24 BULLETIN 55, U. S. DEPARTMENT OF AGRICULTURE. 
by the parenchyma nearest to the center of the leaf. In the former 
case the ducts are termed peripheral; in the latter, medial. The 
fibro-vascular bundles lie in the center of the leaf and are surrounded 
by an imperfect bundle sheath. 
The leaf structure affords a reliable means for distinguishing one 
species of fir from another. Only Alpine fir {Abies lasiocarpa) and 
Fraser fir (Abies fraseri) are likely to be confused with balsam. The 
range of balsam touches that of Alpine fir in the West and that of 
Fraser fir in the South. These three species are readily distinguished 
Fig. 2— Leaf structure of Abies grandis: D, ducts; B,. bundle sheath; F, fibro-vascular bundle; M, 
phyll; E, epidermis; S, strengthening cells. 
from the rest of the firs, such as Abies grandis (fig. 2) and Abies con- 
color, by the position of the resin ducts. In balsam fir (fig. 3), Alpine 
fir (fig. 4), and Fraser fir they lie nearer the center, while in the other 
species they lie close to the periphery of the leaf, as observed by cut- 
ting through a fir needle and observing the exudation of the resin. 
Balsam is distinguished from Alpine and Fraser fir by the presence 
of only a few or the entire absence of strengthening cells, which, in 
the two other species, occur in considerable number. 1 
i The Resin Ducts and Strengthening Cells of Abies and Picea, by Herman B. Dorner. 
Indiana Academy of Science, 1897, p. 116. 
Proceedings of 
