The Bark
Bark is the outer part of the cortex that has perished, having been cut off from nourishment by the thin hard plates of the bark-scales. In the late and early bark-formation is found a general but by no means an exact distinction between Soft and Hard Pines. In the Soft Pines the cortex remains alive for many years, adjusting itself by growth to the increasing thickness of the wood. The trunks of young trees remain smooth and without rifts. In the Hard Pines the bark-formation begins early and the trunks of young trees are covered with a scaly or rifted bark. The smooth upper trunk of older trees is invariable in Soft Pines, but in Hard Pines there are several exceptions to early bark-formation. These exceptions are easily recognized in the field, and the character is of decisive specific importance (glabra, halepensis, etc.).
Among species with early bark-formation are two forms of bark: 1, cumulative, sufficiently persistent to acquire thickness and the familiar dark gray and fuscous-brown shades of bark long exposed to the weather; 2, deciduous, constantly falling away in thin scales and exposing fresh red inner surfaces. The latter are commonly known as Red Pines, as distinguished from Black Pines with dark cumulative bark. Deciduous bark changes after some years to cumulative bark, and the upper trunk only of mature trees is red. Red Pines, although usually recognizable by their bark, are by no means constant in this character. Oecological or pathological influences may check the fall of the bark-scales, and then the distinction between the upper and lower parts of the trunk becomes lost.
The various characters that have been considered in the previous pages may be classified under different heads, some of them applicable to the whole genus, others to larger or smaller groups of species.
Generic Characters
Several characters, quite distinct from those of other genera, are common to all the species.
The primary leaf—appearing as a scale or bract throughout the life of the tree.
The bud—its constant position at the nodes.
The internode—its three distinct divisions.
The secondary leaves—in cylindrical fascicles with a basal sheath.
The pistillate flower—its constant nodal position and its verticillate clusters.
The staminate flower—its constant basal position on the internode and its compact clusters.
The cone—its clearly defined annual growths.
Pinus is also peculiar in the dimorphism of shoots and leaves and in their constant interrelations with the diclinous flowers. Evolutionary processes develop features peculiar to Pinus alone (the oblique cone, etc.), but confined to a limited number of species.
Sectional Characters
There are several characters that actually or potentially divide the genus into two distinct sections, popularly known as Soft and Hard Pines.
1. The fibro-vascular bundle of the leaf, single or double.
2. The base of the bract subtending the leaf-fascicle, non-decurrent or decurrent
3. The phyllotaxis of the cone, simple or complex.
4. The flower-bud, its less or greater development.
Some characters indicate the same distinction but are subject each to a few exceptions.
5. The fascicle-sheath, deciduous or persistent.
6. The walls of the ray-tracheids, smooth or dentate.
7. The connective of the pollen-sacs, large or small.
8. The formation of bark, late or early.
Subsectional Characters
An exact subdivision of the Soft Pines is possible on the following characters.
The umbo of the cone-scales, terminal or dorsal.
The scales of the conelet, mutic or armed.
The pits of the ray-cells, large or small.
Evolutional Characters
The progressive evolution of the fruit of Pinus, from a symmetrical cone of weak tissues, bearing a wingless seed, to an indurated oblique cone with an elaborate form of winged seed and an intermittent dissemination, appears among the species in various degrees of development as follows—
The seed
wingless.
with a rudimentary wing.
with an effective adnate wing.
with an ineffective articulate wing.
with an effective articulate wing.
with an articulate wing, thickened at the base of the blade.
The cone
indehiscent.
dehiscent and deciduous.
dehiscent and persistent.
persistent and serotinous.
and as to its form symmetrical.
subsymmetrical.
oblique.
These different forms of the seed and, to some extent, of the cone, are available for segregating the species into groups of closely related members; while the gradual progression of the fruit, from a primitive to a highly specialized form of cone and method of dissemination, points to a veritable taxonomic evolution which is here utilized as the fundamental motive of the systematic classification of the species.
Specific Characters
All aspects of vegetative and reproductive organs may contribute toward a determination of species, but the importance of each character is often relative, being conclusive with one group of species, useless with another. Characters considered by earlier authors to be invariable with species, such as the dimensions of leaf or cone, the number of leaves in the fascicle, the position of the resin-ducts, the presence of pruinose branchlets, etc., prove to be inconstant in some species. In fact, as the botanical horizon enlarges, the varietal limits of the species broaden and many restrictions imposed by earlier systems are gradually disappearing.
Variation is the preliminary step toward the creation of species, which come into being with the elimination of intermediate forms. Variation in a species may be the result of its participation in the evolutionary processes culminating in the serotinous Pines, or it may result from the ability of the species to adapt itself to various environments by sympathetic modifications of growth, or it may arise from some peculiarity of the individual tree.
Evolutionary variation is associated with the gradual appearance of the persistent, the oblique and the serotinous cone, and of the multinodal spring-shoot. For these conditions appear in less or greater prevalence among the species of the genus.
Variation induced by environment finds familiar illustrations among the species that can survive at the limits of vegetation and can meet these inhospitable conditions by a radical change of all growing parts. Such variations are mainly of dimensions, but, with some species, the number of fascicle-leaves is affected and the shorter growing-season may modify the cone-tissues. In Mexico and Central America are found extremes of climate within small areas and easily within the range of dissemination from a single tree. The cause of the bewildering host of varietal forms, connecting widely contrasted extremes, seems to lie in the facile adaptability of those Pines, which are able to spread from the tropical base of a mountain to a less or greater distance toward its snow-capped summit.
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.