ENGEI.MANN — A SYNOPSIS OF THE AMERICAN FIRS. 595 



which have been named pseudo-bast cells, but are now generally 

 known as hypoderm cells. They are almost always present on the 

 edges and the keel of the leaf, there sometimes crowded in 3-5 

 layers, and they often form a more or less interrupted stratum on 

 the upper side. Where stomata pierce the epidermis, the hypo- 

 dermic stratum is incomplete, or entirely absent. Only in a few 

 species (A. bracteata and religiosa and the Asiatic^rwa) we find 

 such cells also in the interior of the leaf, a case which is common 

 in true pines. In some species the diameter of these cells is equal 

 to that of the epidermis cells ; in some it is smaller, and in a very 

 few larger. Their presence, distribution, and relative size, is tol- 

 erably constant, and furnishes good specific characters. 



I do not describe the parenchymatous cells containing chloro- 

 phyll, nor their variety the so-called pallisade cells (elongated 

 cells perpendicular to the upper side of the leaf), as no essential 

 characters are derived from them. But of great diagnostic im- 

 portance are the resin ducts, of which there are always two in 

 the Abies leaf, readily seen in a horizontal section. In some spe- 

 cies they are placed on the lower side of the leaf, close to the 

 epidermis and mostly near the edges ; in others we find them in 

 the parenchyma, about equidistant from the upper and lower 

 surface. 



The fibro-vascular bundle occupies the centre of the leaf either 

 single (in the more square leaves of the 4th section), or mostly 

 divided in two distinct bundles (in the flat leaves). Both cases 

 occur sometimes in the same species. The bundles show the 

 larger (ligneous) cells above and the smaller (bast-) cells below ; 

 they are surrounded by small pith-like cells, and the whole sepa- 

 rated from the parenchyma by a sheath of larger cells. 



On the differences of the leaf-structure we can base the sub- 

 divisions of the genus with much greater certainty than on the 

 length of the bracts, as was formerly done. 



Sec. I. Balsame.e : Resin ducts within the parenchyma, in the inte- 

 rior of the leaf; leaves on lower branches notched, and mostly without 

 stomata on the upper side, on fertile branches entire, obtuse, or often 

 acute, mostly with a few or more stomata above, towards the tip.— Two 

 eastern and one northwestern species. 



* Exserta: bracts protruding, recurved. 

 I. A. Fraseri. 



