174 



er proportion of Callirrkoe triangulata and Cracca virginiana. 

 The latter is extremely abundant, and grows in dense circular 

 patches ten feet or more in diameter. Callirrhoe triangulata is 

 likewise abundant, and, although not forming dense patches 

 like Cracca, displays its showy purple flowers in profusion, pro- 

 ducing an effect rivaled only by the yellow beds of Cassia 

 chama?crista on the prairies. Other prominent members of the 

 flora are Opuntia humifusa, Allionia nyctaginea, Cassia chamce- 

 crista, Froelichia campestris, Meibomia sessilifolia, Helianthus 

 occidentalis, and Helianthemum majus, all of which are found also 

 on the prairies. There are, however, a number of species which 

 apparently do not occur beyond the forest. Most important 

 among these are the following: 



Pteridium aquilinum Anychia canadensis 



Polygonum cristatum Erysimum arhansanum 



Tali n a m rugospermum Cassia nictitans 



Meibomia nudiilora Hypericum sphcerocarpum 



Meibomia paniculata Lechea villosa 



fpomwa pandurdta Pentstemon hirsutus 



JVabalus asper Galium pilosum 



Artemisia caudata Helianthus illinoensis 



The structure of the vegetation of the black-jack association 

 is remarkably uniform, the only variations being due to differ- 

 ences in the light intensity. In natural clearings there is a 

 preponderance of Cracca virginiana, Ilcliaathas occidentalis, and 

 Rhus aromatica, while a few species of the bunch-grass associa- 

 tion, such as Cassia chamwcrista, Ambrosia psilostachy a, and Mo- 

 narda punctata, may be found with them. In the clearings 

 Crotbnopsis linearis is the prevailing form, associated with a 

 number of other species. 



Tables X. to XII. give counts in the black-jack association. 



A tension zone, with intermediate ecological characters and 

 a mixture of species is not developed between the black-jack 

 association and the prairie. Its absence is an indication of the 

 relatively slight ecological differences between the earlier 

 stages of the black-jack and the mature prairie, but with the 

 gradual development of the forest great changes occur. The 



