THE PLANTING OF STREET-TREES 



99 



top of the branch pipe may end slightly below the surface of 

 the ground, the opening covered with a cap, and the soil 

 brought to grade. When watering, the soil is stirred aside 

 and the cap lifted, and when through the cap is again re- 

 placed and covered with soil so as not to show on the outside. 

 The ground in which the tree is planted must be thor- 



Fig. 15. Sectional view and plan of a subirrigation device of 4-inch tile pipes laid 

 with open joints. A, Tile pipe. B, Layer of broken stone. C, Branch pipe. 



oughly settled before the drain is placed. Then a channel is 

 dug for the drain and the tile pipes are laid perfectly level 

 and separated from the earth by a layer of broken stone or 

 coarse gravel three or four inches in thickness. This inter- 

 mediary layer is necessary to facilitate the flow of water and 

 to prevent the soil from washing into the drain. 



Such irrigation devices are costly and are not always 

 efficient. They become clogged with soil and roots of the 



