SPRUCE AND BALSAM FIR TREES. 35 
ground, the lower limbs standing out horizontally and those above 
trending upward. Six to ten year old trees have not yet assumed this 
sharp-pointed form of crown, and all of their branches are commonly 
at right angles to the stems (PL XXIII, d) . On old trees the lower- 
crown branches droop conspicuously, as do also those of the middle 
crown ; while branches above this trend upward. In old age the ex- 
tremely slow or practically arrested growth of the leader and the con- 
tinued growth of the upper side branches forms a rounded top. 
Twigs of a season's growth are smooth, shiny, and yellowish-green, 
later becoming ashy or brownish-green. The scaly winter buds are 
from one-eighth to three-sixteenths of an inch in diameter and cov- 
ered with resin. 
The color of the foliage varies in different parts of the tree's range 
from a yellow-green (with distinct bluish cast when young) to pale 
yellow -green with whitish tinge. The newly-matured foliage of old 
trees is, however, often decidedly bluish for a season, but later be- 
comes much paler. In general, trees of the southern Kockies are apt 
to have much bluer foliage than those grown farther north, while, as 
a rule, the foliage of Rocky Mountain trees is whiter or bluer than is 
the case with Pacific slope trees. The leaves are flat, plump, blunt, 
or pointed (usually not pointed on lower and middle-crown branches, 
but sometimes pointed on lower branches of old trees) and are 
crowded, apparently, on the upper sides of the twigs and branches 
(Pis. XX, XXI, and XXII) . On the lower branches of youi\g trees 
they are horizontally flat, straight, and stand out more or less dis- 
tinctly from two sides of tohese branches by a twist at their bases (PL 
IX) . Very commonly, however, the leaves of the lower branches of 
young trees, and nearly always of old trees, are curved upward and 
stand erect somewhat in two lines from the top side of the twigs (Pis. 
IX and XX). Leaves of the upper crown, especially on the topmost 
branches, are strongly curved or sickle-shaped and appear to grow 
from the upper sides of the branchlets (Pis. XXI and XXII). The 
leaves of upper branches and those on leaders are always sharp- 
pointed. Lower-branch leaves are usually from 1J to 3 inches long, 
while those of the upper branches are commonly from about 1 to 1J 
inches long. There is also great variation in the length, form, and 
thickness of the leaves of this fir in different parts of its wide range. 1 
A cross-section of a leaf shows two resin ducts each one situated very 
near the corner of the section and on the lower side. Both surfaces 
1 The changes in form from straight, horizontally flattened leaves to vertically flattened 
ones, either straight or curved like a sickle blade, are curious and unexplained, except 
perhaps by the fact that the latter form may present less leaf surface to the direct rays 
of the sun, thus fitting the tree to better endure the hot, dry climate in which this form 
of foliage is most common. Some authors hold that white fir of the Rocky Mountains 
bears longer leaves, and more commonly pointed ones, than do trees of the Pacific forests. 
The writer has, however, seen white fir in the latter region with quite as long leaves, 
while blunt leaves are not infrequent on trees in the Rockies. 
