THE NORTH TEMPERATE ZONE 



105 



nois, there is a mingling of forest and prairie floras. In alluvial 

 soils there may be a well developed forest of the usual type, but 

 with fewer species than further east. In the lighter soils, forests 

 when present are much more open, and the predominant trees 

 are oaks of relatively small size. These "oak openings'' are 

 familiar features of the transition regions of northern Illinois 

 and Wisconsin. In the sandy soil at the foot of Lake Michigan, 

 there are white pines growing on the dunes, among the oaks, and 

 in the lower ground the pepperidge (Nyssa sylvatica), is also a 



Fig. 23. — Woodland and prairie near Chicago; Sumac invading grass-land. Photo., 



Dr. A. G. Vestal. 



feature of the formation. For the most part, however, the forest 

 is a strictly deciduous one. With the oaks there are sometimes 

 hickories amd walnuts. The trees of the oak openings are rather 

 scattered with thin under-growth, except for grasses, and a variety 

 of other herbaceous plants. 



The soil of the typical oak openings is usually a sandy one, and 

 not favorable for the growth of moisture-loving plants. Where 

 there has been a sufficient accumulation of humus, a few woodland 

 species become established, but they are few in number of species, 

 and less luxuriant than those of the typical deciduous forest. 



