Factors Influencing Plant-Life on Sandy Sea-shores. 45 
Evaporation of Moisturf. 
Sand, as mentioned above, ranks lowest in water-holding 
capacity among soils, and this fact is due, to a great extent, to the 
facility with which evaporation takes place from a sandy surface. 
Kerner’s observations, already referred to, make it evident that the 
coarser the sand the greater the rate of evaporation. The principal 
factors regulating this evaporation are :— 
(1) the temperature of the evaporating fluid. 
(2) the temperature of the atmosphere. 
(3) the wind, and 
(4) the degree of humidity in the air. 
The rate of evaporation is :— 
(1) proportional to the evaporating surface. 
(2) proportional to the difference between the highest 
pressure of vapour at saturation and the pressure 
prevailing at the moment, and 
(3) inversely proportional to the atmospheric pressure. 
A loose and open soil, such as sand, leaves the capillary tubes 
open to the surface, and evaporation therefore can proceed from them 
so freely that the underground store of moisture may be insufficient 
to supply the continuous demand of the vegetation covering. It is 
somewhat moderated by the amount of mould which acts as soil 
mulch, but as we have shown, the fine humus particles deposited on 
the sandy surface are rapidly carried downward into the ground 
with the percolating rain, when wind has not already taken hold of 
them, and transported them far away, and sand therefore contains 
very little humus. 
Sometimes the evaporation of moisture from the leaves goes 
on more rapidly than the roots take up fresh supplies from the soil. 
This can often be seen on hot sunny afternoons in the latter part of 
the summer when the leaves are all limp and drooping. As evening 
approaches and the sun’s evaporative power lessens, the supply of 
water from the soil again equals the demand of the leaves, and the 
latter assume their crisp character because the tissues become 
turgid with water. 
The changes in evaporation which necessarily follow the 
changes in atmospheric humidity and pressure, vary with the 
diurnal oscillations of temperature, and are especially great on the 
sea coast. Suffiicient data are not yet at hand to discuss in detail 
this matter of the march of evaporation on coastal sands, but 
enough is known to make it clear that greatly varying conditions 
