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 apical
group consisting of a terminal bud with a whorl of subterminal buds
about its base. On multinodal branchlets the inner nodes bear lateral
buds which may be latent.
Fig. 7 represents a magnified bud of P. resinosa, first immersed in
alcohol to dissolve the resin, then deprived of its scales. This bud
contains both fascicle-buds, destined for secondary leaves, and larger
paler buds at its base. These last are incipient staminate flowers,
sufficiently developed for recognition. Such flower-bearing buds are
characteristic of the Hard Pines in distinction from the Soft Pines
whose staminate flowers cannot be identified in the bud.
The want of complete data leaves the invariability of this distinction
in question, but with all species that I have examined, the flowers of
Hard Pines are further advanced at the end of the summer. In the
following year they open earlier than those of Soft Pines in the same
locality. The staminate flowers of some Hard Pines (resinosa,
sylvestris, etc.,) are not apparent without removing the bud-scales,
but, with most Hard Pines, they form enlargements of the bud (fig. 9).
Invisible or latent buds are present at the nodes and at the apex of
dwarf shoots. The former are the origin of the numerous shoots that
cover the trunk and branches of P. rigida, leiophylla and a few other
species (fig. 10). The latter develop into shoots in the centre of a
leaf-fascicle (fig. 11) when the branchlet, bearing the fascicle, has
been injured.
The size, color and form of buds, the presence of resin in quantity,
etc., assist in the diagnosis of species. Occasionally a peculiar bud,
like that of P. palustris, may be recognized at once.
THE BRANCHLET. Plate I, figs. 12-14.
The branchlet, as here understood, is the whole of a season's growth
from a single bud, and may consist of a single internode (uninodal, fig.
12-a) or of two or more internodes (multinodal, fig. 13), each internode
being defined by a leafless base and a terminal node of buds.
The spring-shoot is uninodal in all Soft Pines and in many Hard Pines,
but, in P. taeda and its allies and in species with serotinous cones, it
is more or less prevalently multinodal.
The uninodal spring-shoot may remain so throughout the growing season
and become a uninodal branchlet. Or a summer-shoot may appear on
vigorous branches of any species with the result of converting a
uninodal spring-shoot into an imperfect multinodal branchlet. The
summer-shoot may be recognized, during growth, by its green, not
scarious bracts and, at the end of the season, by the imperfect growth
of its wood and foliage (fig. 14).
The perfect multinodal branchlet is formed in the winter-bud (fig. 8-a)
and the spring-shoot is multinodal. It is gradually evolved among the
Hard Pines, where it may be absent, rare, frequent or prevalent,
according to the species. In fact there is, in Pinus, an evolutionary
tendency toward multinodal growth, with its beginnings in the
summer-shoot and its culmination in the multinodal winter-bud, most
prevalent among the serotinous Pines.
The multinodal shoot is never invariable in a species, but is rare,
common or prevalent. This condition prevents its employment for grouping
species. For Pines are not sharply divided into multinodal and uninodal
species, and no exact segregation of them, based on this difference, is
possible. In fact the character is unequally developed among closely
related species, such as P. palustris and caribaea. Both produce
multinodal shoots, but the former so rarely that it should be classed as
a uninodal species, while the latter is characteristically multinodal.
The multinodal spring-shoot, however, has a certain correlative value in
its relation to other evolutionary processes that are obvious in the
genus.
The length of the branchlet is much influenced by different soils and
climates. In species able to adapt themselves to great changes, the
length of the internode may vary from 50 cm. or more to 1 cm. or less.
In the latter case the branch is a series of very short leafless joints
terminated by a crowded penicillate tuft of leaves (fig. 12-b). Such a
growth may be seen on any species (ponderosa, albicaulis, resinosa,
etc.) that can survive exposure and poor nourishment.
The presence of wax, as a bloom on the branchlet, is associated with
trees in arid localities, especially Mexico, where it is very common.
With several species the character is inconstant, apparently dependent
on environment, and is a provision against too rapid transpiration.
