STREET TREES. 41 



roach maturity under normal conditions form very large oval-headod 

 handsome trees, but under the artificial conditions existing in cities 

 it is necessary to prune them quite severely when young to remove 

 the long vigorous growths and make the heads more compact. This 

 pruning stimulates more vigorous growth, which must be removed or 

 they will form long branches with heavy tops, that are especially 

 liable to be injured by windstorms. The more they are pruned the 

 greater the tendency to an undesirable form of growth. They begin 

 dropping their leaves early in the summer and lose them very early 

 in the autumn. Their root growth is especially vigorous, so that 

 they are liable to make trouble in sewers in the manner already 

 mentioned. It is largely on this account that many cities prohibit 

 the planting of these trees. Except in regions 6, 7, or 8 or in loca- 

 tions where smoke and fumes in the air prevent the growing of 

 other trees, they should not be planted. 



The Lombardy poplar^ is a tall columnar tree adapted for use on 

 very narrow streets (fig. 14). It is short lived in many places, due 

 largely to the European poplar canker, but otherwise is a satisfactory 

 tree for these conditions in all parts of the United States. The trees 

 may be planted as close together as 30 feet. 



None of the other poplars have m.uch to recommend them for 

 street planting. 



RUBBER TREE. 



The rubber tree^ is a large-headed handsome evergreen, suitable 

 for regions 3 and 5 and the southern parts of regions 2 and 13 when 

 the use of an- evergreen tree is warranted. 



SILK OAK. 



The silk oak,^ or Australian fern, is a large, handsome tree that 

 succeeds well in regions 2, 3, and 13; also in region 5 if provided 

 with a reasonable amiount of moisture, as it stands drought remark- 

 ably well. It is covered in early summer with orange-colored 

 flowers. 



SWEET GUM. 



The sweet gum* is adapted to regions 11, 12, and 13, especially 

 on sandy lands. It forms an oval-headed, handsome tree with star- 

 shaped leaves that assume a particularly brilliant hue in the autumn. 

 It is better adapted to suburban conditions than to the heart of a 

 city. Toward the northern limits of its successful cultivation it is 

 difficult to transplant, while in the warmer sections of the country 

 it can be moved with comparative ease. It should be transplanted 

 only in the spring. 



'■Populus italica (Du Roi) Moench. 3 Qrcvillea robusta A. Cuun. 



2 Ficus elastica Roxb. * Liquiddmbar styraciflua L. 



