DUNE VEGETATION. Plants which in habit sandy areas that are or have been worked over by the wind. They form one of the leading edaphie groups of xerophytes (q.v.). Com paratively few plant species are naturally adapted for life in such severe conditions; not only must the plants be pronounced xerophytes, but they must be able to endure partial sub mergence in the sand. Probably the most char acteristic of all beaeh-dune plants is the sand rced (Ammophila or Psamma), a plant which grows naturally on dunes all over the Northern Hemisphere; this genus is often planted to aid in holding dunes. Suceessfnl Iteneli-dune plant: usually have extensive underground stems, and in this way they often cover wide areas.
The most interesting of all dunes are dering dunes, since they present IR-cull:ir ecologi cal conditions. As a dune moves over a region it often suhm•rges the vegetation in its path way, whether it he lowland or upland. Very few plants can adapt themselves to the new conditions, though a few specie., especially wil lows. are able to do so; in this latter case the buried stem. send out roots. As a dune passes
on. the buried plant societies often appear again soil and sometimes dead trees are shown—a sort of natural graveyard. On the dune itself the vegetation is very slight. because the unstable soil prevents plants from securing a foothold. A few short-lived annuals are ocea sionally seen, and perennials may be found in protected depressions. if a dune for any reason ceases its movement, vegetation easily finds a foothold, and it is not long before the twee wan dering dune becomes. permanently estahlished and covered with a rich vegetation. These facts show that it is the instability of dunes. rather than their xeroidlytie character. which excludes plant life. In tropical regions. the dune plants, of course, belong to quite different species. yet their ecological are the same. There, as in the United States. xerophytic grasses, with limo underground stems which have the power to flourish when half buried in the sand, are still the dominating forms. Sec EDAPHIC ; XEROPHYTES ; DESERT.