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