of transpiration.  For instance, plants grown for some time under arid conditions greatly modify their rate of transpiration, as shown by Spalding, who reports that a plant reared under humid conditions gave off 3.7 times as much water as the same plant reared under arid conditions.  This very interesting observation tends to confirm the view commonly held that plants grown under arid conditions will gradually adapt themselves to the prevailing conditions, and in spite of the greater water dissipating conditions will live with the expenditure of less water than would be the case under humid conditions.  Further, Sorauer found, many years ago, that different varieties of the same crop possess very different rates of transpiration.  This also is an interesting subject that should be more fully investigated in the future.

Ninth, the vigor of growth of a crop appears to have a strong influence on transpiration.  It does not follow, however, that the more vigorously a crop grows, the more rapidly does it transpire water, for it is well known that the most luxuriant plant growth occurs in the tropics, where the transpiration is exceedingly low.  It seems to be true that under the same conditions, plants that grow most vigorously tend to use proportionately the smallest amount of water.

Tenth, the root system—­its depth and manner of growth—­influences the rate of transpiration.  The more vigorous and extensive the root system, the more rapidly can water be secured from the soil by the plant.

The conditions above enumerated as influencing transpiration are nearly all of a physical character, and it must not be forgotten that they may all be annulled or changed by a physiological regulation.  It must be admitted that the subject of transpiration is yet poorly understood, though it is one of the most important subjects in its applications to plant production in localities where water is scaree.  It should also be noted that nearly all of the above conditions influencing transpiration are beyond the control of the farmer.  The one that seems most readily controlled in ordinary agricultural practice will be discussed in the following section.

Plant-food and transpiration

It has been observed repeatedly by students of transpiration that the amount of water which actually evaporates from the leaves is varied materially by the substances held in solution by the soil-water.  That is, transpiration depends upon the nature and concentration of soil solution.  This fact, though not commonly applied even at the present time, has really been known for a very long time.  Woodward, in 1699, observed that the amount of water transpired by a plant growing in rain water was 192.3 grams; in spring water, 163.6 grams, and in water from the River Thames, 159.5 grams; that is, the amount of water transpired by the plant in the comparatively pure rain water was nearly 20 per cent higher than that used by the plant growing