MICHIGAN ACADEMY OF SCIENCE. 115 



with Typha latifolia, Carex retrorsa, C. hystricina, C. rosea, Scirpus atro- 

 virens occupies the area between. The more hydrophytic of these plants 

 owe their presence to the spring near b}'. 



As the ravine widens and deepens a variety of trees become dominant. 

 The vegetation as a whole. is that of the "Bluff society," but the series of 

 changes taking place downward is so rapid that definite societies cannot be 

 distinguished except in the more extreme conditions, i. e., where the ver- 

 tical succession begins with a bluff society and culminates in a flood-plain 

 societ}'. The most characteristic trees of this bluff society are Quercus 

 velutina, Q. alba, Hicoria minima, H. glabra, H. ovata, Juglans nigra, with 

 an occasional elm (Ulmus americana), some ash (Fraxinus nigra), and poplar 

 (Populus tremuloides, P. grandidentata.) The dominant shrub is Corylus 

 americana; several others occur, the chief of which are Viburnum lentago, 

 Ribes cynolDasti, R. floridum, Celastrus scandens, Cornus stolonifera, C. 

 alternifolia, Dioscorea villosa, Rhus radicans, making up an association 

 characteristically mixed and densely vine-clad. De]3endent upon the pro- 

 tection and the shade of the trees and shrubs are Collinsonia canadensis, 

 Phryma leptostachya, Scrophularia marylandica, Helianthus decapetalus, 

 Caulophyllum thalictroides, Geum canadense, Geranium maculatum, and 

 the usual vernal forms such as Thalictrum dioicum, Sanguinaria canadensis, 

 Trillium erectum, Podophyllum, Anemone, Viola, etc.; also various mosses 

 and liverworts. 



A striking difference from this condition is found in places where lateral 

 erosion is greater. These sections may be compared to a denuded locality 

 whose plant covering has been recently laid bare, i. e., during the early spring- 

 rains. The soil, a sand}^ clay, contains approximately 2 % to 3%. of water 

 more than the adjoining upland, but about 4% less than the shaded sec- 

 tions within the ravine. Subject to more exposed conditions, especially 

 with reference to climatic factors, and differing also as to water content and 

 soil temperature, the sections are particular!}' adapted to the study of in- 

 vasion, consequent competition, and succession. The vegetation near the 

 crest consists of Rhus sirta, Rubus canadensis, Solidago canadensis; further 

 down Solanum dulcamara, Sambucus canadensis, Mtis vulpina, \^erbena 

 urticifolia, Mentha canaxlensis, Potentilla canadensis, Alsine media, Nepeta 

 cataria, Dioscorea villosa, Cornus stolonifera. Where the soil is more sandy 

 and contains a larger per cent of gravel, the flora is that of the adjoining 

 hill; the plants just mentioned are replaced by Onagra biennis, Apocynum 

 androsaemifolivmi, Linaria linaria, and various thistles, weeds and grasses. 

 Marked differences are seen from year to year as the action of erosion or the 

 influence of the invaders JDecomes more effective. 



Where the slopes become better covered with humus, and the shade of 

 trees is dense, protection from rapid changes in temperature and moisture 

 is more pronounced, and the vegetation also is more luxuriant. But though 

 topography and soil-water content, as determined by the imderlying com- 

 pact till, are two of the necessary and dominant factors for the development 

 of a mesophytic vegetation, the changes in vegetation and the actual grouj^ing 

 of the plants into minor associations depend upon various factors — both 

 physical and biological. It may be of interest to point out the chief species 

 occurring in areas of very similar conditions of soil and topograph^^ For 

 example, the notes of a cross-section taken from a station about 700 feet 

 (212 m.) from the head of the ravine contain among others the following 

 species on the east side: i\Ieibomia grandiflora, M. marylandica, Aster later- 

 iflorus, A. leevis, A. macrophyllus, Rudbeckia laciniata, Ratibida pinnata, 



