ay 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 illustration 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