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and relics of a former prairie occupation. It is always difficult and 

 usually impossible to distinguish the two. It may be assumed that 

 most of the annuals and the more abundant perennials with effective 

 means of dispersal are invaders, while rare or solitary perennials are 

 relics. Aster sericcus is one of the most reliable examples of this 

 type. As the forest increases in density, the sun-loving prairie species 

 become more and more restricted to the small openings between the 

 trees (PI. XIV, Fig. 2). There the number and character of the 

 species vary very regularly with the size of the opening. For illus- 

 tration, in the Oquawka area openings 12-15 ft. (4-5 m.) across 

 usually contain Opiinfia Rafinesquii and Rudbeckia hirta, while others 

 of twice the diameter may have in addition Pmiiciun pseitdopubescens, 

 Carex Mnhlenhcrgii, and Bouteloua hirsiita. In general, the per- 

 ennial members of the group extend farther into the shade, and the 

 interstitial annuals are more intolerant ; few species appear, and they 

 are usually limited to the larger openings between the trees. This 

 feature of distribution is probably correlated with the duration of life 

 of the plants, and the demands of their seedlings for certain definite 

 light conditions. Some species do not occur beyond the more open 

 woods or the larger open spots. Such are Petalosfemiim purpureum, 

 Pciitstcmon hirsufits, Tradescantia reUexa, Lcspcde::a capitata, Lith- 

 ospermiim Gmclini, Viola pedata, and many others. Some species 

 grow well and reach a normal size in the sun, while in the shade 

 they are stunted or sterile. Shade plants of Coreopsis palmata are 

 weak and lax, with thin divaricately lobed leaves; Physoste^ia dcn- 

 ticidata is weak and thin-leaved, contrasting sharply with the stout, 

 thick-leaved form in full sun ; Tephrosia zirginiana grows with single 

 stems instead of dense bunches ; the bunch-grasses are loose and lax, 

 and tend to lose their bunch habit. Andropogon furcatiis and A. 

 scopariiis are more tolerant than the other bunch-grasses, but in the 

 shade they lose their bunch habit completely, sending up single culms 

 which have a few long spreading" leaves and are always sterile. R^id- 

 beckia hirta and Poa pratensis seem to have about the same light 

 requirements. Both are found only in the more open woods or in 

 sunny places, and are veiy frequently associated. Notwithstanding 

 the limitations in their distribution, a few of these plants plav an im- 

 portant part in the composition of the association, and are almost as 

 characteristic of the forest as some members of the forest group 

 proper. They are Lithospernnun Gmelini and Rudbeckia hirta, be- 

 cause of their conspicuous showy flowers, Tephrosia znrginiana, be- 

 cause of its bunch habit, and Lespedeaa capitata, because of its great 

 frequency and abundance. These four occur in every area of the as- 

 sociation. 



