will be used more or less in this paper, it will be advisable to 

 briefly explain their significance. In 1895 Warming 1 , a Euro- 

 pean botanist, adopted these terms in his "Plant Geography." 

 He divided the plants of the world into four great groups: 

 Hydrophytes, or the plants which grow in water or wet places; 

 Xerophytes, the plants which grow in dry habitats; Mesop- 

 hytes, the plants which grow in places of. medium moisture; 

 such as ordinary forests and meadows; and Halophytes, or 

 plants which grow in salt water or alkaline soil. Most botan- 

 ists have accepted Warming's classification of plant societies 

 as a more or less complete organization of this part of the 

 physiographic-ecological field. The type of vegetation called 

 mesophytic, requires protection from the cold and heat in ex- 

 cess, as well as good moisture conditions. The black and 

 white oaks with an occasional hickory usually form the forest 

 covering on the southern and western slopes. As one passes 

 over the crest of these hills, however, a transition in the vege- 

 tative types is easily discernible. Descending the north- 

 ern slope towards the bottom of Cherry Creek near the Biolo- 

 gical Station, one first marks the elms. Following the elm 

 comes the aspen. Then the sycamore comes in still lower. 

 As one descends lower, the ash prevails. When the hill slope 

 merges with the flood-plain, the walnut and red-bud are more 

 predominant. Along the- creek's margin, always depending 

 upon the nature of the slope, the hazel, linden or basswood, 

 and the willow (salix nigra) occupy the space. The poplars 

 occur here too, but irregularly. These trees furnish the obser- 

 ver with a direct clue to tne topography It is now well known 

 that each particular topographic form has its own peculiar 

 vegetation. To make direct use of the words of one of the 

 foremost students and authors on the subject, "The reason 

 that the type of vegetation is so closely related to the type of 

 topography is that the soil conditions upon which the plants 

 depend are determined by the surface geology and geography. 

 From the standpoint of vegetation the topographic relations 

 are more important than the geological. The topographic 

 conditions determine the exposure, the presence or absence of 

 drainage, and the humus content of the soil, and are thus of 

 overshadowing importance. Having related the vegetation 

 largely to the topography, we must recognize that topography 



