53 



competition with the bunch-grasses is defensive rather than offensive; 

 they can resist the encroachment of a grass, but are not able to dis- 

 place it. Some typical plants of this habit are Aster linariif oliiis , 

 Lithospennuin Guidim, Aster sericeus, Tephrosia virginiaua (PI. 

 IX, Fig. i), and Chrysopsis villosa. Others have more slender stems, 

 several of which arise from a common base and spread divergently, 

 somewhat resembling in habit the looser bunches of Carex Muhlen- 

 bergii or Cyperus Schweinitrj'ii. Good examples of this type are 

 furnished by CallirJioe triangiilata, Fctalosteniuin purpureiim, and 

 Petalostenium candidiun. Still others have erect stems which tend 

 to grow in clusters, as Coreopsis palinata, Solidago missouriensis, 

 Solidago nemoralis, and Hcliantheinum majns. A fourth type is fur- 

 nished by Physalis nirginiana, Baptisia hractcata, and Tradescautia 

 reflexa, with solitaiy stems which branch freely or bear widely 

 spreading leaves toward the top. A fifth type is illustrated by Bu- 

 phorhia coroUata or the species of Liatris, whose slender erect stems 

 grow singly and occupy very little ground space. This type ap- 

 proaches most nearly the third group of interstitials. One member of 

 the group, Breiveria Pickcringn, has ven^ numerous long decumbent 

 stems, fomiing an elevated mass at the center, and spreading out in 

 all directions on the sand. 



The shrubs of the association are for convenience classified in 

 this group. They include RJiits canadensis, var. illinoensis, forming- 

 dense rounded masses 3-15 feet (1-5 m.) across and three feet (i m.) 

 high, and excluding all other vegetation; Aniorpha canescens and 

 Ccanothus auiericanus, undershrubs with several erect or ascending 

 stems one to three feet {y^ dm.) high; and Ceanothus ovatiis (PI. 

 II, Fig. 2), with several ascending stems forming an irregular bushy 

 shrub two or three feet (i m.) tall. The two species of Ceanothus 

 are notable for their immense woody roots, frequently six inches 

 (1.5 dm.) in diameter and extending downward to great depths. 

 They are cro\\ned by a few live stems, which are of comparatively 

 short life, and with the dead and decaying bases of many others of 

 previous years. 



It is needless to say that the vast majority of these plants present 

 obvious xerophytic adaptations, the most general of which are a re- 

 duction of surface to narrow or small leaves, and a protective cover- 

 ing of silvery or canescent hairs or scales. Their general tone is 

 grayish green, amid which the vivid green of Buphorbia coroUata 

 and Tradescantia reflexa appears strangely out of place. The various 

 types described do not include all the species of the gTOup, but omit 

 some of the less frequent. Neither do all perennials belong to this 



