114 NINTH REPORT. 



however, the heaviest rainfall occurred in March. Low water is more fre- 

 quent in November, gradually increasiing until the ground is frozen. The 

 prevailing rains and winds are from the west and south-west, the winds 

 averaging an hourly velocity of about ten miles (160 km.) for the year 19.04, 

 and a maximum velocity of 56 miles during July. The number of rainy 

 days is considerabh'' larger than the number of clear days. 



Among the chief factors governing the distribution of plant association?- 

 the edaphic and physiographic agencies should be considered also; but 

 here, as with certain climatic data, the importance of these features is best 

 dealt with in the particular description of the localities studied. Neither 

 is it necessary to discuss the early history or the varieties of ravines, since 

 this is set forth in the excellent work of Cowles (6). 



Of the ravines studied thus far in this locality, four have been chosen for 

 a preliminary report, on account of the prominence of fom- phases in ravine 

 processes. For convenience they may be treated under the following as- 

 pects: (1) a locally typical ravine; (2) a ravine influenced by man; (3) a 

 ravine of arrested development due to captured territory; (4) a rejuvenated 

 ravine. As illustrations of dynamics in botany no better examples around 

 here could be cited. What additional phases may be present, the brief 

 period of investigation has made impossible to show. Proceeding now to 

 a more detailed account of the ravines and their vegetation, we may con- 

 veniently follow them in their natural order of succession. 



I. A LOCALLY TYPICAL RAVINE. 



The crest outline of the ravine in School-girl glen is approximately that 

 of the 860 feet (260 m.) contour line, (above sea-level) (*), leading into the 

 Huron river by a north north-eastern direction; 1100 feet (330 m.) can be 

 regarded as the length of it, while near the mouth the greatest width is about 

 300 feet (90 m.), and the depth 110 feet (33 m.). Surrounding the ravine 

 on the east, south and west side and sloping into it, is a peach orchard sev- 

 eral acres in extent. The soil consists of a heterogeneous sandy loam in- 

 clining in places to a clay structure, and occasionally with a large per cent 

 of gravel. The ravine has been cleared somewhat about twenty 3'ears ago, 

 and the consequent changes doubtless have led to a marked modification 

 in vegetation. But the undisturbed trees and the characteristic physical 

 conditions have been effective in the restoration of the original flora, and 

 toda}^ the ravine presents a fairly advanced stage in the process. The water 

 entering it is derived chiefly from drainage during seasons of maximum 

 precipitation and, by an underground channel, from a spring near the head 

 of the ravine. Bordering the east and west side occur (f) Corylus ameri- 

 cana, Crataegus spp., Rhus hirta, R. glabra, Rubus nigrobaccus, R. cana- 

 densis, Helianthus divaricatus. A dense growth of Solanum dulcamara, 

 Vitis vulpina, Sambucus canadensis, Micrampelis lobata, Hystrix hystrix 



1 Cowles, H. C. The physiographic Ecology of Chicago and vicinity. Bot. Gaz. 31: 

 73-81. 



* A topographic map, known as the "Ann Arbor Quadrangle," has been recently com- 

 pleted and published by the U. S. G. S. in cooperation with the Geol. survey of the 

 State of Michigan. 



t The nomenclature is that of Britton's Manual of the Flora of the Northern States 

 and Canada, 1901. 



