126 SUCCESSION 



of pushing up rapidly through a covering of sand. They 

 react by fixing the sand with their roots, thus preventing 

 its blowing about, and also by catching the shifting particles 

 among their culms and leaves, forming a tiny area of 

 stabilisation, in which the next generation can establish a 

 foothold. The gradual accumulation of vegetable detritus 

 serves also to enrich the soil, and makes possible the advent 

 of species requiring better nourishment. Blowouts 

 {anernodia) are almost exact duplicates of dunes in so far as 

 the steps of re vegetation are concerned: while one is a 

 hollow, and the other a hill, in both the reaction operates 

 upon a wind-swept slope. Sandhills (amathia) and deserts 

 (eremia) show similar though less marked reactions, except 

 where they exhibit typical inland dunes. Sandbinders, 

 while usually classed as xerophytic or halophytic, are in 

 reality dissophytes. Their roots grow more or less super- 

 ficially in moist sand, and are morphologically raesophytic, 

 while their leaves bear the stamp of xerophytes. The 

 direction of movement in successions of this kind is 

 normally from xerophytes to mesophytes, i. e., it is 

 mesotropic. In sandhills and deserts, the succession oper- 

 ates wholly within the xerophytic (dissophytic) series. 

 Along sea coasts, the mesophytic terminus is regularly 

 forest, except where forests are remote, when it is grass- 

 land. 



(3) Succession by reducing run-off, and 

 preventing 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. Eegions 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 rain- 

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

 dysgeogenous soils, on the contrary, absorb most of the 



