The Genus Pinus Part 2

DIMENSIONS OF THE CONE.

The cone is small, medium or large in different species, but varies greatly under the influences of environment or of individual peculiarities. The character possesses relative value only, for great variation is possible in the same locality and even on the same tree.

THE PEDUNCLE.

All conelets are pedunculate, but in some species the peduncle, even when long (patula), may become overgrown and concealed by the basal scales of the ripe cone. Articulation usually takes place between the peduncle and the branch, sometimes with the loss of a few basal scales which remain temporarily on the tree (ponderosa, pal.u.s.tris, etc.). With P. Nelsonii, and to a less degree with P. Armandi, there is articulation between the cone and its peduncle.

There are several species bearing persistent cones with no articulation.

This condition appears in other genera, such as Larix and Picea, but without obvious significance. In Pinus, however, the gradual appearance of the persistent cone, for it is rare, common, prevalent or invariable in different species, and its essential a.s.sociation with the serotinous cone, suggest an evolution toward a definite end.

THE UMBO.

The exposed part of the scale of the conelet is the umbo of the ripe cone, a small definite area representing the earlier part of the biennial growth of the cone. The position of the umbo on the apophysis is the basis of Koehne's subdivision of the section Haploxylon.

1. Umbo terminal Subsection Cembra fig. 46-a.

2. Umbo dorsal Subsection Paracembra fig. 46-b.

Two other characters a.s.sist in establis.h.i.+ng these subsections--the conelet, unarmed in Cembra, armed in Paracembra--the pits of the ray-cells of the wood, large in Cembra, small in Paracembra.

THE APOPHYSIS.

The apophysis represents the later and larger growth of the cone-scale.

With a terminal umbo the margin of the apophysis is free and may be rounded (fig. 49) or may taper to a blunt point (fig. 52), and any extension of the scale is a terminal extension. With the dorsal umbo all sides of the apophysis are confined between other apophyses, and any extension is a dorsal thickening of the apophysis or a dorsal protuberance. The outline of an apophysis with a dorsal umbo is quadrangular, or it is irregularly pentagonal or hexagonal, the different forms depending on the arrangement of the contiguous scales, whether of definite or indefinite phyllotactic order, a distinction to be considered later.

The two positions of the umbo result from the relative growth of the dorsal and ventral surfaces of the cone-scale. With the terminal umbo the growth of both surfaces is uniform, with the dorsal umbo the growth is unequal. A true terminal umbo rests on the surface of the underlying scale, although several species with terminal umbos show the first stages of the dorsal umbo. The umbo of P. Lambertiana or of P. flexilis does not touch the surface of the scale below, and a small portion of the under side of the apophysis is brought into view on the closed cone.

The cone of P. albicaulis (Plate VIII, fig. 90) shows all degrees of development between a terminal umbo near the apex of the cone and a dorsal umbo near its base.

The growth of the apophysis may be limited and constant (strobus, echinata, etc.) or exceedingly variable, ranging from a slight thickness to a long protuberance (pseudostrobus, montana, etc.). The protuberance is usually reflexed from the unequal growth of the two surfaces. With the terminal umbo the protuberance lengthens the scale, with the dorsal umbo it thickens the scale. It is sometimes a specific character (ayacahuite, longifolia) appearing on all cones of the species, sometimes a varietal form, a.s.sociated in the same species with an unprolonged apophysis (sylvestris, montana).

On different parts of the same cone, base, centre or apex, the dimensions of the apophyses differ, but at each level the scales may be uniform on all sides of the cone. That is to say, the cone is symmetrical with reference to any plane pa.s.sing through its axis. This, the symmetrical cone, is characteristic of all other genera of the Abietineae, and is invariable among the Soft Pines and in many Hard Pines (figs. 47, 48, 52, 54). But among the Hard Pines there is gradually developed a new form of cone with smaller flatter apophyses on the anterior, and larger thicker apophyses on the posterior surface.

This is the peculiar oblique cone of Pinus (figs. 50, 51, 53), symmetrical with reference to one plane only, which includes the axis of both cone and branch. The oblique cone is a gradual development among the Hard Pines; in some species it is a.s.sociated as a varietal form with the symmetrical cone, and finally, in some serotinous species, it is the constant form.

