72 SEVENTH REPORT. 



The distribution of the plant groups is conditional on several factors: 

 The first is the drainage. In general the height above water level divides 

 the vegetation into upland and lowland floras. The first of these is main- 

 ly the one we have to deal with. 



Usually the banks are dryer than the lowlands, but this is locally 

 varied by the slope of the bank, steep banks not retaining water as well 

 as slopes ; again the moisture content varies with the soil, clays and water 

 retaining soils may sustain mesophytic or hydrophytic groups of plants 

 even on steep slopes; the collection of humus increases the water retain- 

 ing power of soil, old banks even of steep slope supporting mesophytic 

 plants. 



Secondly, the physical nature of the soil also affects the succession in 

 another way. The penetration of clays and hard soils by root stalks and 

 seedlings is more difficult than of loose soils and these plants therefore 

 appear later in clay. On the other hand the wet clay invites surface 

 plants, as mosses, etc., and they readily develop. The pioneer herbs and 

 shrubs which have rapid growth, are limited in clay. 



Third, the atmospheric conditions affect the moisture content of the 

 soil. The air is dryer and has a greater range of humidity on the banks 

 than in the lowlands. 



The temperature of both air and soil varies more on the high bank than 

 in the lowland, while the soil of the bog below the surface has a lower 

 and more constant temperature than is found elsewhere. 



The light intensity probably excludes some plants from young ravines 

 which develop when shade is afforded. 



The most conspicuous factor affecting the succession of ravine plants is 

 erosion. The cutting away of the soil desrtoys the old flora or carries it 

 into the ravine and the cycle may be interrupted at any jioint and the 

 succession started anew. 



Adrian, Michigan, March 30, 1905. 



