296 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Important in itself is the steepness of a slope, for it largely deter- 

 mines the stability of the surface and retention of water, and also 

 the effect of aspect or exposure — especially in the higher latitudes. 

 Thus in the northern hemisphere a steep south-facing slope will 

 receive the strong midday sun's rays more or less perpendicularly, 

 while a steep north-facing slope may receive only oblique and weak 

 morning and evening rays, or perhaps none at all. These differences 

 often have a marked effect, especially on the water and temperature 

 conditions in the two places, and consequently on the vegetation. 

 For besides the intensity, the duration, quality, and quantity of 

 incident light are at the same time affected. 



Slope can also greatly affect the character as well as the amount 

 of soil which accumulates. This, like the nature of the underlying 

 rock, is often reckoned as an edaphic factor ; but in so far as either 

 determines or results in topographic change, it is to be considered 

 as physiographic. Different textures and types of rocks will produce 

 different topographies which bring about local climatic differences 

 that are physiographically engendered. Such differences may also 

 affect the water conditions, including the level below which the 

 ground is waterlogged or frozen, and so again drastically affect 

 the habitat. 



Geodynamic agencies are particularly active in mountain districts 

 and about coasts, causing all manner of changes in topography— 

 sometimes almost from day to day. Steep slopes and river banks 

 are constantly being eroded, the material sliding down as talus, etc., 

 or being washed down and deposited elsewhere. Such activities 

 often cause the formation of new ' open ' habitats, both in the 

 places whence the eroded material came and in the areas in which 

 it comes to rest. Frosts may help disintegration through their 

 heaving, splitting, and other erosive tendencies, while avalanches 

 often clear off the surface materials from considerable areas. And 

 even as mountain slopes change their surfaces, and river-beds alter 

 their outlines and vary courses, so do sea-coasts and cliffs vary in 

 their conformation, erosion being widely at work. In other places 

 there are silting salt-marshes or gradually moving sand-dunes or 

 shingle-banks — to mention only a few of the geodynamic sources of 

 physiographic change. 



Edaphic 



The edaphic factors are those which are dependent on the soil 

 as such — on its constitution, water and air content, inhabiting 