THE OBLIQUE CONE.

When the oblique cone is merely a varietal form (halepensis, etc.), it gives the impression of an accident, resulting from the reflexed position of the cone and the consequent greater development of the scales receiving a greater amount of light and air. But with the serotinous cones (radiata, attenuata), the advantages of this form become apparent. The cones of these species are in crowded nodal cl.u.s.ters, reflexed against the branch (fig. 50). The inner, anterior scales are perfectly protected by their position, while the outer, posterior scales are exposed to the weather. These last only are very thick; that is to say, there is an economical distribution of protective tissue, with the greatest amount where it is most needed. The oblique form is peculiarly adapted for a cone destined to remain on the tree for twenty years or more and to preserve its seeds unimpaired. Like the persistent cone, the oblique cone finds in a.s.sociation with the serotinous cone a definite reason for existence.

[Ill.u.s.tration: PLATE IV. THE CONE]

PHYLLOTAXIS. Plate V.

There is an obvious difference between the cones of the two sections of the genus. Those of the Soft Pines (figs. 55, 56) have larger and fewer scales, those of the Hard Pines (figs. 57, 58) have more numerous and smaller scales, in proportion to the size of the cone. The former condition represents a lower, the latter condition represents a higher, order of phyllotaxis.

DEFINITE PHYLLOTAXIS.

On a cylindrical axis with scales of the same size, the spiral arrangement would appear as in fig. 62, where the scales are quadrangular and any four adjacent scales are in mutual contact at their sides or angles. These four scales lie on four obvious secondary spirals (fig. 59, a-a, b-b, c-c, d-d). According to the phyllotactic order of the scales these may be the spirals of 2, 3, 5, 8 or of 3, 5, 8, 13 or of 5, 8, 13, 21 etc., etc., from which combinations the primary spiral, on which the scales are inserted on the cone-axis, can be easily deduced. Four quadrangular scales in mutual contact represent the condition of definite phyllotaxis. If the cone is conical, definite phyllotaxis would be possible among all the scales only when the size of the scales diminishes in equal measure with the gradual diminution of the cone's diameter. Such a hypothetical cone is shown in fig. 61.

INDEFINITE PHYLLOTAXIS.

On an imaginary cone of conical form and with scales of equal size throughout, there must be more scales about the base than about the apex of the cone. The phyllotactic conditions must differ, and the obvious spirals, in pa.s.sing from base to apex, must undergo readjustment. If the scales at the base are in definite phyllotactic order and those at the apex are in the next lower order, it is evident that intermediate scales, in the gradual change from one condition to the other, must represent different conditions of indefinite phyllotaxis, while those in a central position on the cone may belong equally to either of two orders.

A Pine cone is never absolutely cylindrical nor do its scales vary in size proportionately to the change of diameter. Most of the scales of a cone are in indefinite phyllotactic relation, while definite phyllotaxis is found only at points on the cone.

As an extreme ill.u.s.tration, the cone of P. pinaster (fig. 60) shows four mutually contiguous quadrangular apophyses at (a), lying on the obvious spirals 5, 8, 13, 21, at (b) four similar apophyses on the spirals 3, 5, 8, 13, and at (c) four others on the spirals 2, 3, 5, 8. Between these three points are apophyses of irregular pentagonal or hexagonal outline, with three scales only in mutual contact (figs. 63, 64). Such are the majority of the scales of the cone and represent more or less indefinite conditions of phyllotaxis.

The cones of Hard Pines, by reason of relatively more and smaller scales and of a more conical form, attain a higher phyllotaxis and a more complex condition, two or even three orders being represented on a single cone; while the cones of Soft Pines, by reason of relatively fewer and larger scales and a more cylindrical form, are of lower phyllotaxis, with one order only more or less definitely presented.

Therefore phyllotaxis furnishes another distinction between the two sections of the genus, but its further employment is exceedingly restricted on account of the constant repet.i.tion of the same orders among the species.

