165] ENGELMANN—REVISION OF THE GENUS PINUS, ETC. 5 
both surfaces in the Pixaster section; only P. Balfourtiana has 
none on the back, and thus approaches S¢rvdus in this as it does 
in many other respects. In S¢rodus we find on the back no or but 
few stomata, or sometimes a single or an interrupted line of them. 
P. Lambertiana only has numerous stomata on the back, thus 
approaching Pinaster. 
I will have to dwell somewhat extensively on the internal struc- 
ture of the leaves, as it provesto be of the greatest impor- 
tance for the classification of the species. We distinguish in a 
transverse section the thick epidermis, the chlorophyll- bearing 
parenchyma cells, and in the centre the fibro-vascular bundle. 
This latter is single in the terete and mostly in the quinate 
leaves; it is double in the broader triangular or ternate, and 
‘in the semi-terete or binate leaves. This difference, however, 
is of very little diagnostic importance, as we find occasionally 
single or double bundles in the same species. The fibro-vascular 
bundles always show wood cells on the upper or ventral, and bast 
cells on the lower or dorsal side, traversed by delicate medul- 
lary rays, usually obliquely diverging from the lower to the upper 
side. The bundles are imbedded in a mass of small (medullary?) 
cells, free of chlorophyll, and are together with those surrounded 
and separated from the parenchyma by a sheath of larger cells, 
also destitute of chlorophyll. 
Within the parenchyma of the leaf a smaller or larger number 
of longitudinal tubes or ducts are found, the RESIN DUCTS, nor- 
mally probably two, but very often more, even as many as a 
dozen or more. These ducts occupy a certain definite position 
within the leaf. They lie (1) close to the epidermis, peripheral 
ducts, in some species more on the ventral, in others more on the 
dorsal side of the leaf; or (2) they occupy a place within the 
parenchyma and surrrounded by it on all sides, parenchymatous 
ducts; or (3) they lie close to the sheath which surrounds the . 
vascular bundles, zzternal ducts. This position of the ducts is 
so constant, and seems to be so intimately connected with the 
essential character of the plant, that I venture to adopt it as one 
of the principal characters for the subdivision of the genus. I 
must add, however, that in some few species smaller, accessory 
ducts do sometimes occupy an abnormal position. Thus I find 
occasionally in some Strodz, especially in P. exce/sa, where there 
