REACTIONS 127 



rainfall ; the run ofE is small and erosion slight, except 

 where the slope is great, a rare condition on account ol the 

 imperfect cohesion of the uarticlds. In compact soils, the 

 plants of the initial formations not merely break the impact 

 of the rain-drops, but, what is much more important, they 

 delay the downward movement of the water, and produce 

 namberless tiny streams. The delayed water is largely 

 absorbed by the soil, and the reduction of the run-off pre- 

 vents the formation of rills of sufficient size to cause 

 erosion. As in dunes, such plants are usually perennial 

 grasses, though composites are frequent: the root system is, 

 however, more deeply seated, and a main or tap root is often 

 present. On sand and gravel slopes, the loose texture of 

 the soil results generally in the production of sandblnaers 

 with fibrous roots. Unlike dunes, such slopes exhibit a 

 large number of mats and rosettes with tap roots, which are 

 efiective in preventing the slipping or washing of the sand, 

 and run little danger of being covered, as is the case with 

 dune-formers. In both instances, each pioneer plant serves 

 as a center of comparative stabilisation for the establish- 

 ment of its own offspring, and of such invaders as find their 

 way in. From the nature of these, slopes almost invariably 

 pass through grassland stages before finding their termini 

 in thickets or forests. Bad lands (hydrotribia) furnish the 

 most striking examples of eroded habitats. The rainfall in 

 the bad lands of Nebraska and South Dakota is small (300 

 mm.); yet the steepness of the slope and the compactness of 

 the soil render erosion so extreme, that it is all but im- 

 possible for plants to obtain a foothold. Their reaction is 

 practically negligible, and the vegetation passes the pioneer 

 stages only in the relatively stable valleys. Mountain 

 slopes (ancia), and ridges and hills (lophia) are readily eroded 

 in new or denuded areas. This is especially true of hill and 

 mountain regions, which have been stripped of their forest 

 or thicket cover by fires, lumbering, cultivation or grazing. 

 Where the erosion is slight, the resulting succession may 

 show initial xerophytic stages, or it may be completely 

 mesostatic. Excessively eroded habitats are xerostatic, as 