[Ill.u.s.tration: PLATE V. PHYLLOTAXIS OF THE CONE]

THE CONE-TISSUES. Plate VI.

The axis of the cone is a woody sh.e.l.l, enclosing a wide pith and covered by a thick cortex traversed by resin-ducts. By removing the scales and cortex from the axis (fig. 65) the wood is seen to be in sinuous strands uniting above and below fusiform openings, the points of insertion of the cone-scales. From the wood, at each insertion, three stout strands enter the scale, dividing and subdividing into smaller tapering strands whose delicate tips converge toward the umbo. Fig. 70 represents a magnified cross-section of half the cone-scale of P. Greggii; at (a) is a compact dorsal plate of bast cells; at (e) is a ventral plate of the same tissue but of less amount; at (b) is the softer brown tissue enclosing the wood-strands (d, d) (the last much more magnified in fig.

69) and the resin-ducts (e, e).

WOOD STRANDS.

The wood-strands, forming the axis of the cone, differ in tenacity in the two sections of the genus. Those of the Soft Pines are easily pulled apart by the fingers, those of the Hard Pines are tougher in various degrees and cannot be torn apart without the aid of a tool. This difference is correlated with differences in other tissues, all of them combining in a gradual change from a cone of soft yielding texture to one of great hardness and durability.

If a cone scale of P. ayacahuite is stripped of its brown and bast tissues (fig. 66) and is immersed in water and subsequently dried, there is at first a flexion toward the cone-axis (fig. 67) and then away from it (fig. 68). The wood-strands are hygroscopic and cooperate with the bast tissues in opening and closing the cone. This appears to be true of all species excepting the three species of the Cembrae, whose strands are so small and weak that they are not obviously affected by hygrometric changes.

BAST TISSUE.

With the exception of the three species of the Cembrae the inner part of the cone-scales is protected by sclerenchymatous cells forming hard dorsal and ventral plates (fig. 70, a, c). In Soft Pines these cells are subordinate to the more numerous parenchymatous cells, but in Hard Pines the sclerenchyma increases in amount until, among the serotinous species, it is the predominating tissue of the cone-scale, giving to these cones their remarkable strength and durability.

This bast tissue is hygroscopic and, with its greater thickness on the dorsal surface, there is a much greater strain on that side of the scale, tending to force the scales apart when they are ripe and dry, and subsequently closing and opening the cone on rainy and sunny days.

The cone, during the second season's growth, is completely closed, its scales adhering together with more or less tenacity. In most species the hygroscopic energy of the scales is sufficient to open the cone under the dry condition of its maturity, but with several species the adhesion is so persistent that some of the cones remain closed for many years.

These are the peculiar serotinous cones of the genus.

THE SEROTINOUS CONE.

As an ill.u.s.tration of the area to which the adhesion is confined, a section may be sawed from a cone of P. attenuata (fig. 71). The axis and the scales that have been severed from their apophyses (b) can be easily pushed out of the annulus (a), which is composed wholly of apophyses so firmly adherent that they will successfully resist a strong effort to break them apart. When immersed in boiling water, however, the ring falls to pieces. An examination of these pieces discovers adhesion only on a narrow ventral border under the apophysis and on a corresponding dorsal border back of the apophysis. The rest of the scale is not adherent, so that the seed is free to fall at the opening of the cone.

The serotinous cone is a gradual development, wanting in most species, rare in a few, less or more frequent in others. A similar evolution of the persistent cone, of the oblique cone and of the cone-tissues has been already discussed. All these progressive characters culminate in mutual a.s.sociation in P. radiata and its allies. The result is a highly specialized fruit that should convey taxonomic significance of some kind.

With all serotinous species that I have seen, some of the trees open their cones at maturity, others at indefinite intervals. That is to say, the seed of a prolific year is not at the mercy of a single, perhaps unfavorable season. The chances of successful germination are much increased by the intermittent seed-release peculiar to these Pines. Such a method of dissemination must accrue to the advantage of a species. In other words, this intermittent dissemination and the oblique form of cone with its perfected tissues all mark the highest development of the genus.