1886. | BOTANICAL GAZETTE. 261 
First described by Engelmann in Trans. St. Louis Acad., ii. 209; then re- 
duced by him to a variety of the next species in Bot. Oalif., ii. 225; finally re- 
stored to specific rank by the same author in Bor. Gazerrs, vii. 4. Several 
‘specimens labeled P. aristata in herbaria belong here. 
2. P. flexilis James. Epidermis not so thick-walled: one to 
four rows of dorsal stomata: two dorsal ducts (.030-.045 mm.) ; 
rarely a ventral one: thin-walled cells about ducts equalling those 
next the epidermis: leaves 2—4 in. long. 
Western slope of Rocky Mountains to California. 
The extreme forms of these two species stand well apart, but there are in- 
termediate forms which are hard to determine. 
- + T No stomata on dorsal side of leaf (often present in P. monticola), 
3. P. reflexa Engelm. Three or four rows of stomata on ven- 
tral faces: two dorsal ducts (.025-.040 mm.): number of cells in 
bundle-sheath 16-20: fibro-vascular bundle often quite large, al- 
most filling the fibro-vascular region, which. often has strength- 
ening cells, differing in this respect from the other species of the 
group and resembling P. cembroides: leaves 1—2 in. long. 
High mountains of New Mexico and Arizona. 
First described as a variety of P. flexilis by Engelmann in Bot. Wheeler’s 
Report ; then raised to specific rank in Bor. GAzeErreE, vii. 4. 
4. P. Strobus L. Three to five rows of stomata on ventral 
faces: one to three ducts (.035-.040 mm.), mostly two and sit- 
uated half way between the middle and edge of the dorsal face ; 
when three the odd one is on one of the ventral faces: number 
of cells in bundle-sheath 15-19, mostly 16: leaves 3-4 in. long. 
Along the Alleghanies and in the northern states east of the Mississippi. 
5. P. Ayacahuite Ehrenberg. Much like the last, but with 
a few more cells in the bundle-sheath (18-21), and always two 
dorsal ducts, which are much smaller (.015-.025 mm.). 
Mountains of Mexico. 
6. P. monticola Dong. Two to six rows of ventral stomata, 
often one or two dorsal rows: mostly two dorsal ducts (.025-.050 
mm.), sometimes but one; often a few ventral ducts: number of 
cells in bundle-sheath 20-25: leaves 2-4 in. long. 
Mountains of the Pacific slope. 
The thin-walled layer next the epidermis, which is ch 
group, is not so evident as in the other species, but can be distinguished from 
the strengthening cells. It marks well a transition phase to the next group. 
* # No thin-walled layer next the epidermis: strengthening cells next 
the epidermis and generally about the ducts : leaves one to five. 
teristic of this 
+ Stomata on dorsal side of leaf. 
