during the
darkened half of the day, when the action is not counterbalanced by
the sun's rays. While the sun is high and the air is warm there is a
constant absorption of moisture in large part from the ground or from
the neighbouring water areas, probably in some part from those
suspended stores of water, the clouds, if such there be in the
neighbourhood. We can readily notice how clouds drifting in from the
sea often melt into the dry air which they encounter. Late in the
afternoon, even before the sun has sunk, the radiation of heat from
the earth, which has been going on all the while, but has been less
considerable than the incurrent of temperature, in a way overtakes
that influx. The air next the surface becomes cooled from its contact
with the refrigerating earth, and parts with its moisture, forming a
coating of water over everything it touches. At the same time the
moisture escaping from the warmed under earth likewise drops back upon
its cooled surface almost as soon as it has escaped. The thin sheet of
water precipitated by this method is quickly returned to the air when
it becomes warmed by the morning sunshine, but during the night
quantities of it are absorbed by the plants; very often, indeed, with
the lowlier vegetation it trickles down the leaves and enters the
earth about the base of the stem, so that the roots may appropriate
it. Our maize, or Indian corn, affords an excellent example of a plant
which, having developed in a land of droughts, is well contrived,
through its capacities for gathering dew, to protect itself against
arid conditions. In an ordinary dew-making night the leaves of a
single stem may gather as much as half a pint of water, which flows
down their surfaces to the roots. So efficient is this dew supply,
this nocturnal cloudless rain, that on the western coast of South
America and elsewhere, where the ordinary supply of moisture is almost
wanting, many important plants are able to obtain from it much of the
water which they need. The effect is particularly striking along
seashores, where the air, although it may not have the humidity
necessary for the formation of rain, still contains enough to form
dew.

It is interesting to note that the quantity of dew which falls upon an
area is generally proportioned to the amount of living vegetation
which it bears. The surfaces of leaves are very efficient agents of
radiation, and the tangle which they make offers an amount of
heat-radiati