Chemical and Physical Papers. 79 



north. The south-facing bluffs are usually too dry for forest growth. 

 After the slope is of sufficient steepness much wash accumulates 

 toward the base and affords protection for young trees from fire and 

 a good soil for seedlings to develop. Because of shade, slowness 

 of evaporation, and safety from fire, the steep north-sloping bluff 

 often supports a small growth of trees even where the creek bank 

 is barely beginning to harbor isolated thickets and the first young 

 pioneers among the trees. The trees of the wooded bluffs are 

 mostly hackberry. White elm, wild plum, Rough-leaf dogwood, 

 Smooth sumac, and Choke-cherry, with a number of smaller shrubs 

 and lianas. 



The Ravine. — The ravine valleys are usually so narrow that 

 fires were very destructive, and formerly they contained little be- 

 sides small thickets of Amorpha fruticosa, usually confined to the 

 banks of ponds, Sand-bar willow, elderberry. Smooth sumac, and 

 Poison ivy, which usually grew as a sort of crown-former with an- 

 nual aerial shoots. But at present Peach-leaf willows, cottonwoods, 

 and occasionally White elms, Green ashes, box- elders, and escaped 

 Osage oranges and White mulberries are developing very rapidly, 

 so that where fire and grazing are absent little forest belts are ap- 

 pearing where thirty years ago there was nothing but pure prairie. 

 The abundant sediment deposited from the cultivated fields is 

 probably a very important factor at present in facilitating tree de- 

 velopment in the ravines. The steep north-facing bluffs of the 

 larger ravines are often covered with thickets of wild plum. Rough- 

 leaf dogwood, Smooth sumac, gooseberry, coral-berry, and rasp- 

 berry. Very often, also, Poison ivy and Riverside grape are present, 

 and occasionally a tree. 



