FLORISTIC FEATURES OF A PRAIRIE PROVINCE 385 



Stability, for the latter are seen to be driven out before the 

 former have be^riin to give way. 



The ability" of blue grass to invade formations which for hun- 

 dreds of years have maintained a balanced type is a phenomenon 

 which has not been satisfactorily explained. It is usually asso- 

 ciated with Veriena stricta on the hill areas, -vith Ambrosm arte- 

 mes-iifolia throughout its range and quite recently in the vicinity 

 with Melilotus alha, which has made its denouement along road- 

 sides and railroads and is continuing its march with blue grass 

 into the prairies. While Poa pratensis and Ambrosia artemesi- 

 folia grow luxuriantly in moderately moist conditions, their 



Fig. 159. — Ulmus americana, Populus deltoides, Crataegus mollis, — pioneer 

 forest trees on the edge of a prairie ravine. 



range extends from hilltop to alluvial basin though they do not 

 thrive w'here the soil is constantly saturated nor in stony graveled 

 areas. Blue grass possesses a radially extensive, slender, active 

 rhizome with a fine, close network of node roots which forms a 

 firm sod. Its broad range of habitat combined with its virile 

 rhizome system equips it with unusually good qualities for suc- 

 cessful invasion. The fact that Ambrosia though an annual has 

 an abbreviated tap root which fits easily bet-ween the rcots or 

 rhizomes of other plants while it also has a wide range of 



