The Struggle For Existence 201

shore convolvulus (Calystegia soldanella) as well as
those already mentioned, serves a double purpose. It
checks loss of water by evaporation from the soil and
reduces the surface exposed to the wind. At the same
time it more or less checks the drifting of the sand.
Over considerable sandy areas tree lupins have been
planted and these, partly because in this soil, poor in
nitrogen, they are able to derive their nitrogenous com-

-- Insert image here --
Sand-dune near mouth of the Rangitikei River, showing the sand-binder
pingao (Scirpus frondosus).

pounds indirectly from the air, have established them-
selves and multiplied to form dense thickets that check
the inroads of the sand.

XEROPHYTIC HYDROPHYTE SOCIETIES.
Bogs differ from swamps in the fact that, in them,
the nitrogenous compounds of the organic matter they
contain do not decompose and become available for