3l6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



tends to be least where the vegetation forms least of a ' show ', and 

 especially where plants fail to stabilize the surface, as in the case of 

 many dunes and coastal or desert or high-arctic areas. Plants are 

 again relatively impotent in places of drastic topography and con- 

 sequently strong geodynamic influences and recurrent catastrophe, 

 for here the physical forces of nature usually rule, rather than the 

 vegetation, and our recognition tends to be of habitats rather than 

 of their inhabitants. 



Deserts are areas where the water conditions are too unfavourable 

 (in the sense that the drought is lastingly too severe) to allow the 

 support of any extensive continuous development even of short 

 Grasses or scrub. They cover wide areas of flattish or other topo- 

 graphy and in a sense are simulated on a small scale by areas of 

 porous sand, gravel, shingle, or rock, where arid conditions may 

 prevail even in regions of plentiful precipitation. The so-called 

 cold deserts are the high-polar and high-alpine regions where frozen 

 conditions make water unavailable to plants during most of the 

 year. Even where the precipitation is extremely small in these 

 rigorous regions, as in some high-arctic areas, there is, however, 

 usually plentiful water available from melting snow for fair plant 

 growth in favourable situations, at least early in the growing-season ; 

 moreover there is normally frozen ground-water not far below the 

 surface, so the regional appellation of ' desert ' seems inappropriate. 



In passing, mention should also be made of the so-called ' aero- 

 plankton ', consisting of spores, etc., which float freely and unharmed 

 in the air (although they can scarcely be considered as normally 

 living thus), the microscopic ' cryoplankton ' which really live in 

 and on the surface layers of snow or ice {see Chapter XV), and the 

 ' edaphon ', the flora and fauna of the soil, which actually forms a 

 special habitat for numerous recognized soil organisms. 



Aquatic Habitats 



Even when we divide these into the two main groups of saline and 

 freshwater habitats, there are left intermediate ' brackish ' ones 

 which seem best considered with salt waters. The degree of 

 salinity can, and frequently does, greatly aflFect the habitat and 

 attendant community. Freshwater habitats, apart from the mar- 

 ginal ones already considered, comprise those of lakes, tarns, and 

 ponds where the waters are relatively static, and those of rivers and 

 streams where they are more or less dynamic. But streams can 



