The peculiarities of individual trees that induce abnormally short or long growths, the dwarf or other monstrous forms, the variegations in leaf-coloring, etc., etc., are not available for classification, for they may appear in any species, in fact in any genus of Conifers. These variations are artificially multiplied for commercial and decorative purposes. But inasmuch as they are repeated in all species and genera of the Coniferae that have been long under the observation of skillful gardeners, their significance has a broader scope than that imposed by the study of a single genus.
The following classification is based on the gradual evolution of the fruit from a cone symmetrical in form, parenchymatous in tissue, indehiscent and deciduous at maturity, releasing its wingless seed by disintegration--to a cone oblique in form, very strong and durable in tissue, persistent on the tree, intermittently dehiscent, releasing its winged seeds partly at maturity, partly at indefinite intervals during several years. This evolution embraces two extreme forms of fruit, one the most primitive, the other the most elaborate, among Conifers.
Two sections of the genus, Soft and Hard Pines, are distinguished by several correlated characters, and moreover are distinct by obvious differences in the tissues of their cones as well as in the quality and appearance of their wood.
With the Soft Pines the species group naturally under two subsections on the position of the umbo, the anatomy of the wood and the armature of the conelet. In one subsection (Cembra) are found three species, P. cembra and its allies, with the cone-tissues so completely parenchymatous that the cones cannot release the seeds except by disintegration. In both subsections there is a gradual evolution from a wingless nut to one with an effective wing, adnate in one subsection, adnate and articulate in the other. The different stages of this evolution are so distinct that the Soft Pines are easily separated into definite groups.
Among the Hard Pines a few species show characters that are peculiar to the Soft Pines. These exceptional species form a subsection (Parapinaster) by themselves.
With the remaining species, the majority of the Pines, the distinctions that obtain among Soft Pines have disappeared. The dorsal umbo, the articulate seed-wing, the persistent fascicle-sheath, the dorsal and ventral stomata of the leaf and its serrate margins, the dentate walls of the ray-tracheids have become fixed and constant. But a new form of seed-wing appears, with a thickened blade, assuming such proportions in P. Sabiniana and its two allies that these three constitute a distinct group, remarkable also for the size of its cones.