The branchlet furnishes evidence of the section to which the species
belongs, for the bract-bases persist after the bracts have fallen away.
The color of the branchlet, its lustre, the presence of minute hairs,
etc., are often suggestions for determining species.
[Illustration: PLATE I. PRIMARY LEAF, BUD AND BRANCHLET]
THE SECONDARY LEAF. Plate II.
Secondary leaves, the 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 a leaf is a
sector, its proportional part, of a circle. Theoretically the leaf, in
section, should indicate the number of leaves composing its fascicle.
This is absolutely true for fascicles of two leaves only. No fascicle of
five leaves, that I have examined, is equally apportioned among its five
members. It may be divided in various ways, one of which is shown in
fig. 18, where the leaf (a) might be mistaken for one of a fascicle of
3, and the leaf (b) for one of a fascicle of 6. Therefore if absolute
certainty is required, a fascicle of triquetral leaves is best
determined by actual count.
The transverse section of a leaf may be conveniently divided into three
distinct parts--1, the dermal tissues, epiderm, hypoderm and stomata
(fig. 17-a)--2, the green tissue, containing the resin-ducts (fig.
17-b)--3, the stelar tissues, enclosed by the endoderm and containing
the fibro-vascular bundle (fig. 17-c).
THE DERMAL TISSUES OF THE LEAF.
The stomata of Pine leaves are depressed below the surface and interrupt
the continuity of epiderm and hypoderm. They are wanting on the dorsal
surface of the leaves of several Soft Pines, constantly in some species,
irregularly in others. In Hard Pines, however, all surfaces of the leaf
are stomatiferous. In several species of the Soft Pines the longitudinal
lines of stomata are very conspicuous from the white bloom which
modifies materially the general color of the foliage.
Under the action of hydrochloric acid the hypoderm is sharply
differentiated from the epiderm by a distinct reddish tint, but without
the aid of a reagent the two tissues do not always differ in appearance.
The cells of epiderm and hypoderm may be so similar that they appear to
form a single tissue. In most species, however, the epiderm is distinct,
while the cells of the hypoderm are either uniform, with equally thin
or thick walls--or biform, with very thin walls in the outer row of
cells and very thick walls in the inner row or rows of cells--or
multiform, with cell-walls gradually thicker toward the centre of the
leaf. These conditions may be tabulated as follows--
Cells of epiderm and hypoderm similar fig. 19.
Cells of epiderm and hypoderm distinct.
Cells of hypoderm uniform, thin or thick figs. 20, 21.
Cells of hypoderm biform fig. 22.
Cells of hypoderm multiform fig. 23.
The biform hypoderm is not always obvious (clausa, Banksiana, etc.)
where in some leaves there is but one row of cells. But with the
examination of other leaves one or more cells of a second row will be
found with very thick walls. Among Hard Pines there is no Old World
species with a biform hypoderm. But there are a few American species
with uniform hypoderm (resinosa, tropicalis, patula and Greggii); while,
in some leaves of the few American Hard Pines with multiform hypoderm,
the uniform hypoderm is a variation.
THE GREEN TISSUE.
In this tissue are the resin-ducts, each with a border of cells,
corresponding in appearance and in chemical reaction with the cells of
the hypoderm and with thinner or thicker walls. With reference to the
green tissue the foliar duct may be in one of four positions.
1. External against the hypoderm fig. 24.
2. Internal against the endoderm fig. 28.
3. Medial in the green tissue, touching neither
hypoderm nor endoderm fig. 26.
4. Septal touching both endoderm and hypoderm,
forming a septum fig. 30.
Among the Soft Pines the external duct is invariable in the subsection
Paracembra. It is also characteristic of the Strobi, where it is
sometimes associated with a medial duct. In the Cembrae and the
Flexiles, however, the ducts are external in some species, or medial or
both in others, without regard to the affinities of these species.
