SUCCESSION 259 



reactions, except where they exhibit typical inland dunes. Sand-binders, 

 while usually classed as xerophytic or halophytic, are in reality dissophytes. 

 Their roots grow more or less superficially in moist sand, and are morpholog- 

 ically mesophytic while their leaves bear the stamp of xerophytes. The di- 

 rection of movement in successions of this kind is normally from xerophytes 

 to mesophytes. i. e., it is mesotropic. In sand-hills and deserts, the succes- 

 sion operates wholly within the xerophytic (dissophytic) series. Along sea- 

 coasts, the mesophytic terminus is regularly forest, except where forests 

 are remote, when it is grassland. 



318. Succession by reducing run-off and erosion. All bare or denuded 

 habitats that have an appreciable slope are subject to erosion by surface 

 water. The rapidity and degree of erosion depend upon the amount of 

 rainfall, the inclination of the slope, and the structure of the surface soil. 

 Regions of excessive rainfall, even where the slope is slight, show great, 

 though somewhat uniform erosion; hill and mountain are deeply eroded 

 even when the rainfall is small. Slopes consisting of compact eugeogenous 

 soils, notwithstanding the marked adhesion of the particles, are much eroded 

 where the rainfall is great, on account of the excessive run-off. Porous 

 dysgeogenous soils, on the contrary, absorb most of the rainfall; the run-off 

 is small and erosion slight, except where the slope is great, a rare condition 

 on account of the imperfect cohesion of the particles. 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 move- 

 ment of the water, and produce nvimberless 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 re- 

 sults generally in the production of sand-binders with fibrous roots. .Un- 

 like dunes, such slopes exhibit a large number of mats and rosettes with 

 tap-roots, which are effective 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 com- 

 parative stabilization for the establishment of its own offspring, and of such 

 invaders as find their way in. From the nature of these, slopes almost in- 

 variably pass through grassland stages before finding their termini in thick- 

 ets or forests. Bad lands (tiria) 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 compact- 



