440 PRACTICAL BOTANY . 



deeply placed, are in contact with the moist soil from which 

 a supply of water may be secured. The depth to which roots 

 are known to go in regions where the water is found only at 

 great depths is discussed in Sect. 27. 



For a long time it was supposed that the chief reason for 

 cultivating plants was to keep down the weeds, but we here 

 see that this is but a small part of the truth. Weeds have 

 been of much advantage to agriculture, since in keeping them 

 down the farmer has tilled the soil so as to help regulate the 

 moisture supply for the growing plants. 



406. Dry-land farming. It has been shown that the culti- 

 vable area of the earth may be extended by drainage of un- 

 used swamp areas ; but it may be greatly extended if water in 

 proper quantity and at the proper times is placed upon arid 

 lands. It is said that approximately two fifths of the area of 

 the United States is too dry for cultivation without irrigation. 



Dry-land farming is one method now being tried in regions 

 where there is some rainfall, but an amount that is insufficient 

 to produce a good crop. By careful tillage of the soil the 

 scanty rainfall is conserved, and in this way most of the rain- 

 fall of two or more years may be used for one crop. Good 

 crops have been grown in this way, but it is evident that 

 much work over a long period is necessary in order to accu- 

 mulate enough water for one crop. It is hoped that drought- 

 enduring and drought-resisting varieties of economic plants, 

 especially wheat and other cereals, may be found or devel- 

 oped, thus increasing the outlook for dry-land farming. 



407. Irrigation. The practice of irrigating lands is in some 

 parts of the earth a very old one. Its extensive use in the 

 United States is recent and both the government and private 

 enterprises have expended enormous sums of money in sup- 

 plying water from lakes and rivers to lands which previously 

 were non-productive. In some cases this has involved damming 

 the mountain streams and diverting the water over, around, 

 or through mountains, and finally to the valleys to be culti- 

 vated. With control of the water supply, fertile soil, abundant 