Among the Hard Pines the external duct is characteristic of the Old
World, there being but two American Pines with this character (resinosa
and tropicalis). The internal duct is peculiar to Hard Pines of the New
World, its presence in Old World species being extremely rare. The
medial duct is common to species of both hemispheres, either alone or in
association with ducts in other positions (figs. 25, 27). The septal
duct is peculiar to a few species (oocarpa, tropicalis, and less
frequently Pringlei and Merkusii). I have also seen it in a leaf of P.
canariensis. The internal and septal ducts appear to be confined to the
species of warm-temperate or tropical countries.
The number of resin-ducts of a single leaf may be limited to two or
three (strobus, koraiensis, etc.), but in many species it is exceedingly
variable and often large (pinaster, sylvestris, etc.). Eighteen or more
ducts in a single leaf have been recorded. Such large numbers are
peculiar to Pinus. Occasionally a single leaf, possibly the leaves of a
single tree, may be without ducts, but this is never true of all the
leaves of a species.
THE STELAR TISSUES.
The walls of the endoderm are, in most species, uniform, but, with P.
albicaulis and some species of western North America, the outer walls of
the cells are conspicuously thickened (fig. 32). Both thin and thick
walls may be found among the leaves of the group Macrocarpae and of the
species longifolia.
The fibro-vascular bundle of the leaf is single in Soft Pines, double in
Hard Pines. This distinction is employed by Koehne as the basis of his
two sections, Haploxylon and Diploxylon. The double bundle is usually
obvious even when the two parts are contiguous, but they are sometimes
completely merged into an apparently single bundle. This condition,
however, is never constant in a Hard Pine, and a little investigation
will discover a leaf with a true double bundle.
Some cells about the fibro-vascular bundle acquire thick walls with the
appearance and chemical reaction of the hypoderm cells. Among the Soft
Pines this condition is most obvious in the group Cembroides. Among the
Hard Pines it appears in all degrees of development, being absent (figs.
24, 25), sometimes in irregular lines above and below the bundle (figs.
26, 27, 30, 31), or forming a conspicuous tissue between and partly
enclosing the two parts of the bundle (figs. 28, 29).
The leaf-section furnishes sectional and other lesser distinctions. It
is often decisive in separating species otherwise difficult to
distinguish (nigra and resinosa or Thunbergii and sinensis, etc.).
Sometimes it is sufficiently distinct to determine a species without
recourse to other characters (tropicalis, oocarpa, Merkusii, etc.). An
intimate knowledge of the leaf-section, with an understanding of the
limits of its variation, is a valuable equipment for recognizing
species.
[Illustration: PLATE II. SECONDARY LEAVES]
THE FLOWERS. Plate III, figs. 33-39.
The flowers in Pinus are monoecious, the pistillate in the position of a
long shoot, taking the place of a subterminal or lateral bud, the
staminate in the position of a dwarf-shoot, taking the place of a
leaf-fascicle but confined to the basal part of the internode.
Pistillate flowers are single or verticillate. On multinodal shoots they
are often multiserial, appearing on two or more nodes of the same
spring-shoot (fig. 33). On uninodal shoots they are necessarily
subterminal (fig. 34), the lateral pistillate flower being possible only
on multinodal shoots (fig. 35) where it is often associated with the
subterminal flower (fig. 33). Like the multinodal shoot, on which its
existence depends, the lateral pistillate flower cannot be employed for
grouping the species. It is merely the frequent, but not the essential,
evidence of condition of growth that is more perfectly characterized by
the shoot itself.
Staminate catkins are in crowded clusters, capitate or elongate (figs.
36, 37), but with much variation in the number of catkins in each
cluster. In P. rigida I have found single catkins or clusters of all
numbers from two to seventy or more. In P. Massoniana and P. densiflora
a cluster attains such unusual length (fig. 37) that this character
becomes a valuable distinction between these species and P. sinensis,
which has short-capitate clusters. The catkins differ much in size, the
largest being found among the Hard Pines.
In the connective of the binate pollen-sacs there is a notable
difference (figs. 38, 39), the smaller form being characteristic of the
Soft Pines. But this is not invariable (excelsa, sylvestris, etc.), and
the absence of complete data does not permit an accurate estimate of its
importance.
