correlation
with climatic variation, may argue, not for specific distinction, but
for specific identity. The remarkable variation in the species may be
attributed partly to this adaptability, partly to a participation, more
or less pronounced, in the evolutionary processes that culminate in the
serotinous Pines.

PART I

CHARACTERS OF THE GENUS

THE COTYLEDON. Plate I, figs. 1-3.

The upper half of the embryo in Pinus is a cylindrical fascicle of 4 to
15 cotyledons (fig. 1). The cross-section of a cotyledon is, therefore,
a triangle whose angles vary with the number composing the fascicle.
Sections from fascicles of 10 and of 5 cotyledons are shown in figs. 2
and 3. Apart from this difference cotyledons are much alike. Their
number varies and is indeterminate for all species, while any given
number is common to so many species that the character is of no value.

THE PRIMARY LEAF. Plate I, figs. 4-6.

Primary leaves follow the cotyledons immediately (fig. 4) and assume the
usual functions of foliage for a limited period, varying from one to
three years, secondary fascicles appearing here and there in their
axils. With the permanent appearance of the secondary leaves the green
primaries disappear and their place is taken by bud-scales, which in the
spring and summer persist as scarious bracts, each subtending a fascicle
of secondary leaves. At this stage the bracts present two important
distinctions.

1. The bract-base is non-decurrent, like the leaf-base
of Abies fig. 5.
2. The bract-base is decurrent, like the leaf-base of Picea fig. 6.

The two sections of the genus, Haploxylon and Diploxylon, established by
Koehne on the single and double fibro-vascular bundle of the leaf, are
even more accurately characterized by these two forms of
bract-insertion. The difference between them, however, is most obvious
on long branchlets with wide intervals between the leaf-fascicles.

The bracts of spring-shoots are the scarious bud-scales of the previous
winter; but the bracts of summer-shoots have the form and green color of
the primary leaf.

THE BUD. Plate I, figs. 7-11.

The winter-bud is an aggregate of minute buds, each concealed in the
axil of a primary leaf converted into a scarious, more or less
fimbriate, bud-scale. Buds from which normal growth develops appear only
at the nodes of the branches. On uninodal branchlets they form an ap