" The stomata are
delicately balanced valves, exceedingly sensitive to external
influences. They are more numerous on the lower side than on the
upper side of plants. In fact, there is often five times more on the
under side than on the upper side of a leaf. It has been estimated
that 150,000 stomata or more are often found per square inch on the
under side of the leaves of ordinary cultivated plants. The stomata
or breathing-pores are so constructed that they may open and close
very readily. In wilted leaves they are practically closed; often
they also close immediately after a rain; but in strong sunlight
they are usually wide open. It is through the stomata that the gases
of the air enter the plant through which the discarded oxygen
returns to the atmosphere.
It is also through the stomata that the water which is drawn from
the soil by the roots through the stems is evaporated into the air.
There is some evaporation of water from the stems and branches of
plants, but it is seldom more than a thirtieth or a fortieth of the
total transpiration. The evaporation of water from the leaves
through the breathing-pores is the so-called transpiration, which is
the greatest cause of the loss of soil-water under dry-farm
conditions.
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