e permanent foliage of Pines, are borne on
dwarf-shoots in the axils of primary leaves. They form cylindrical
fascicles, rarely monophyllous, prevalently of 2, 3 or 5 leaves,
occasionally of 4, 6, 7, or 8 leaves. The scales of the fascicle-bud
elongate into a basal sheath, deciduous (fig. 15) in all Soft Pines
except P. Nelsonii, persistent (fig. 16) in all Hard Pines except P.
leiophylla and Lumholtzii. Inasmuch as these three species are easily
recognized, the fascicle-sheath is useful for sectional distinctions.

EXTERNAL CHARACTERS.

The number of leaves in the fascicle is virtually constant in most
species, the variations being too rare to be worthy of consideration.
With some species, however, heteromerous fascicles are normal. The
influences that cause this variation are not always apparent (echinata,
etc.), but with P. ponderosa, leiophylla, sinensis and others, the
number of leaves in the fascicle is, in some degree, dependent on
climatic conditions, the smaller number occurring in colder regions. In
Mexico, for example, where snow-capped mountains lie on subtropical
table-lands and extremes of temperature are in juxtaposition, the
conditions are favorable for the production of species with heteromerous
fascicles, and the number of leaves in the fascicle possesses often
climatic rather than specific significance.

Among conifers, the leaf of Pinus attains extraordinary length with
great variation, from 5 cm. or less to 50 cm. or more, the maximum for
each species being usually much more than twice the minimum. Climate is
the predominating influence; for the shortest leaves occur on alpine and
boreal species, the longest leaves on species in or near the tropics.

The length of the leaf is complicated by the peculiarities of individual
trees and by pathological influences; as a general rule, however, the
length of leaves is less or greater according to unfavorable or
favorable conditions of temperature, moisture, soil and exposure.
Therefore the dimensions of the leaf may be misleading. It can be said,
however, that certain species always produce short leaves, others leaves
of medium length, and others very long leaves.

Persistence of the leaf varies with the species and with the individual
tree. But it is noteworthy that the longest persistence is associated
with short leaves (Balfouriana, albicaulis, montana, etc.).

INTERNAL CHARACTERS.

Since the leaf-fascicle is cylindrical, the cross-section of