THE CONELET. Plate III, figs. 40-45.
After pollination the pistillate flower closes and becomes the conelet,
the staminate flowers withering and falling away. The conelet makes no
appreciable growth until the following year. Like the pistillate flower
it may be subterminal or lateral, but a subterminal pistillate flower
may become a pseudolateral conelet by reason of a summer-growth (fig.
40-a). Such a condition may be recognized on the branchlets of the
present, and of the previous year (fig. 40-b), by the very short
internode and short leaves beyond the fruit.
The conelet offers some distinctions of form, of color, and of length of
peduncle, while in some species (sylvestris, caribaea, etc.) its
reflexed position is an important specific character. The most important
distinctions, however, are found in its scales, which may be
1. entire subsection Cembra fig. 41.
2. tuberculate tropicalis, etc. fig. 42.
3. short-mucronate sylvestris, glabra, etc. fig. 43.
4. long-mucronate aristata, contorta, etc. fig. 44.
5. spinescent taeda, pungens, etc. fig. 45.
[Illustration: PLATE III. FLOWERS AND CONELET]
THE CONE. Plate IV.
The cone of Pinus shows great differences of color, form and tissue;
these are useful for specific and sectional distinctions, while the
gradual change from the primitive conditions of the Cembrae to the
elaborate form, structure and mode of dissemination of some serotinous
species are obvious evidence of an evolution among the species of
remarkable taxonomic range. A form new among Coniferae appears, the
oblique cone, and a new condition, the serotinous cone, both appearing
at first alone and, finally, in constant association.
COLOR OF THE CONE.
With few exceptions the color of the ripe cone may be classified under
one of the following shades of brown or yellow.
Nut-brown The stain of the walnut-husk.
Rufous brown A pronounced reddish nut-brown.
Fulvous brown A yellowish nut-brown.
Tawny yellow The color of the lion.
Orange Ochre-yellow to red-orange.
These colors may be paler or deeper. They may be obscured by a fuscous
shade or may be modified by a dull or lustrous surface. The presence of
two or more of these shades in a single species and the inherent
difficulties of color description lessen the value of the character.
Nevertheless certain allied species, such as P. nigra and Thunbergii, or
P. densiflora and Massoniana, may be distinguished by the prevalent
difference in the color of their cones.
DIMENSIONS OF THE CONE.
The cone is small, medium or large in different species, but varies
greatly under the influences of environment or of individual
peculiarities. The character possesses relative value only, for great
variation is possible in the same locality and even on the same tree.
THE PEDUNCLE.
All conelets are pedunculate, but in some species the peduncle, even
when long (patula), may become overgrown and concealed by the basal
scales of the ripe cone. Articulation usually takes place between the
peduncle and the branch, sometimes with the loss of a few basal scales
which remain temporarily on the tree (ponderosa, palustris, etc.). With
P. Nelsonii, and to a less degree with P. Armandi, there is articulation
between the cone and its peduncle.
There are several species bearing persistent cones with no articulation.
This condition appears in other genera, such as Larix and Picea, but
without obvious significance. In Pinus, however, the gradual appearance
of the persistent cone, for it is rare, common, prevalent or invariable
in different species, and its essential association with the serotinous
cone, suggest an evolution toward a definite end.
THE UMBO.
The exposed part of the scale of the conelet is the umbo of the ripe
cone, a small definite area representing the earlier part of the
biennial growth of the cone. The position of the umbo on the apophysis
is the basis of Koehne's subdivision of the section Haploxylon.
1. Umbo terminal Subsection Cembra fig. 46-a.
2. Umbo dorsal Subsection Paracembra fig. 46-b.
Two other characters assist in establishing these subsections--the
conelet, unarmed in Cembra, armed in Paracembra--the pits of the
ray-cells of the wood, large in Cembra, small in Paracembra.
THE APOPHYSIS.
The apophysis represents the later and larger growth of the cone-scale.
