This condition is again to be
noted by the dry-farmer, for the dry-farm districts are
characterized by an abundance of sunshine.
Fifth, transpiration is decreased by the presence in the soil-water
of large quantities of the substances which the plant needs for its
food material. This will be discussed more fully in the next
section.
Sixth, any mechanical vibration of the plant seems to have some
effect upon the transpiration. At times it is increased and at times
it is decreased by such mechanical disturbance.
Seventh, transpiration varies also with the age of the plant. In the
young plant it is comparatively small. Just before blooming it is
very much larger and in time of bloom it is the largest in the
history of the plant. As the plant grows older transpiration
diminishes, and finally at the ripening stage it almost ceases.
Eighth, transpiration varies greatly with the crop. Not all plants
take water from the soil at the same rate. Very little is as yet
known about the relative water requirements of crops on the basis of
transpiration. As an illustration, MacDougall has reported that
sagebrush uses about one fourth as much water as a tomato plant.
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