226 TYPICAL EXAMPLES OF PLANT SUCCESSION 



berries {Rubus sp.). The shrub stages would be followed by a xeric 

 tree community in which the dominant species would be the black 

 and the w^hite oaks and the shagbark and the pignut hickories 

 (Carya ovata and C. cordiformis) . 



In the tall grass prairie region, which includes a large part of 

 Illinois and several states west of the Mississippi River, one of the 

 dominant species of the climax community is the tall blue-stem grass 

 (Andropogon fiircahis). The xerarch succession here starts much 

 the same as in the forest region, but the pioneer herbs are followed 

 by the short blue-stem grass {Andropogon scoparius) and this in 

 turn by the climax tall blue-stem association. 



The hydrarch succession may be traced as it occurs in a slough. 

 Here the bulrush (Scirpus flumatilis) is at first the most abundant 

 plant, though it may be accompanied by many other kinds of water 

 plants. The bulrushes begin to die as the slough becomes drier 

 through filling and are replaced by sedges (Carex sp.) which dominate 

 the second stage. The sedges are followed by slough grass {Spartina 

 michmixiana) . Sometimes the slough grass follows the bulrush 

 directly without an intervening sedge stage. The slough grass is 

 followed by blue joint-grass (Calamagrostis canadensis) and tall 

 panicum {Panicum virgatum), or by panicum alone, which, in turn, 

 is finally replaced by tall blue-stem. 



137. Successions on Rock.— Areas of bare rock are relatively 

 infrequent and of limited extent. They are for the most part rocks 

 that have been exposed through erosion by water or by glaciers. 

 Rocks vary greatly in hardness and this factor is very important in 

 determining the rapidity with which the earlier stages of plant suc- 

 cession may take place. Rocks are classified as igneous, sedimentary, 

 and metamorphic. Igneous rocks are those which were melted and 

 have solidified by cooling. They are glassy or crystalline in char- 

 acter and are usually very hard. Granite is a familiar example. 

 Sedimentary rocks are those which have been laid down under water 

 by mechanical, chemical, and biotic processes. They are composed 

 of more or less rounded and worn fragments, are seldom crystalline, 

 and frequently are quite soft. The common sandstones, limestones 

 and shales are sedimentary rocks. Metamorphic rocks are those 

 which have been changed from igneous or sedimentary rocks, usually 

 by heat and pressure. They are usually harder than the rocks from 

 which they are formed. INIarble, for example, is metamorphosed 

 limestone and slate is formed from shale. 