With a terminal umbo the margin of the apophysis is free and may be
rounded (fig. 49) or may taper to a blunt point (fig. 52), and any
extension of the scale is a terminal extension. With the dorsal umbo all
sides of the apophysis are confined between other apophyses, and any
extension is a dorsal thickening of the apophysis or a dorsal
protuberance. The outline of an apophysis with a dorsal umbo is
quadrangular, or it is irregularly pentagonal or hexagonal, the
different forms depending on the arrangement of the contiguous scales,
whether of definite or indefinite phyllotactic order, a distinction to
be considered later.
The two positions of the umbo result from the relative growth of the
dorsal and ventral surfaces of the cone-scale. With the terminal umbo
the growth of both surfaces is uniform, with the dorsal umbo the growth
is unequal. A true terminal umbo rests on the surface of the underlying
scale, although several species with terminal umbos show the first
stages of the dorsal umbo. The umbo of P. Lambertiana or of P. flexilis
does not touch the surface of the scale below, and a small portion of
the under side of the apophysis is brought into view on the closed cone.
The cone of P. albicaulis (Plate VIII, fig. 90) shows all degrees of
development between a terminal umbo near the apex of the cone and a
dorsal umbo near its base.
The growth of the apophysis may be limited and constant (strobus,
echinata, etc.) or exceedingly variable, ranging from a slight thickness
to a long protuberance (pseudostrobus, montana, etc.). The protuberance
is usually reflexed from the 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 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.
[Illustration: PLATE V. PHYLLOTAXIS OF THE CONE]
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).
WOOD STRANDS.
The wood-strands, forming the axis of the cone, differ in tenacity in
the two sections of the genus. Those of the Soft Pines are easily pulled
apart by the fingers, those of the Hard Pines are tougher in various
degrees and cannot be torn apart without the aid of a tool. This
difference is correlated with differences in other tissues, all of them
combining in a gradual change from a cone of soft yielding texture to
one of great hardness and durability.
If a cone scale of P. ayacahuite is stripped of its brown and bast
tissues (fig. 66) and is immersed in water and subsequently dried, there
is at first a flexion toward the cone-axis (fig. 67) and then away 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 these progressive characters culminate in
mutual association in P. radiata and its allies. The result is a highly
specialized fruit that should convey taxonomic significance of some
kind.
With all serotinous species that I have seen, some of the trees open
their cones at maturity, others at indefinite intervals. That is to say,
the seed of a prolific year is not at the mercy of a single, perhaps
unfavorable season. The chances of successful germination are much
increased by the intermittent seed-release peculiar to these Pines. Such
a method of dissemination must accrue to the advantage of a species. In
other words, this intermittent dissemination and the oblique form of
cone with its perfected tissues all mark the highest development of the
genus.
THE SEED. Plate VI. Figs. 72-79.
The seed of Pinus contains an embryo, with the cotyledons clearly
defined, embedded in albumen, which is protected by a bony testa with an
external membranous spermoderm, produced, in most species, into an
effective wing. While the seed of other genera of the Abietineae shows
no striking difference among the species, that of Pinus is remarkably
variable, presenting alike the most primitive and the most elaborate
forms among the Conifers. These differences are valuable for the
segregation of kindred species and for some specific distinctions.
WINGLESS SEEDS.
With wingless seeds the main distinction is found in the spermoderm,
which is entire in one species only, P. koraiensis. In P. cembra it is
wanting on the ventral surface of the nut, but on the dorsal surface, it
is adnate partly to the nut, partly to the cone-scale. The nut of P.
albicaulis and that of P. cembroides are quite bare of membranous cover.
The spermoderm of P. flexilis is reduced to a marginal border, slightly
produced into a rudimentary wing adnate to the nut.
THE ADNATE WING.
In P. strobus, longifolia and their allies and in P. Balfouriana the
spermoderm is prolonged into an effective wing-blade from a marginal
adnate base like that of P. flexilis. This adnate wing cannot be
detached without injury.
THE ARTICULATE WING.
