Transpiration

As water evaporates through the breathing-pores from the leaves it necessarily follows that a demand is made upon the lower portions of the plant for more water.  The effect of the loss of water is felt throughout the whole plant and is, undoubtedly, one of the chief causes of the absorption of water from the soil.  As evaporation is diminished the amount of water that enters the plants is also diminished.  Yet transpiration appears to be a process wholly necessary for plant life.  The question is, simply, to what extent it may be diminished without injuring plant growth.  Many students believe that the carbon assimilation of the plant, which is fundamentally important in plant growth, cannot be continued unless there is a steady stream of water passing through the plant and then evaporating from the leaves.

Of one thing we are fairly sure:  if the upward stream of water is wholly stopped for even a few hours, the plant is likely to be so severely injured as to be greatly handicapped in its future growth.

Botanical authorities agree that transpiration is of value to plant growth, first, because it helps to distribute the mineral nutrients necessary for plant growth uniformly throughout the plant; secondly, because it permits an active assimilation of the carbon by the leaves; thirdly, because it is not unlikely that the heat required to evaporate water, in large part taken from the plant itself, prevents the plant from being overheated.  This last mentioned value of transpiration is especially important in dry-farm districts, where, during the summer, the heat is often intense.  Fourthly, transpiration apparently influences plant growth and development in a number of ways not yet clearly understood.

Conditions influencing transpiration

In general, the conditions that determine the evaporation of water from the leaves are the same as those that favor the direct evaporation of water from soils, although there seems to be something in the life process of the plant, a physiological factor, which permits or prevents the ordinary water-dissipating factors from exercising their full powers.  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:—­