UNDER-DRAINS AND PLANT ROOTS 



261 



established. Fine gravel or coarse sand is a more per- 

 manent filtering medium. 



Plant roots sometimes enter the joints of tile drains, 

 and develop so as to stop the flow of water. This occurs 

 most readily where the tile carries ''living water/' as 

 where a permanent spring 

 is drained. During dry 

 periods and in naturally 

 well-drained soil, water per- 

 colates from the joints of 

 the tile into the adjacent 

 soil, which conditions at- 

 tract roots and may lead 

 them into the tile at the 

 joints. Depth is not a 

 decided protection against 

 this difficulty unless it be 

 excessive. 



There are many points 

 about the construction of 

 a tile-drain system about 

 which special precaution 

 should be taken. Some of 

 these are: (1) Uniformity 

 of grade. (2) Avoid lead- 

 ing a lateral into a main 

 with a less fall unless silt 

 basins are used. (3) Pro- 

 tection of outlets against 



jp , ,s Fig. 92. Ditch cut by the ma- 



caving and freezing. (4) chineshownin Fig . 91 . goiiaheavy 



Protection Of the Outlet cl ay- Depth 4 feet. 



