48 THE PLANT SOCIETIES OF 



left behind; we have only the completed product, the mesophytic 

 forest. 



Where these mesophytic forests are disturbed we may perhaps get 

 some notion of what took place in the first postglacial centuries. On 

 the clay banks along the drainage canal, and also on recent river bluffs, 

 one may follow in rapid succession a series of plant societies leading 

 to the forest. There is here no pronounced lichen or moss stage as on 

 rock hills, but the first vegetation consists of xerophytic annuals and 

 perennial herbs. Xerophytic shrubs, especially Salix and Populus, 

 soon appear. It is not long before there is an extensive thicket forma- 

 tion with an herbaceous undergrowth. Humus accumulates with great 

 rapidity, and we soon have almost a mesophytic vegetation in which 

 the dominant thicket species are likely to be the aspen (Populus tremu- 

 loides), wild crab (Pyrus coronaria), red haw (Crattegus punctata, C. 

 coccinea, etc.). Such a thicket is the immediate forerunner of the oak- 

 hickory type of mesophytic forest. When a forest of oak and hickory 

 is cut down or destroyed by fire, it returns after a comparatively short 

 interval, but the first stages in the clearing are thicket stages much 

 like those just described. Of course it takes much longer to develop 

 a forest from naked clay soil than from a forest land that has been 

 cleared. Whether the stages that led up to the first postglacial forests 

 are such as have been described is very doubtful. It is much more 

 likely that the first forests were of slow growth and were coniferous in 

 character, such as are found farther north. Fig. 28 shows a typical 

 morainic hill forest of the above type. Here the dominant tree is the 

 red oak; a white oak is seen at the right. 



Among the shrubs of these morainic forests there may be men- 

 tioned, apart from the crabs and haws, the hazel (Corylus Americana), 

 and various species of Viburnum. Many herbaceous plants are found, 

 among which are Podophyllum (May%pple), Claytonia (Spring beauty), 

 various species of Aster, Trillium, Geranium maculatum, Viola pubes- 

 cens (Yellow violet). Anemone nemorosa, etc. Sometimes the bur oak 

 {Quercus macrocarpa) is the dominant tree in these morainic forests, 

 though in such cases the habitat is usually more moist or else the 

 drainage is less perfect. A bur oak forest is shown in fig. 2g. The 

 transition from this type to the morainic swamp forests, already men- 

 tioned, is an easy one, and bur oaks are often found with- the swamp 

 white oak and other species characteristic of such places. 



In spite of the abundance of the type of morainic forest described 



