That the evaporation of water
from the soil or from a free water surface is not the same as that
from plant leaves may be shown in a general way from the fact that
the amount of water transpired from a given area of leaf surface may
be very much larger or very much smaller than that evaporated from
an equal surface of free water exposed to the same conditions. It is
further shown by the fact that whereas evaporation from a free water
surface goes on with little or no interruption throughout the
twenty-four hours of the day, transpiration is virtually at a
standstill at night even though the conditions for the rapid
evaporation from a free water surface are present.
Some of the conditions influencing the transpiration may be
enumerated as follows:--
First, transpiration is influenced by the relative humidity. In dry
air, under otherwise similar conditions, plants transpire more water
than in moist air though it is to be noted that even when the
atmosphere is fully saturated, so that no water evaporates from a
free water surface, the transpiration of plants still continues in a
small degree.
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