he 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, associated 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 passing 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 associated 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
clusters, 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 association with the
serotinous cone a definite reason for existence.

[Illustration: PLATE IV. THE CONE]

PHYLLOTAXIS. Plate V.

There is an obvious difference bet