The articulate wing can be removed from the nut and can be replaced
without injury. An ineffective form of this wing is seen in the
Gerardianae and in P. pinea, where the blade is very short and the base
has no effective grasp on the nut.
The base of the effective articulate wing contains hygroscopic tissue
which acts with the hygroscopic tissue of the cone-scales. The dry
conditions that open the cone and release the seeds cause the bifurcate
base of the wing to grasp the nut more firmly.
This articulate wing is found in P. aristata and in all Hard Pines
except P. pinea, longifolia and canariensis. The wing-blade is usually
membranous throughout, but in some species there is a thickening of the
base of the blade that meets the membranous apical part in an oblique
line along which the wing is easily broken apart. This last condition
attains in P. Coulteri and its associates a remarkable development.
Plate VI, fig. 72 shows the wingless seed of P. cembroides; fig. 73
represents the seed of P. flexilis, with a rudimentary wing; fig. 74
shows two seeds of P. strobus, intact and with the wing broken away;
fig. 75 represents the articulate wing, whose bifurcate base when wet
(fig. 76) tends to open and release the nut. When dry (fig. 77) the
forks of the base, in the absence of the nut, close together and cross
their tips; figs. 78, 79 show the peculiar reinforced articulate wing of
P. Coulteri.
Such wide variation in so important an organ suggests generic
difference. But here we are met by the association of the different
forms in species evidently closely allied. The two Foxtail Pines are so
similar in most characters that they have been considered, with good
reason, to be specifically identical; yet the seed-wing of P.
Balfouriana is adnate, that of P. aristata articulate. P. Ayacahuite
produces not only the characteristic wing of the Strobi, adnate, long
and effective, but also, in the northern variety, a seed with a
rudimentary wing, the exact counterpart of the seed of P. flexilis. In
both sections of the genus are found the effective adnate wing (Strobi
and Longifoliae) and the inefficient articulate wing (Gerardianae and
Pineae). A little examination of all forms of the seed will show that
they blend gradually one into another.
The color of the wing is occasionally peculiar, as in the group
Longifoliae. There is usually no constancy in this character, for the
wing may be uniform in color or variously striated in seeds of the same
species. The length and breadth of the seed-wing, being dependent on the
varying sizes of the cone-scale, differ in the same cone. They are also
inconstant in different cones of the same species, and of this
inconstancy the seed of P. ayacahuite furnishes the most notable
example.
[Illustration: PLATE VI. CONE-TISSUES AND SEEDS]
THE WOOD. Plate VII.
With the exception of the medullary rays, a very small proportion of the
whole, the wood of Pinus, as seen in cross-section (fig. 82), is a
homogeneous tissue of wood-tracheids with interspersed resin-ducts. In
tangential section the medullary rays appear in two forms, linear,
without a resin-duct, and fusiform, with a central resin-duct. In radial
section the cells of the linear rays are of two kinds, ray-tracheids,
forming the upper and lower limits of the ray, characterized by small
bordered pits, and ray-cells, between the tracheids, characterized by
simple pits.
The walls of the ray-tracheids may be smooth or dentate; the pits of the
ray-cells may be large or small. These conditions admit of four
combinations, all of which appear in the medullary rays of Pinus, and of
which a schematic representation is given in Plate VII. These
combinations are
Ray-tracheids with smooth walls. Soft Pines.
Ray-cells with large pits Subsection Cembra fig. 80.
Ray-cells with small pits Subsection Paracembra fig. 81.
Ray-tracheids with dentate walls. Hard Pines.
Ray-cells with large pits Group Lariciones fig. 83.
Ray-cells with small pits Other Hard Pines fig. 84.
This, the simplest classification of Pine-wood, is not without
exceptions. P. pinea of the Hard Pines resembles, in its
wood-characters, P. Gerardiana and P. Bungeana of the Soft Pines. The
dentate ray-tracheids of P. longifolia are not always obvious. The
tracheids of P. luchuensis, according to Bergerstein (Wiesner Festschr.
