1 8 THE PLANT SOCIETIES OF 



limestone ravine resembles a sandstone ravine far more than a lime- 

 stone ravine resembles an exposed limestone bluff, or a sandstone 

 ravine resembles an exposed sandstone bluff. We may make the above 

 statements in another form. Rock as such, or even the soil which 

 comes from it, is of less importance in determining vegetation than 

 are the aerial conditions, especially exposure. And it is the stage in 

 the topography which determines the exposure. 



All of the preceding statements as to topographic stages, whether 

 young or old, refer not to times but to constructional forms. Two 

 ravines, equally youthful from the topographic standpoint, may differ 

 widely as to actual age in years or centuries, since erosion is more 

 rapid in one rock than in another. In our region, however, elements 

 of actual time are not very important, except as between rock and clay, 

 since the limestone is less soluble and the sandstone is more easily 

 eroded than is often the case. 



B. The river bluff. — As, a valley deepens and widens, the conditions 

 outlined above undergo radical changes. From this point it will be 

 necessary to discuss two phases in the growing river, the bluff phase 

 and the bottom phase. We have left the clay ravine bluffs in a state 

 of temporary climax, clothed with luxuriant mesophytic forest trees 

 and with a rich undergrowth of vernal herbs. More and more the 

 erosive processes are conspicuous laterally, and widening processes 

 prevail over the more primitive deepening. As a result, the exposure 

 to wind, sunlight, and changes of temperature increases; the moist- 

 ure content of the slopes becomes less and less. The rich meso- 

 phytic herbs, including the liverworts and mosses, dry up and die. 

 The humus oxidizes more rapidly, and a xerophytic undergrowth 

 comes in. In place of Hepatica and its associates, we find Antennaria, 

 Foa compressa (Wire grass), Equisetum hyemale (Scouring rush), and 

 other xerophytic herbs ; Polytrithum also replaces the mesophytic 

 mosses. The first signs of the new xerophytic flora are seen at the top 

 of the ravine slope ; indeed, the original xerophytic plants may never 

 have been displaced here by the ravine mesophytes. As the ravine 

 widens, the xerophytic plants creep down the slope, often almost to 

 the water's edge. Some of the young ravines between Evanston and 

 Waukegan show xerophytes at the summits of the slopes. Fig. j 

 shows a widening ravine at Beverly Hills ; the vegetation is much less 

 luxuriant than that shown in the young ravine oifig. 2. 



After a few years have passed, xerophytic shrubs appear on the 



