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

FOREST TREES OF THE PACIFIC SLOPE. 205
rowed to about three-fourths of an inch, gradually increasing in width, toward
and above the middle, to about 1 or 1 inches; then tapering to a stiff sharp
point (fig. 81, b). The blade is thin and the edges are strongly curled or rolled
from above the middle to the point. The edges are, moreover, conspicuously
marked with thread-like fibers, which are frayed from the borders as the leaf
grows older. A single branched cluster (about 14 to 16 inches long) of flowers
is borne from among the uppermost green leaves. The pulpy, sweetish fruit
(fig. 81, c), ripened late in August or early in September, is on a slender drooping
stem. It is 3i or 4 inches long, blunt at the ends, the top end having a short
thick point. The fiat seeds, packed in 6 chambers of the fruit, are black (fig.
81, d). Wood: Nothing can now be said of the wood of this yucca, except that
it is lighter and somewhat softer than that of the Joshua tree.
LONGEVITY-No definite statement can be made concerning the age limit of
this yucca, which, however, can hardly be less long-lived than the Joshua tree.
Messrs. C. R. Orcutt and S. B. Parish, who know the tree yuccas from long
observation, both inform the writer that the Mohave yucca is an exceedingly
persistent but very slow grower in its native habitat, scarcely any change having
been perceived in trees under observation for the last twenty-five years.
RAAGE.
From northeastern Arizona and southern Nevada across the Mohave Desert into California,
and from the southern base of the San Bernardino Mountains to the coast and
northward to Monterey, sometimes ascending mountain slopes to 4,000 feet,
Similar to Joshua Iree.
OCC URRENCE.
SimOcca tTYohuc
DICOTYL ED ONE S.
The trees of the great dicotyledonous class are so called because the germinating
seeds produce two seed-leaves, or cotyledons. They have broad leaves,
with a central vein and a network of smaller connected veins. They are further
characterized by having the non-resinous wood of their trunks in annual layers,
which appear as concentric rings on a cross-section of the trunk. Each layer is
formed just beneath the living bark and over the layer produced the previous
year. This mode of diameter growth gave rise in earlier days to the class name
" exogens," or outside growers, in contradistinction to " endogens," or inside
growers, a class name then given to the trees we now more generally call " monocotyledones."
The two terms, " endogens " and " exogens," originated when
knowledge of how members of the two classes grow was incomplete. Later
studies show that the term " exogens " is still correctly applicable to all dicotyledonous
trees, but that the term " endogens " does not express the manner in
which monocotyledones actually make their diameter growth. Monocotyledones
were once thought to increase in diameter by the addition, each year, of
scattered woody fibers at the center or pith of the tree, thus gradually crowding
the woody tissue previously formed to the outside of the trunk. In other words,
the outside of the trunk was believed to have once occupied the center of the
stem. We now know, however, that these trees grow in diameter by laying on
tissue outside of that formed the previous year, but not in a distinguishable
layer as in dicotyledonous trees. Diameter growth of the gymnosperms (pines,
spruces, firs, etc.) is produced in exactly the same way as in dicotyledonous
trees, but the oleo-resinous woods of the former distinguish them from the latter
class. It is true, -indeed, that the wood of some of our broadleaf trees contains
r6sinous matter, but it is not in any high degree oleo-resinous, as in gymno-
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