All the species are of the Western Hemisphere, and among them may be found the biform hypoderm of the leaf, the internal resin-duct, and the total absence of external resin-ducts, characters common in American Hard Pines. The eastern species are quite constant in their characters and present no varietal forms; the western species, on the other hand, are very variable. This difference may be due to the even level and slight climatic differences of the Atlantic states and to the remarkable diversity of altitude and climate of the western states and Mexico.
Outer walls of the leaf-endoderm thick. Cones large, attaining 12 cm. or more in length. Prickles of the cone inconspicuous. Bark-formation late 37. pseudostrobus Bark-formation early 38. Montezumae Prickle of the cone conspicuous 39. ponderosa Cones small, 7 cm. or less in length 40. teocote Outer walls of the leaf-endoderm thin. Spring-shoots mostly uninodal. Prickle of the cone slender, sometimes deciduous. Cones mostly oblique 41. Lawsonii Cones symmetrical 42. occidentalis Prickles of the cone stout and persistent 43. palustris Spring-shoots multinodal. Resin-ducts internal 44. caribaea. Resin-ducts mostly medial. Prickle of the cone stout 45. taeda Prickle of the cone slender. Bark-formation late 46. glabra Bark-formation early 47. echinata
1839 P. pseudostrobus Lindley in Bot. Reg. xxv. Misc. 63. 1839 P. apulcensis Lindley in Bot. Reg. xxv. Misc. 63. 1842 P. tenuifolia Bentham, Pl. Hartw. 92. 1846 P. orizabae Gordon in Jour. Hort. Soc. Lond. i. 237, f.
Spring-shoots uninodal, conspicuously pruinose. Bark-formation late, the cortex of young trees smooth. Leaves in fascicles of 5, sometimes of 6, from 15 to 30 cm. long, drooping; resin-ducts medial, hypoderm variable in amount, often in very large masses, the outer walls of the endoderm thick. Conelets mucronate. Cones from 7 to 14 cm. long, ovate or ovate-conic, symmetrical or oblique, deciduous and often leaving a few basal scales on the trees; apophyses rufous or fulvous brown, flat, elevated or, in one variety, prolonged in various degrees, the prolongations nearly uniform or much more prominent on the posterior face of the cone, the mucro usually deciduous.