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fortuitous distribution of seeds. Moreover, the advent of the 

 ultimate stage may be hastened or retarded by the abundance or 

 scarcity of water in the superficial layers of the soil. Thus shade 

 conditions in an oak-hickory forest favor the conservation of the 

 soil water, but at the same time the demands of the trees upon 

 the available supply may be so great as to practically exhaust 

 the water near the surface. And where, as along the summits of 

 the trap ridges, water is never very abundant, it is not surprising 

 to find that while the arborescent species in the forest, with their 

 deeply penetrating root systems, may be relatively mesophytic, 

 the shrubs and herbaceous plants, with their shallow root systems, 

 are quite xerophytic. The point to be emphasized in this 

 connection is that, although the mature trees of the climax forest 

 may be able to utilize water at some depth below the surface* 

 their seedlings are dependent on the supply near the surface; so 

 that, unless there is sufficient water in these surface layers to 

 enable the climax trees to tide over the critical seedling stages, 

 they cannot establish themselves and the oak-hickory stage may 

 be of indefinite duration. As will be brought out in a later 

 paragraph, the accumulation of humus may have an important 

 bearing on the phenomena of succession, particularly during 

 these more advanced stages. 



Along the trap ridges the climax forest of this region is best 

 developed on the lower slopes, where there is a relatively constant 

 supply of ground water, available throughout the growing 

 season.* By far the most abundant and most characteristic 

 tree here is the chestnut {Castanea dentata). With it are com- 

 monly associated Liriodendron Tulipifera and some of the more 

 mesophytic trees of the preceding stage, e. g., Quercus rubra, 

 Quercus alba, and Acer rubrum, while scattered through the forest, 

 sometimes abundant locally, are other mesophytic trees, such as 

 Acer saccharum, Fagus grandifolia, Fraxinus americana, Prunus 

 serotina, Tilia americana, and Tsuga canadensis. As species of 

 secondary importance may be mentioned Carpinus caroliniana, 



* It should perhaps be remarked at this point that the slopes of the trap ridges, 

 especially toward the base, are usually covered to a greater or less degree with 

 glacial debris. As will be brought out in a later paragraph, this fact has an im- 

 portant bearing on the rapidity of the succession. 



