The Genus Pinus

by George Russell Shaw

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 passing 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 illustration, 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 repetition of the same orders among the species.

THE CONE-TISSUES. [Plate VI].

The axis of the cone is a woody shell, 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).