THE PLANT SOCIETIES. 



A. THE INLAND GROUP. 

 I. THE RIVER SERIES. 



A. The ravine. — No topographic forms lend themselves so well 

 to a physiographic sketch of the vegetation as do those that are con- 

 nected with the life history of a river. Beginning with the ravines, 

 which are deep and narrow, because of the dominance of vertical cut- 

 ting, we pass to the broader valleys, where lateral cutting becomes 

 more pronounced. From this stage on we have to deal with two phases 

 of river action, the destructive, which is concerned with the life history 

 of the bluff, and the constructive, which has to do, with the develop- 

 ment of the flood plain. 



Wherever there is an elevated stretch of land adjoining a body of 

 water,'' such as a lake bluff, one is apt to find excellent illustrations of 

 the tieginning of a ravine. Fig. i shows an embryonic ravine of a 

 type that may be seen frequently along the clay bluffs between Evans- 

 ton and Waukegan. A ravine of this type is essentially a desert, so 

 far as plant life is concerned. The exposure to wind and to alterna- 

 tions of temperature and moisture is excessive. The lack of vegeta- 

 tion, however, is due chiefly to the instability of the soil ; this 

 instability is particularly great in the case of clay bluffs such as these, 

 where the seepage of water causes extensive landslide action. No 

 plants can get a foothold in such a place, unless it be a few species 

 that may be able to make their appearance between periods of land- 

 slide action ; among these plants annuals particularly predominate. 

 The perennials that may be found in such places are almost entirely 

 plants which have slid down the bank. Near the center of fig. / is a 

 clump of shrubs that have slid down in this way. Ravines of a similar 

 type jnay also be seen at many places inland, and wherever found the 

 poverty of vegetation on the slopes is the most striking character. 



As a ravine extends itself inland the conditions outlined above 

 may be always seen about its head, but toward the mouth of the ravine 

 the slopes are less precipitous. Torrents cut down the bed of the 

 ravine until a depth is reached approaching the water level at its mouth. 



