MICHIGAN ACADEMY OF SCIENCE. 119 



plants check the numerous rills due to the downward flow of the water, caus- 

 ing deposition of part of the eroded soil. Accelerated growth, partly of the 

 plant, partly on account of the fixation of soil, gives rise to the well-known 

 hummocks of swamps. Building, enriching the soil, and occasional fires 

 are represented by many and diverse changes. The repeated burnings do 

 not injure so much the vegetation of the pasture, as the invading plants of 

 the adjacent woods. However, it is necessary to investigate more definitely 

 the various factors and stages determining the invasion between these two 

 areas. This ravine and several others of a like type, excellently illustrate 

 that the stage of a i^rocess in ravine life is more often clue to biological than 

 to physical causes, and that a description or an understanding of a life his- 

 tory of plant associations is inadequate, unless the human factor is taken 

 into account. As an influence for introducing and eliminating many species, 

 and complicating or interfering with the scene of action of ravine life, the 

 human agency, is certainly of increasing importance. 



III. A RAVINE OF ARRESTED DEVELOPMENT DUE TO CAPTURED TERRITORY. 



In Cascade Glen, as in the preceding section, and the one next to be de- 

 scribed, woods occur some distance from the edge of the bluffs and down to 

 the river margin. Throughout the whole area oaks and hickories are the 

 dominant trees. The region is topographically very much like that of School- 

 girl Glen, except that the direction of the first and larger ravine is more par- 

 allel with the bluff, i. e., in a direction west to east-north-east. Springs and 

 the effect of cultivation in adjoining fields have been the cause of the more 

 rapid development, thus aiding in the ca]:)ture of the neighboring territory, 

 forcing a shifting of divides, and leading the increased drainage into the main 

 ravine stream bed. This directed the formative forces into new channels, and 

 resulted in the arrested development of neighboring ravines. The physio- 

 graphic features are not so marked in the larger ravine as in some others of 

 this vicinity. The vegetation is similar to that of the ravine first described, 

 but noteworth}^ is the fact that the flora is increased in variety by a large 

 number of herbaceous plants. The lowland of the ravine is damp, shady, 

 and rich in humus. The shade of the trees is much greater on the south 

 than on the north side; hence the flora of the former is much richer in species, 

 especially of the early flowering kind, while the flora of the north side con- 

 tains a high proportion of late flowering Compositse. This difference is also 

 observable in regard to ferns, mosses and liverworts. A few preliminary 

 experiments show that the water content of the soil depends largely upon 

 drainage, the firmness of the soil particles, and upon accumulated humus; 

 it is the principal element in determining the character of the growth, while 

 the ecological distribution of the vegetation is determined by conditions 

 of water content of soil as modified by topographic environment, and 

 plant reaction. Various other ecological factors such as the historic one, 

 are of importance also, but the data on hand are so limited that at present 

 a consideration of these and other c^uestions can not be attempted. 



As an example of arrested development due to captured territory, the ra- 

 vine nearest the headwaters of the one just mentioned, is of more special 

 interest. The ravine indicates various changes, all of which have an evi- 

 dent effect on the vegetation. The conditions of plant life are very distinct; 

 the amount of water available is scanty, partly on account of the divide ex- 

 isting, partly because the vegetation carpet reduces instability of soil, and 

 erosion is confined to side-wash. Where soil and vegetation are more com- 



