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This gives the grove an aspect not unHke an old apple orchard (PI. 

 I, Fig", i). The absence of low branches is possibly due to the action 

 of fires, since natural pruning does not seem veiy effective. The trees 

 are uniformly low, rarely exceeding 35 ft. (10 m.) in height or 

 one foot (3 dm.) in diameter. In the Havana and Oquawka areas 

 the bitter nut hickoiy, Carya cordiforinis, also occurs. 



Since the oak trees are the dominant members of the association, 

 they determine to a large extent the ecological nature of the forest 

 floor, and many peculiarities in the growth or distribution of the 

 herbaceous or shrubby members are directly correlated with the char- 

 acter of the forest. In the young oak woods the ground is bare sand, 

 covered by leaves only around fallen branches or sumach thickets. 

 In forests of greater area or wider extent there is a greater accumu- 

 lation of leaves, leading to the formation of a thin layer of leaf-mold. 

 The thickness of the leaf-mold is a crude index to the age of the for- 

 est. The surface layers of sand thus gradually attain a greater capac- 

 ity for holding water and a g-reater amount of organic matter. Even 

 in the young forests the surface layers of sand are in general moister 

 than on the prairie, because of the lower intensity of light and the 

 slight exposure to the wind. The intensity of the light is much re- 

 duced, although the foliage of the trees is less dense than in the more 

 typical forests of the state. These two features, soil and light, are 

 the most important environmental factors in the association. 



The herbaceous and shrubby vegetation, after a few ubiquitous 

 weeds and naturalized plants are excluded, may for convenience be 

 referred to two groups. The prairie group includes those species 

 more abundant in and more typical of that fomiation, and the forest 

 group includes species more characteristic of the forest and rare or 

 absent in the prairies. It is impossible, but also unnecessary, to 

 draw a sharp line between the two groups. It is evidently the light 

 relation that determines the distribution of the members of the prairie 

 group, since they occur in leaf-mold in the more open woods, but are 

 absent from dense woods with a pure sand substratum. The leaf- 

 mold, on the other hand, seems to be of chief importance to the true 

 forest species, since they frequently occur in open woods with a thin 

 layer of mold on the surface, but seldom in shady woods with a sand 

 floor. Dense woods without leaf-mold are therefore very poor in 

 species and individuals, while the most luxuriant herbaceous vegeta- 

 tion is developed in relatively open woods with a thin layer of mold. 



The species of the prairie group persist within the edge of the 

 forest, and are at first dominant. Farther back from the margin 

 the prairie species may include both invaders from the bunch-grass 



