374 PLANT PHYSIOLOGY 



drought resistance. But as the capacity of enduring permanent 

 wilting is not the only index of drought resistance, the method 

 of wilting does not give reliable results. 



87. Physiological Basis of Irrigation. — In hot arid regions, the 

 precipitation of the period of vegetation together with the mois- 

 ture reserves remaining from winter is insufficient for the 

 cultivation of crop plants. The only means of raising the 

 productivity of such land for rearing cattle that feed on the wild 

 xerophytic vegetation and for cultivating crops is the application 

 of artificial irrigation. In such dry regions as. are sufficiently 

 provided with rivers that carry water from mountains or from 

 more humid regions, artificial irrigation of the field has been 

 applied from prehistoric times. But even in parts of the country 

 where dry farming is possible but w^here sufficiently high yields 

 cannot be obtained, the application of artificial irrigation is of 

 great public significance as one of the most reliable methods of 

 the control of drought that occurs periodically in such regions. 



The technique of the irrigation of large tracts of land differs 

 widely from the methods of watering of small vegetable gardens. 

 One of its chief purposes is a periodic deep moistening of the soil 

 under cultivation, to form a water reserve that will supply the 

 plants during several weeks. In irrigation, either the surface is 

 flooded with water, which is gradually absorbed by the soil; or, 

 if the field is situated on a slope, water is allowed to flow along 

 the surface or through special furrows between the plants 

 until the soil is moistened to a sufficient depth, at least 70 to 

 100 cm. 



During the application of water and immediately afterward, 

 the plants are in a saturated soil; but during the comparatively 

 long interval between irrigations, they gradually consume all 

 the moisture, and toward the end of this interval, the soil is 

 usually quite dried. Hence, plants developed under periodic 

 irrigation have to endure drought that somewhat retards growth. 

 The longer the intervals between irrigation, the greater this 

 retardation. With very long periods between water applications 

 or with too early cessation of irrigation, which may be caused by 

 insufficiency of irrigation water or the absence of a systematic 

 planning of its distribution, the plants, overwatered during the 

 early irrigations, may be in worse condition than those that have 

 been hardened in the beginning of their vegetation. 



