18 BULLETIN 327. U. S. DEPARTMENT OF AGRICULTURE. 
fir trees are. with the exception of one species (Abies venusta), 
coated with resin. 
The leaves of firs, spirally arranged on the branches, persist for 
from 5 to 10 years (usually 9). after which the oldest set of leaves 
(a season's growth) gradually disappears. Branches from which 
the leaves have fallen are conspicuously marked by smooth circular 
scars, points where the leaves were once attached. In this respect the 
firs differ radically from the spruces, the leaves of which when shed 
leave a projecting portion of their bases attached to the branch, giv- 
ing the latter a distinctly rough feel and appearance. Leaves on 
the lower branches of our native firs are mostly flat (in one species 
triangular) and rounded, notched, or blunt at the end (one species 
only having needle-pointed leaves). In some species the leaves of 
lower branches appear to grow more or less distinctly in two ranks 
(somewhat like the teeth of a comb) on opposite sides of the twigs 
(PL VII), while in other species they clothe the top of the branch 
(PL X). The firs that have a distinctly two-ranked arrangement of 
their foliage on the lowest branches produce leaves on their middle 
and upper-crown branches which appear more and more to grow 
only from the upper sides of the twigs. In the case of firs whose 
leaves thickly clothe the upper sides of the lower branches there is 
little change from this arrangement in the middle and upper crown. 
In all cases, however, leaves of the extreme upper branches are stouter, 
crowded, and strongly curved toward the upper side of the horizontal 
twigs, and often keenly pointed or somewhat sharp-pointed (PL IX). 
The stout leaders also have larger, keenly pointed leaves (Pis. VIII. a 
and XII. a). It is important, therefore, to note the very dissimilar 
form, habit, and character of leaves in these three parts of the crown, 
for leaves of the middle-crown branches (PL VIII. h) are sometimes 
different in form and arrangement from those of either the lower or 
upper crown branches. A cross section of the leaves of firs shows 
two resin ducts (a distinctly marked ring of cells) usually near the 
lower surface and close to the edges. In the case of some of our firs. 
however, these ducts are in the interior of the leaf's tissue, about 
midway between the upper and lower surface of the leaf. 1 The 
under surface of the leaves has one or several rows of stomata or 
minute pores on each side of the raised midrib, and sometimes also on 
the upper surface. 
1 Eesin ducts are scarcely visible to the naked eye, but they can be readily seen in a 
thin cross section or slice of a fresh leaf viewed under a simple pocket lens. The location 
of resin ducts is always indicated by two minute drops of resin which ooze from the 
ducts when a fresh leaf is cut in two. As a rule there is little variation in the charac- 
teristic position of resin ducts in leaves taken from different parts of the tree. In the 
case of three exotic firs (Abies nordmanniana, A. ceplialonica, and A. pectinata) resin 
ducts of the lower and middle-crown branches are near the margin of the leaves, while 
in leaves of the topmost or fruit-bearing branches the ducts are situated in the interior of 
the leaf's tissue. (Fide cf. Guinier and Maire, Bull. Soc. Bot. France. LV. 1S9. 190S.) 
