Whether plants, especially cultivated ones, are capable of adapting themselves to arid conditions so as to be capable of producing satisfactory crops with less water than is actually consumed in the humid region, has not been directly determined. Such is, however, the impression produced by farming experience; and the fact that among the common weeds of arid California are mustard and rape, cited by Wollny[69] as requiring over three times as much water as does maize for the production of one part of dry matter, lends color to the supposition that in some manner these, and probably other plants, use more water in humid than in dry climates (see this chapt. p. 212). [69] See Wollny’s experiments, Forsch. Agr. Phys. Vol. 20, p. 58. It is therefore impossible to assign a definite figure for the amount of water required by vegetation _at large_; and even for one and the same plant, only approximations conditioned upon climatic factors can be given. We can in many cases, however, assign for one plant, or for certain groups of plants, the amounts of water producing the best results (“optimum”) and the least amount (“minimum”) compatible with a paying crop, that must be furnished during the growing season, to produce certain results. For when instead of fruiting, it is desired that the crop should produce the largest possible amount of vegetable substance, as in the case of forage crops, a larger amount of water will usually be serviceable. _Different conditions of Soil-Water._—Water may be contained in the soil in three different conditions, viz.: 1. From absorption of water vapor; Hygroscopic water. 2. Liquid water held suspended between the soil particles so as to exert no hydrostatic pressure; capillary water, or water of imbibition. 3. Liquid water seeking its level; bottom, ground or hydrostatic water. HYGROSCOPIC WATER. Soils artificially dried so as to deprive them of all their moisture, when exposed to moist air absorb water vapor with great energy at first; both the rapidity of absorption and the amounts absorbed, when full time is given, varying greatly with their nature. Sandy soils, broadly speaking, absorb the smallest amounts; while clayey soils, and those containing much humus, or finely divided ferric hydrate, take up the largest proportion. The figure expressing the amount of aqueous vapor absorbed at the standard temperature of 15° Cent., is called the _coefficient of moisture absorption_. For one and the same substance, this coefficient rises as the grain becomes finer, the surface being correspondingly increased (see chapt. 6).