CROP DEVELOPMENT UNDER IRRIGATION 161 



developed and a sufficient quantity of materials has been 

 stored in the various plant organs, growth diminishes, 

 flowers and, later, seeds are developed. This is the 

 natural course of plant-growth. It is indispensable that 

 in the beginning the plant be given every possible chance 

 to develop its root-system. 



It has long been known that a dry soil is more com- 

 pletely filled with roots than is a wet one. Under dry- 

 farm conditions, for instance, wheat roots penetrate 

 heavy clay soils to a depth of 8 feet or more. No special 

 attention was at first given to this observation, because, 

 under the humid conditions prevailing in the earlier inves- 

 tigations of agriculture, there seemed to be no practical 

 method of regulating the quantity of water in the soil 

 during the growing season. The rain came as it willed, 

 irrespective of the needs of the farmer. During the 

 last few years, however, this matter has been given 

 quantitative investigation. Von Seelhorst and Tucker 

 found that, of the whole oat plant, including the under- 

 and above-ground parts, when little water was used, 

 about 13 per cent was contained in the roots; when 

 much water was used, about 7.5 per cent was found 

 in the roots. With barley, wheat, peas, and other simi- 

 lar crops, it has likewise been shown that the total and 

 relative weights of roots are largest when little water 

 is used. In the Utah work, it was found that the propor- 

 tion of sugar beets or potatoes to the parts above ground 

 was not greatly affected by the quantity of water used. 

 In fact, the tendency seems to be that specialized roots 

 and tubers increase slightly in proportion to the whole 

 plant as the quantity of water is increased. It may, 

 however, be stated, as a law fairly well established, that 

 the roots of plants, at least of annual plants, always form 



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