Water-requirements of Growing Plants.—The amount of water contained in any plant at one time, however large, is but a small proportion of what is necessary to carry it through its full development. When we measure the amount of water actually evaporated through the plant in the course of its normal growth, we find it to be several hundred times the quantity of dry vegetable substance produced; varying according to the extent and structure of the leaf-surface, the number and size of the breathing pores (stomata) of the leaves, and the climatic conditions (including specially the duration of active vegetation, and temperature during the same), from 225 to as much as 912 times the weight of the mature, dry plant.
The following are extreme figures for water consumption of different plants as reported by different observers, viz., Lawes and Gilbert in England, Hellriegel in northern Germany, Wollny in Southern Germany (Munich), and King in Wisconsin: Wheat, 225 to 359; barley, 262 to 774; oats, 402 to 665; red clover, 249 to 453; peas, 235 to 447; mustard and rape, 845 to 912 respectively; the latter figure being the maximum thus far reported. The highest figures given are throughout very nearly those of Wollny, working in the very rainy climate of Munich.
Evaporation from Plants in Different Climates.—It might be expected that in countries where the air is usually moist, the evaporation will, other things being equal, be less than where it is commonly far below the point of saturation. But the “guardian cells” (stomata) of the leaf pores possess the power of regulating, to a certain extent, the evaporation from the leaf-surface in accordance with temporarily prevailing conditions, so as to allow free evaporation in moist air, but to prevent the wilting and drying-up of the leaf in hot and dry air, save in extreme cases. Moreover, plants adapted to arid conditions are usually provided with additional safeguards in the form of thick, non-conducting layers of surface cells, or long channels connecting the interior tissue with the breathing-pores on the surface. Often hairy, scaly or viscous coverings serve the same end. On the other hand, when the air is very moist, so as to check evaporation, water is sometimes found secreted in minute droplets around the breathing-pores of the leaves, since its ascent is a necessary condition of nutrition and development.
Relation between Evaporation and Plant-growth.—There is not in all cases any direct relation between the amount of evaporation and plant growth; but experience, as well as numerous rigorous experiments have shown that under ordinary conditions of culture, and within limits varying for different soils and crops, production is almost directly proportional to the water supply during the period of active vegetation.
On the basis of Hellriegel’s results, showing that wheat uses (in Germany) about 435 tons, or nearly four acre-inches of water in the production of one ton of dry matter, and assuming the ratio of grain to straw to be 1:1.5, King calculates the following table of probable production under different moisture conditions (Physics of Agriculture, page 140):
| YIELD PER ACRE. | ||||
|---|---|---|---|---|
| Number of Bushels. | Weight of Grain. Tons. | Weight of Straw. Tons. | Total Weight. Tons. | Water used. Acre-inches. |
| 15 | .45 | .675 | 1.125 | 4.498 |
| 20 | .60 | .90 | 1.500 | 5.998 |
| 25 | .75 | 1.125 | 1.875 | 7.497 |
| 30 | .90 | 1.350 | 2.250 | 8.997 |
| 35 | 1.05 | 1.575 | 2.625 | 10.495 |
| 40 | 1.20 | 1.800 | 3.000 | 12.000 |
S. Fortier has made several series of tests to determine the actual yield of grain crops under field conditions when supplied with different amounts of water. Two of these were made at the Montana experiment station in 1902 and 1903, (see reports of these years), in large tanks placed in a field, level with the ground. The results of the last year’s experiments are shown graphically in the [figure below], from which it will be seen that the yield increased quite regularly with the amount of water supplied, up to the depth of 36 inches of water.
Fig. 37.—Experiments on Cereal production with
various amounts of water
(Fortier, Report Mont. Expt. Sta., 1903).
It should be noted that in this case (and as usual) not only the quantity but the quality of the grain was greatly improved as the water-supply increased, it becoming larger and more uniform in size.