156 J. E. CRIBBS 



ravine occurs upon the glacial drift at an altitude of 600 feet, or 

 upon unglaciated clays at an altitude of 1500 feet. The differ- 

 ence in the factor or temperature is but slight; the subjection to 

 desiccating influences is very similar whether it be a question of 

 access of wind or sun. The relative humidity, in so far as 

 there is any appreciable difference, resolves itself into a question 

 of soil moisture. So far as soil composition is concerned, the 

 variations are in no instance striking. The greatest factor 

 determining the composition of the associations is soil moisture, 

 and the greatest differences in the disposition of the vegetation 

 may be assigned either directly or indirectly to this factor. 



In the case of ponds and swamps there occurs an interesting 

 relation. The aquatic condition, or the high saturation of the 

 soil, inhibits the process of oxidation; so that the acids and other 

 by-products of the plant's activities accumulate more rapidly 

 than they can be removed by chemical reduction. These sub- 

 stances perhaps have a direct influence upon the vegetation 

 developing there. It is at least apparent that the consumption 

 of what oxygen is available by the process of decay, lowers the 

 amount in the water so that it becomes more difficult for the 

 plants to secure a sufficient amount. It is primarily because of 

 the undrained condition of such areas, however, that these 

 detrimental materials are retained in an objectionable form, 

 for they are soon leached out of soils when they are properly 

 drained. 



The determination of an evergreen forest or a deciduous one 

 as found in ravines and valleys, appears also to be closely associ- 

 ated with soil moisture. There seems to be a modifying factor 

 here, — presumably not so much the temperatiu-e of the atmos- 

 phere as the relative coldness of the waters which seep from 

 the ravine slopes. 



Similar topographies usually mean similar combinations of 

 physical factors; and like combinations of physical conditions 

 mean the recurrence of a given type of vegetation. Although 

 always of the same type, there usually occur slight differences in 

 composition ; a fact attributable to unequal or local distribution. 



The vertical disposition of the three types of forest on the 



