larger roots, which spread near the
    surface, upset the sidewalk or prevent the growth of other
    vegetation on the lawn, while its finer rootlets, in their eager
    search for moisture, penetrate and clog the joints of neighboring
    water and sewer pipes.  The tree is commonly attacked by the
    oyster-shell scale, an insect which sucks the sap from its bark
    and which readily spreads to other more valuable trees like the elm. 
    The female form of this tree is even more objectionable than the
    male, because in the early spring the former produces an abundance
    of cotton from its seeds which litters the ground and often makes
    walking dangerous.  The only justification for planting the Carolina
    poplar is in places where the conditions for tree growth are so poor
    that nothing else will grow, and in those cases the tree should be
    cut back periodically in order to keep it from becoming too tall and
    scraggly.  It is also desirable for screening in factory districts
    and similar situations.

[Illustration:  FIG. 42.—­Bud of the Carolina Poplar.]

The silver or white poplar (Populus alba) may be told from the other poplars by its characteristic smooth, whitish-green bark, often spotted with dark blotches, Fig. 43.  The leaves are silvery-white and downy on the under side.  The twigs are dark green in color and densely covered with a white down.  It grows to very large size and forms an irregular, wide-spreading, broad head, which is characteristically different from that of any of the other poplars.

[Illustration:  FIG. 43.—­Bark of the Silver Poplar.]

The quaking aspen (Populus tremuloides), the large-toothed aspen (Populus grandidentata) and the balsam poplar or balm of Gilead (Populus balsamifera) are other common members of the poplar group.  The quaking aspen may be told by its reddish-brown twigs, narrow sharp-pointed buds, and by its small finely toothed leaves.  The large-toothed aspen has thicker and rather downy buds and broader and more widely toothed leaves.  The balsam poplar has a large bud thickly covered with a sticky, pungent, gelatinous substance.

GINGKO OR MAIDENHAIR TREE (Gingko biloba)

[Illustration:  FIG. 44.—­Gingko Trees.]

Distinguishing characters:  The peculiar branches of this tree emerge
    upward from a straight tapering trunk at an angle of about 45 deg.
    and give to the whole tree a striking, Oriental appearance, which is
    quite different from that of any other tree, Fig. 44.

Leaf:  Like that of a leaflet of maidenhair fern, Fig. 45.

Range:  A native of northern China and introduced into eastern North
    America.

Soil and location:  The gingko will grow in poor soils.