112), have smooth walls. My specimen shows dentate tracheids. There is
also evidence of transition from small to large pits (I. W. Bailey in
Am. Nat. xliv. 292). Both large and small pits appear in my specimen of
P. Merkusii.
Of other wood-characters, the presence or absence of tangential pits in
the tracheids of the late wood establishes a distinction between Soft
and Hard Pines. These pits, however, while always present in Soft Pines,
are not always absent in Hard Pines. The single and multiple rows of
resin-ducts in the wood of the first year may prove to be a reliable
sectional distinction, but this character has not been sufficiently
investigated to test its constancy. The wood-characters, therefore,
however decisive they may be for establishing the phylogenetic relations
of different genera, must be employed in the classification of the Pines
with the same reservations that apply to external characters.
Ray-tracheids with dentate walls and ray-cells with large pits are
peculiar to Pinus. Therefore the presence of these characters, alone or
in combination, is sufficient evidence for the recognition of Pine-wood.
But the combination of smooth tracheids with small pits (subsection
Paracembra) Pinus shares with Picea, Larix and Pseudotsuga.
Among Hard Pines the size of the pits has a certain geographical
significance. The large pits are found in all species of the Old World
except P. halepensis and P. pinaster; the small pits in all species of
the New World except P. resinosa and P. tropicalis. The Asiatic P.
Merkusii with both large and small pits is not strictly an exception to
this geographical distinction. The four exceptional species by this and
by other characters unite the Hard Pines of the two hemispheres.
THE BARK.
Bark is the outer part of the cortex that has perished, having been cut
off from nourishment by the thin hard plates of the bark-scales. In the
late and early bark-formation is found a general but by no means an
exact distinction between Soft and Hard Pines. In the Soft Pines the
cortex remains alive for many years, adjusting itself by growth to the
increasing thickness of the wood. The trunks of young trees remain
smooth and without rifts. In the Hard Pines the bark-formation begins
early and the trunks of young trees are covered with a scaly or rifted
bark. The smooth upper trunk of older trees is invariable in Soft Pines,
but in Hard Pines there are several exceptions to early bark-formation.
These exceptions are easily recognized in the field, and the character
is of decisive specific importance (glabra, halepensis, etc.).
Among species with early bark-formation are two forms of bark: 1,
cumulative, sufficiently persistent to acquire thickness and the
familiar dark gray and fuscous-brown shades of bark long exposed to the
weather; 2, deciduous, constantly falling away in thin scales and
exposing fresh red inner surfaces. The latter are commonly known as Red
Pines, as distinguished from Black Pines with dark cumulative bark.
Deciduous bark changes after some years to cumulative bark, and the
upper trunk only of mature trees is red. Red Pines, although usually
recognizable by their bark, are by no means constant in this character.
Oecological or pathological influences may check the fall of the
bark-scales, and then the distinction between the upper and lower parts
of the trunk becomes lost.
[Illustration: PLATE VII. THE WOOD]
SUMMARY
The various characters that have been considered in the previous pages
may be classified under different heads, some of them applicable to the
whole genus, others to larger or smaller groups of species.
GENERIC CHARACTERS
Several characters, quite distinct from those of other genera, are
common to all the species.
1. The primary leaf--appearing as a scale or bract throughout the
life of the tree.
2. The bud--its constant position at the nodes.
3. The internode--its three distinct divisions.
4. The secondary leaves--in cylindrical fascicles with a basal
sheath.
5. The pistillate flower--its constant nodal position and its
verticillate clusters.
6. The staminate flower--its constant basal position on the
internode and its compact clusters.
7. The cone--its clearly defined annual growths.
Pinus is also peculiar in the dimorphism of shoots and leaves and in
their constant interrelations with the diclinous flowers. Evolutionary
processes develop features peculiar to Pinus alone (the oblique cone,
etc.), but confined to a limited number of species.
SECTIONAL CHARACTERS
There are several characters that actually or potentially divide the
genus into two distinct sections, popularly known as Soft and Hard
Pines.
