438 PEACTICAL BOTANY 



of danger to plants, and surplus water may also be injurious. 

 Some of this surplus water may run off the surface without 

 ever entering the soil (Sect. 408). If much of it enters the 

 soil and remains for a long time as free water, it may drown 

 the roots of plants. Sometimes the slope of the land surface 

 is such that the free water of the soil runs off with sufficient 

 rapidity to prevent drowning of plant roots, but in most cases 

 growth of plants is enhanced by artificial methods of under- 

 ground drainage. Ditches are made and earthen tile placed 

 in them, thus forming drainage courses which hasten the nat- 

 ural underground flow of the water. The cereals ordinarily 

 thrive best in soils which contain from 50 to 60 per cent of 

 their total water-holding capacity. 



In swampy places artificial drainage, which furthers the 

 growth of economic plants, also restrains the growth of those 

 swamp plants which ordinarily thrive in wet soils. Much of 

 our best land has been made available by drainage, and there 

 are enormous areas that would be most valuable if only they 

 were properly drained. It has been estimated by Professor 

 Shaler that along the Atlantic coast alone there are over 

 3,000,000 acres of swamp lands that it is possible to reclaim 

 by drainage. The estimates of the United States government 

 indicate that in our country there are nearly 100,000,000 acres 

 of swamp land. There are thirty-five states in the eastern half 

 of the United States in which there are over 30,000,000 acres 

 of swamp lands. Much of this vast area can be drained, and 

 may then become the growing place for valuable economic 

 plants instead of the relatively valueless swamp plants. 



When underground drainage for ordinary cultivated fields 

 was first advocated, opponents asserted that while it might be 

 helpful in removing surplus water during times of abundant 

 rain, the same drains would be the means of depletion of the 

 water during times of drought. Practice has shown, however, 

 that if surplus water is removed in rainy seasons, plant roots 

 grow deeper into the soil and are thereby better placed for 

 enduring subsequent dry periods. Furthermore, the thin films 



