=317. Succession by binding aeolian soils.= Dunes (_thinia_) are classic examples of the reaction of pioneer vegetation upon habitats of wind-borne sand. The initial formations in such places consist exclusively of sand-binders, plants with masses of fibrous roots, and usually also with strong rootstalks, long, erect leaves, and a vigorous apical growth. They are almost exclusively perennial grasses and sedges, possessing the unique property 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 stabilization, 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 (_anemia_) are almost exact duplicates of dunes in so far as the steps of revegetation are concerned; while one is a hollow, and the other a hill, in both the reaction operates upon a wind-swept slope. Sand-hills (_amathia_) and deserts (_eremia_) show similar though less marked 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 morphologically mesophytic 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 sand-hills and deserts, the succession operates wholly within the xerophytic (dissophytic) series. Along seacoasts, the mesophytic terminus is regularly forest, except where forests are remote, when it is grassland.