1. The fibro-vascular bundle of the leaf, single or double.
2. The base of the bract subtending the leaf-fascicle, non-decurrent
or decurrent
3. The phyllotaxis of the cone, simple or complex.
4. The flower-bud, its less or greater development.
Some characters indicate the same distinction but are subject each to a
few exceptions.
5. The fascicle-sheath, deciduous or persistent.
6. The walls of the ray-tracheids, smooth or dentate.
7. The connective of the pollen-sacs, large or small.
8. The formation of bark, late or early.
SUBSECTIONAL CHARACTERS
An exact subdivision of the Soft Pines is possible on the following
characters.
1. The umbo of the cone-scales, terminal or dorsal.
2. The scales of the conelet, mutic or armed.
3. The pits of the ray-cells, large or small.
EVOLUTIONAL CHARACTERS
The progressive evolution of the fruit of Pinus, from a symmetrical cone
of weak tissues, bearing a wingless seed, to an indurated oblique cone
with an elaborate form of winged seed and an intermittent dissemination,
appears among the species in various degrees of development as follows--
The seed
1. wingless.
2. with a rudimentary wing.
3. with an effective adnate wing.
4. with an ineffective articulate wing.
5. with an effective articulate wing.
6. with an articulate wing, thickened at the base of the blade.
The cone
1. indehiscent.
2. dehiscent and deciduous.
3. dehiscent and persistent.
4. persistent and serotinous.
and as to its form
5. symmetrical.
6. subsymmetrical.
7. oblique.
These different forms of the seed and, to some extent, of the cone, are
available for segregating the species into groups of closely related
members; while the gradual progression of the fruit, from a primitive to
a highly specialized form of cone and method of dissemination, points to
a veritable taxonomic evolution which is here utilized as the
fundamental motive of the systematic classification of the species.
SPECIFIC CHARACTERS
All aspects of vegetative and reproductive organs may contribute toward
a determination of species, but the importance of each character is
often relative, being conclusive with one group of species, useless with
another. Characters considered by earlier authors to be invariable with
species, such as the dimensions of leaf or cone, the number of leaves in
the fascicle, the position of the resin-ducts, the presence of pruinose
branchlets, etc., prove to be inconstant in some species. In fact, as
the botanical horizon enlarges, the varietal limits of the species
broaden and many restrictions imposed by earlier systems are gradually
disappearing.
Variation is the preliminary step toward the creation of species, which
come into being with the elimination of intermediate forms. Variation in
a species may be the result of its participation in the evolutionary
processes culminating in the serotinous Pines, or it may result from the
ability of the species to adapt itself to various environments by
sympathetic modifications of growth, or it may arise from some
peculiarity of the individual tree.
Evolutionary variation is associated with the gradual appearance of the
persistent, the oblique and the serotinous cone, and of the multinodal
spring-shoot. For these conditions appear in less or greater prevalence
among the species of the genus.
Variation induced by environment finds familiar illustrations among the
species that can survive at the limits of vegetation and can meet these
inhospitable conditions by a radical change of all growing parts. Such
variations are mainly of dimensions, but, with some species, the number
of fascicle-leaves is affected and the shorter growing-season may modify
the cone-tissues. In Mexico and Central America are found extremes of
climate within small areas and easily within the range of dissemination
from a single tree. The cause of the bewildering host of varietal forms,
connecting widely contrasted extremes, seems to lie in the facile
adaptability of those Pines, which are able to spread from the tropical
base of a mountain to a less or greater distance toward its snow-capped
summit.
The peculiarities of individual trees that induce abnormally short or
long growths, the dwarf or other monstrous forms, the variegations in
leaf-coloring, etc., etc., are not available for classification, for
they may appear in any species, in fact in any genus of Conifers. These
variations are artificially multiplied for commercial and decorative
purposes. But inasmuch as they are repeated in all species and genera of
the Coniferae that have been long under the observation of skillful
gardeners, their significance has a broader scope than that imposed by
the study of a single genus.