial retreats probably represent periods during 

 which the snowfall was less than the loss by melting. 

 The changes, then, which accompanied both advance and 

 retreat must have been gradual. As the ice retreated it 

 left a bare area composed of ground up rock fragments. 

 This bare area was invaded by the mosses and lichens 

 of the tundra because they can grow on such a substra- 

 tum and can endure the severe climatic conditions found 

 immediately below the glacial ice. As the ice retreated 

 farther and farther to the north, other plants invaded 

 the tundra. The dessicating action of the wind pre- 

 vented tree development in exposed situations, so the 

 trees invaded the protected slopes along streams and 

 morainal ridges while grasses occupied the intervening 

 areas. Swamps grasses and sedges undoubtedly were pio- 

 neers but, as the higher ground dried out, it probably was 

 occupied by xerophytic grasses and the pioneers were 

 restricted to the depressions which remained wet 

 throughout the summers. In the Plains Eegion xerophy- 

 tic grasses invaded the tundra and then moved east as 

 far as Ohio, occupying all the high ground except the pro- 

 tected places where trees were able to grow. 



Two kinds of tree vegetation invaded the tundra; the 

 bottomland vegetation along the flood plains of the 

 larger streams and the upland vegetation along morainal 

 slopes and stream bluffs. In the former the pioneers are 

 the willows followed by the river maple, cottonwood, ash, 

 the elms, linden, the walnut and butternut, and the pig- 

 nut. The pioneers of the latter are pines or xerophytic 

 shrubs, followed by oaks, then maples, and, in some 

 places, the beech. Pines have undoubtedly advanced 

 along some of the stream bluffs and some remain as 



