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 numberless 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- 
