116 NINTH REPORT. 



Heliopsis scabra, H. helianthoides, Helianthus decapetalus, H. divaricatiis, 

 Monarda fistiilosa, Leptandra virginica, Hystrix hystrix, Coreopsis tripteris, 

 Asclepias exaltata. No trees are found here. A few feet farther down 

 several oaks, (Ouercus veliitina) occupy aj^osition about midway between 

 the upper and lower margin of the slope. On this cross-section the vegeta- 

 tion above and around the oaks is clistinctly xerophytic and light-loving 

 in character; the plants within the shadow cast by the oaks, and toward the 

 lower edge of the ravine are Leptandra virginica, Coreopsis tripteris, Hama- 

 melis virginiana, Vagnera racemosa, Dioscorea villosa, Vitis vulpina, Aster 

 la^vis, A. prenanthoides, Collinsonia canadensis, MitQlla d3'philla, Asplenium 

 filix-fa'mina, Osmunda cinnamomea, Adiantum pedatum, and various 

 mosses, such as Hypnum and Mnium. A similar section on the west side, 

 under more exposed conditions, contained among others Rhus hirta, Pteris 

 ar|uilina, Euphorbia corollata and Apocynum cannabinum dominant, while 

 toward the more shaded environment were seen Smilax hispida, Corylus 

 americana, Pteris aquilina, Pleliopsis scabra, Dasystoma laevigata, Euphorbia 

 corallata, Verbena urticifolia, Solidago canadensis, Onagra biennis, Hystrix 

 hystrix, and at the base Salomonia commutata, averaging a height of 6 to 

 7 feet. The differences in temi^erature between the upland and the base of 

 the ravine vary from 2o-°-9°C. on sunny days: on cloudy or windy days 

 the differences are less. Occasionally when the direction of the wind is 

 from the north the reverse holds true, i. e., the temperature within the ravine 

 is from l^°-8° higher than that of the adjoining fields. The soil contains 

 from 12% to 14% organic matter and a physical water content averaging 

 3% to 5% higher as compared with the upland. It is inadequate to express 

 these changes in vegetation in terms of water content merely, since various 

 factors are involved, and the results arise from the united action of these (7-8). 

 Conditions obtain whose water-content and characteristic distribution of 

 plants is largely determined by light; the development of jjlants as well as 

 their structiu'e and density change in accordance with the varying light 

 intensity. The reactions of the plants are ec[ually great and profound — 

 the habitat in turn undergoing marked changes also. For instance, the in- 

 creasing diffuseness of light in some places, due to luxuriant growth, precludes 

 nearly all undergrowth, increases the humidity of the air, thus lessening 

 again aeration, transpiration, and absorption of soil- water — reactions 

 through Avhich light, himiidity, temperature and soil-water content are most 

 distinctly modified. In other places the plants produce seeds and seedlings ' 

 with difficulty. The decomposed remains of an earlier vegetation lead to 

 mechanical and chemical changes in the soil. But though increasing the 

 soil in nutrition content and water-content, in their extent these changes 

 are more effective toward breaking up the flora into a heterogeneous forma- 

 tion, accompanied with a frequent changing of one dominant group by 

 another. In other and dryer situations the more mesophytic of these plants 

 differ in appearance, but especially in the extent and branching of the root 

 system. The same physiological adjustments, though in a converse sense, hold 

 true for the more xerophytic plants in the moister regions of the ravine. 

 However, it is not the purpose of this paper to indicate in detail the exten- 

 sive physiological reactions both functional and structural occurring here. 

 On comparing the composition of these societies with that of others, it is 

 clearly seen that the mutual interaction of plants and environment leads 



7 Warming, E. Lehrbuch der okologischen Plfanzengeogi'aphie, 1896, p. 105-106. 

 S.'Schimper, A. F. W. Pflanzen-Geographie auf Physiologischer Grundlage, 1898, p. 

 204-205, 



