174 



er proportion of Cnllirrlioe fridnf/iifufti and Crnrni rirr/itiia/Hi. 

 The latter is extremely abundant, and grows in dense circular 

 patches ten feet or more in diameter. Cdllirrlioi' triinitjnhitd is 

 likewise abundant, and, although not forming dense patches 

 like Craccfi. display's its showy purple flowers in profusion, pro- 

 ducing an effect rivaled only by the yellow beds of Ctissin 

 c/uDtim'risfd on the prairies. Other prominent members of tlie 

 flora are Opioitid liimilfuxn. AUIojiid iiiiridfi'nien, Cdssia tlidiide- 

 cristd, Froclicliid cdiupestris, Meihoiiiid se.ssl/ifolid, Hi'lidiifhiis 

 (tecidentaJia, and Helirnithcnnou majits, 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 followiny;: 



^p 



Pteridium aquilinui/i Anychia canadensis 



Polygonum cristatum Erysimum arkansamim 



Talinum rucfospermum Cassia nidi fans 



Meibomia nuditfora Hypericum sp/uproearpum 



Meihomia paniculata Lechea villosa 



/pomoea pandurata ■ Pentstemon hirsutus 



Xahahis asper Galium pilosum 



Artemisia caudata Helianthus iUinoensis 



The structure of the vegetation of the black-jack association 

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

 ences in the light inten.sity. In natural clearings there is a 

 preponderance of Craccd rirf/iiiiaitd, Hididiiflnis occideidalis, and 

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

 tion, such as Cd.snid rhdintcrrififd, Aiiihrosid jisilosfiicln/d. and Mo- 

 n'trd'i jiinicfdfd. may be found with them. In the clearings 

 Crotottop^is lined ris is the prevailing form, associated with a 

 number of other species. 



Tal)les 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 lilack-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 



