This is explained by the observation that since the
life process of a plant produces a certain amount of heat, the plant
is always warmer than the surrounding air and that transpiration
into an atmosphere fully charged with water vapor is consequently
made possible. The fact that transpiration is greater under a low
relative humidity is of greatest importance to the dry-farmer who
has to contend with the dry atmosphere.
Second, transpiration increases with the increase in temperature;
that is, under conditions otherwise the same, transpiration is more
rapid on a warm day than on a cold one. The temperature increase of
itself, however, is not sufficient to cause transpiration.
Third, transpiration increases with the increase of air currents,
which is to say, that on a windy day transpiration is much more
rapid than on a quiet day.
Fourth, transpiration increases with the increase of direct
sunlight. It is an interesting observation that even with the same
relative humidity, temperature, and wind, transpiration is reduced
to a minimum during the night and increases manyfold during the day
when direct sunlight is available.
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