This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Evergreen trees with two kinds of leaves, the primary ones linear or scale-like, deciduous, the secondary ones forming the ordinary foliage, narrowly linear, arising from the axils of the former in fascicles of 2-5 (rarely solitary in some western species), subtended by the bud-scales, some of which are united to form a sheath. Staminate aments borne at the bases of shoots of the season, the clusters of stamens spirally arranged, each in the axil of a minute scale; filaments very short; anthers 2-celled, the sacs longitudinally dehiscent. Ovule-bearing aments solitary or clustered, borne on the twigs of the preceding season, composed of numerous imbricated minute bracts, each with an ovule-bearing scale in its axil, ripening into a large cone, which matures the following autumn, its scales elongating and becoming woody. Seeds 2 on the base of each scale, winged above, the testa crustaceous. [Name Celtic. The popular names of the species are much confused.]
About 100 species, natives of the northern hemisphere. In addition to the following, 25 others occur in southern and western North America. Type species: Pinus sylvestris L., of Europe. The group of which Pinus Strobus L. is the type is regarded by some authors as a distinct genus.
Leaves 5 in a sheath; cone-scales little thickened at the tip.
Leaves 2 or 3 in a sheath; cone-scales much thickened at the tip.
Cones terminal or subterminal.
Leaves 2 in a sheath; cones 1 1/2'-2 1/2' long, their scales pointless.
Leaves 3 in a sheath; cones 4'-10' long, their scales prickle-tipped.
Cones light, 6'-10' long; leaves 10'-16' long.
Cones very heavy and woody, 3'-4 1/2' long; leaves 3'-6' long.
Cone-scales with neither spine nor prickle; leaves in 2's.
Cone-scales tipped with a spine or prickle.
Leaves some or all of them in 2's.
Cones 1 1/2'-2 1/2' long, their scales tipped with prickles.
Leaves stout, 1 1/2'-2 1/2' long.
Leaves slender, 3' - 5' long.
Cones 3 1/2'-5' long, their scales tipped with very stout short spines.
Leaves in 3's (very rarely some in 2's or 4's).
Cones oblong-conic; leaves 6'-10' long; old sheaths 6"-10" long.
Leaves 3'-5' long; cone-scales with stiff prickles.
Leaves 6'-10' long; cone-scales with small slender deciduous or obsolete prickles.