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.  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.  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.  Even greater differences exist between other plants.  This is one of the interesting subjects yet to be investigated by those who are engaged in the reclamation of dry-farm districts.  Moreover, the same crop grown under different conditions varies in its rate