The Genus Pinus Part 3

THE SEED. Plate VI. Figs. 72-79.

The seed of Pinus contains an embryo, with the cotyledons clearly defined, embedded in alb.u.men, which is protected by a bony testa with an external membranous spermoderm, produced, in most species, into an effective wing. While the seed of other genera of the Abietineae shows no striking difference among the species, that of Pinus is remarkably variable, presenting alike the most primitive and the most elaborate forms among the Conifers. These differences are valuable for the segregation of kindred species and for some specific distinctions.

WINGLESS SEEDS.

With wingless seeds the main distinction is found in the spermoderm, which is entire in one species only, P. koraiensis. In P. cembra it is wanting on the ventral surface of the nut, but on the dorsal surface, it is adnate partly to the nut, partly to the cone-scale. The nut of P.

albicaulis and that of P. cembroides are quite bare of membranous cover.

The spermoderm of P. flexilis is reduced to a marginal border, slightly produced into a rudimentary wing adnate to the nut.

THE ADNATE WING.

In P. strobus, longifolia and their allies and in P. Balfouriana the spermoderm is prolonged into an effective wing-blade from a marginal adnate base like that of P. flexilis. This adnate wing cannot be detached without injury.

THE ARTICULATE WING.

The articulate wing can be removed from the nut and can be replaced without injury. An ineffective form of this wing is seen in the Gerardianae and in P. pinea, where the blade is very short and the base has no effective grasp on the nut.

The base of the effective articulate wing contains hygroscopic tissue which acts with the hygroscopic tissue of the cone-scales. The dry conditions that open the cone and release the seeds cause the bifurcate base of the wing to grasp the nut more firmly.

This articulate wing is found in P. aristata and in all Hard Pines except P. pinea, longifolia and canariensis. The wing-blade is usually membranous throughout, but in some species there is a thickening of the base of the blade that meets the membranous apical part in an oblique line along which the wing is easily broken apart. This last condition attains in P. Coulteri and its a.s.sociates a remarkable development.

Plate VI, fig. 72 shows the wingless seed of P. cembroides; fig. 73 represents the seed of P. flexilis, with a rudimentary wing; fig. 74 shows two seeds of P. strobus, intact and with the wing broken away; fig. 75 represents the articulate wing, whose bifurcate base when wet (fig. 76) tends to open and release the nut. When dry (fig. 77) the forks of the base, in the absence of the nut, close together and cross their tips; figs. 78, 79 show the peculiar reinforced articulate wing of P. Coulteri.

Such wide variation in so important an organ suggests generic difference. But here we are met by the a.s.sociation of the different forms in species evidently closely allied. The two Foxtail Pines are so similar in most characters that they have been considered, with good reason, to be specifically identical; yet the seed-wing of P.

Balfouriana is adnate, that of P. aristata articulate. P. Ayacahuite produces not only the characteristic wing of the Strobi, adnate, long and effective, but also, in the northern variety, a seed with a rudimentary wing, the exact counterpart of the seed of P. flexilis. In both sections of the genus are found the effective adnate wing (Strobi and Longifoliae) and the inefficient articulate wing (Gerardianae and Pineae). A little examination of all forms of the seed will show that they blend gradually one into another.

The color of the wing is occasionally peculiar, as in the group Longifoliae. There is usually no constancy in this character, for the wing may be uniform in color or variously striated in seeds of the same species. The length and breadth of the seed-wing, being dependent on the varying sizes of the cone-scale, differ in the same cone. They are also inconstant in different cones of the same species, and of this inconstancy the seed of P. ayacahuite furnishes the most notable example.

[Ill.u.s.tration: PLATE VI. CONE-TISSUES AND SEEDS]

THE WOOD. Plate VII.

With the exception of the medullary rays, a very small proportion of the whole, the wood of Pinus, as seen in cross-section (fig. 82), is a h.o.m.ogeneous tissue of wood-tracheids with interspersed resin-ducts. In tangential section the medullary rays appear in two forms, linear, without a resin-duct, and fusiform, with a central resin-duct. In radial section the cells of the linear rays are of two kinds, ray-tracheids, forming the upper and lower limits of the ray, characterized by small bordered pits, and ray-cells, between the tracheids, characterized by simple pits.

The walls of the ray-tracheids may be smooth or dentate; the pits of the ray-cells may be large or small. These conditions admit of four combinations, all of which appear in the medullary rays of Pinus, and of which a schematic representation is given in Plate VII. These combinations are

Ray-tracheids with smooth walls. Soft Pines.

Ray-cells with large pits Subsection Cembra fig. 80.

Ray-cells with small pits Subsection Paracembra fig. 81.

Ray-tracheids with dentate walls. Hard Pines.

Ray-cells with large pits Group Lariciones fig. 83.

Ray-cells with small pits Other Hard Pines fig. 84.

This, the simplest cla.s.sification of Pine-wood, is not without exceptions. P. pinea of the Hard Pines resembles, in its wood-characters, P. Gerardiana and P. Bungeana of the Soft Pines. The dentate ray-tracheids of P. longifolia are not always obvious. The tracheids of P. luchuensis, according to Bergerstein (Wiesner Festschr.

112), have smooth walls. My specimen shows dentate tracheids. There is also evidence of transition from small to large pits (I. W. Bailey in Am. Nat. xliv. 292). Both large and small pits appear in my specimen of P. Merkusii.

Of other wood-characters, the presence or absence of tangential pits in the tracheids of the late wood establishes a distinction between Soft and Hard Pines. These pits, however, while always present in Soft Pines, are not always absent in Hard Pines. The single and multiple rows of resin-ducts in the wood of the first year may prove to be a reliable sectional distinction, but this character has not been sufficiently investigated to test its constancy. The wood-characters, therefore, however decisive they may be for establishing the phylogenetic relations of different genera, must be employed in the cla.s.sification of the Pines with the same reservations that apply to external characters.

Ray-tracheids with dentate walls and ray-cells with large pits are peculiar to Pinus. Therefore the presence of these characters, alone or in combination, is sufficient evidence for the recognition of Pine-wood.

But the combination of smooth tracheids with small pits (subsection Paracembra) Pinus shares with Picea, Larix and Pseudotsuga.

Among Hard Pines the size of the pits has a certain geographical significance. The large pits are found in all species of the Old World except P. halepensis and P. pinaster; the small pits in all species of the New World except P. resinosa and P. tropicalis. The Asiatic P.

Merkusii with both large and small pits is not strictly an exception to this geographical distinction. The four exceptional species by this and by other characters unite the Hard Pines of the two hemispheres.

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, c.u.mulative, 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 c.u.mulative bark.

Deciduous bark changes after some years to c.u.mulative 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.

[Ill.u.s.tration: PLATE VII. THE WOOD]

SUMMARY

The various characters that have been considered in the previous pages may be cla.s.sified 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.

1. The primary leaf--appearing as a scale or bract throughout the life of the tree.

2. The bud--its constant position at the nodes.

3. The internode--its three distinct divisions.

4. The secondary leaves--in cylindrical fascicles with a basal sheath.

5. The pistillate flower--its constant nodal position and its verticillate cl.u.s.ters.

6. The staminate flower--its constant basal position on the internode and its compact cl.u.s.ters.

7. 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.