De I. VNER VEW D Forest Trees of the Pacific Slope

FOREST TREES OF THE PACIFIC SLOPE. 201
YUCCA. YUCCAS.
The tree yuccas are characterized by their bristling, stiff, upright, bayonet-like,
sharp-pointed leaves, which are thickly clustered at the top of the unbranched
young trunk or at the ends of the few large club-like branches of old trees.
The edges of the leaves of different species are either smooth (unbroken), rough
with minute teeth, or with a few thread-like fibers. Toward their ends the
edges of the leaves are often conspicuously rolled or turned toward each other
(on the upper side), giving the top end of the leaf a trough-like form. The
leaves of each season's growth remain green for several years; later they begin
slowly to bend down, close to the trunk or branch, and finally they die and
within a few years fall from the tree. Bark of the tree yuccas is rather thick
and cork-like, and furrowed and checked as in ordinary trees. The large waxlike
showy flowers are perfect (male and female organs in same flower), and
appear in big branched clusters from among the topmost leaves. With one exception
(Y. aloifolia, which is self-fertilized), they are so constituted that they
can be cross-fertilized only through the visits of insects (moths). The fruit (of
Pacific species), matured in one season, resembles a small cucumber in form
(about 3 to 4 inches long; fig. 81), or in some species a large butternut of the
same dimensions (fig. 79). In some species it stands erect on the flowering
branches; in others, it hangs down. It is indistinctly 6-sided with 6 separate
cells, each containing numerous thin, flat, black seeds. The fruit may have a
thin, fleshy, outer covering, which dries upon the shell when the seed chambers
do not open of their own accord, or it may consist of a stiff, brittle shell, sometimes
tending to split open at the top, but usually remaining closed. Such fruits
fall from the tree in a dry and brittle condition and are carried some distance
from the parent tree by wind or water. Finally they are broken open
and the seeds scattered. Large quantities of yucca seeds are eaten by rodents.
Birds, which feed on the fleshy fruit covering, also assist in distributing the
seed. Many seeds are blighted by insects (moths), to the attacks of which
flowers and fruits of yuccas are especially subject, but upon which the flowers
depend entirely for fertilization.
The tough, fiber-like wood of yuccas is of little commercial value at present,
but should be useful for paper pulp. The tree merits the forester's attention as
a means of helping to maintain much-needed tree growth in arid southwestern
plains and foothills.
Two tree species occur within the Pacific region, and here only in southern
California; elsewhere they range eastward into the southern Rocky Mountain
region and northward, one extending into Lower California. The yuccas are of
ancient origin, remains of them having been found in the Tertiary period.
Joshua Tree.
Yucca arborescens (Torr.) Trelease.
DISTINGUISHING CHARACTERISTICS.
Of the two tree yuccas indigenous in the Pacific region the Joshua tree, also
called "yucca cactus," is much the larger and more tree-like (fig. 79). From
18 to 25 feet, or occasionally 30 or 35 feet, is the usual height, with a diameter
of from 1 to 2 feet. It is easily the most wild-looking denizen of desert hills and
plains. The keenly pointed bayonet-like leaves, bristling at the ends of big
clumsy branches, defy every intrusion and compel respect from many natural
enemies. Young trees are unbranched (fig. 80, a) until they have produced flow-