18 BULLETIN 327, U. S. DEPARTMENT OP 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, b) 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 Resin 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. cephalonica, and A. pectinata) resin 

 ducts of the l&wer 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, 189, 1908.) 



