OF THE HAWAIIAN ARCHIPELAGO 499 
is quite rapid. All of these conditions tend to greatly reduce the 
available water supply of a beach, even though the latter be ex- 
posed to normal precipitation. In other words, the physical char- 
acteristics of the sandy beach, as has already been suggested, tend 
strongly toward xerophytism. Olsson-Seffer (’10) shows that it 
is the volume of water which a soil is capable of placing at the 
disposal of the plant, which is the limiting factor in the production 
of its vegetative covering and the controlling condition in the 
distribution of this vegetation. Percolation in sand is so rapid 
that were it not for the counteracting influence of surface tension 
very little water would be retained by sand. Permeability in- 
creases as the sand particles increase in size. Internal dew for- 
mation in the sand is the direct cause of a portion of the permanent 
moisture of the strand or dune. It is also to be noted, in this 
connection, that extreme changes in the salinity of the soil water, 
due to flooding by fresh water, are detrimental to the strand 
flora. : 
The two important constituents of the soil water of the sandy 
beach are lime and salt. On coral beaches the percentage of lime 
is very high. It is dissolved out by the rain-water, and ultimately 
forms consolidated limestone. The soluble salt content is coastal 
and is not of as great ecologic importance as was formerly sup- 
posed. An excellent statement is given by Olsson-Seffer (’10). 
Sandy soil yields its water to plants more freely than do other 
soils, and below the superficial layer of dry sand there is always a 
surprising amount of water. Fuller (’12) found this to be more 
than double the wilting coefficient of dune soil. 
Owing to the frequent inundations by waves and subsequent 
rapid changes in evaporation, the soil temperatures of the lower 
sand beach are more variable than on any other formations of the 
sandy strand. On account of the low specific heat of sand, the 
surface layers are rapidly heated in the daytime and quickly 
cooled at night. Thus there is considerable fluctuation of diurnal 
temperatures. 
The list of plants enumerated by Schauinsland (’99) as oc- 
curing on the Laysan atoll may be taken as representative of the 
coral strand flora of the leeward isles. This list includes: 
