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- 
L. Cockayne, Ph.D., F.R.S., phot 
Sand-dune near mouth of the Rangitikei River, showing the sand-binder 
pingao (Scirpus frondosus). 
pounds indirectly from the air, have established them- 
seves 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 
