Chapter II: Part 2

The anatomical structure of the vegetative organs of recent cycads is
of special interest as affording important evidence of relationship
with extinct types, and with other groups of recent plants.
Brongniart, who was the first to investigate in detail the anatomy of
a cycadean stem, recognized an agreement, as regards the secondary
wood, with Dicotyledons and Gymnosperms, rather than with
Monocotyledons. He drew attention also to certain structural
similarities between _Cycas_ and _Ginkgo_. The main anatomical
features of a cycad stem may be summarized as follows: the centre is
occupied by a large parenchymatous pith traversed by numerous
secretory canals, and in some genera by cauline vascular bundles (e.g.
_Encephalartos_ and _Macrozamia_). In addition to these cauline
strands (confined to the stem and not connected with the leaves),
collateral bundles are often met with in the pith, which form the
vascular supply of terminal flowers borne at intervals on the apex of
the stem. These latter bundles may be seen in sections of old stems to
pursue a more or less horizontal course, passing outwards through the
main woody cylinder. This lateral course is due to the more vigorous
growth of the axillary branch formed near the base of each flower,
which is a terminal structure, and, except in the female flower of
_Cycas_, puts a limit to the apical growth of the stem. The vigorous
lateral branch therefore continues the line of the main axis. The pith
is encircled by a cylinder of secondary wood, consisting of single or
multiple radial rows of tracheids separated by broad medullary rays
composed of large parenchymatous cells; the tracheids bear numerous
bordered pits on the radial walls. The large medullary rays give to
the wood a characteristic parenchymatous or lax appearance, which is
in marked contrast to the more compact wood of a conifer. The
protoxylem-elements are situated at the extreme inner edge of the
secondary wood, and may occur as small groups of narrow,
spirally-pitted elements scattered among the parenchyma which abuts on
the main mass of wood. Short and reticulately-pitted tracheal cells,
similar to tracheids, often occur in the circummedullary region of
cycadean stems. In an old stem of _Cycas_, _Encephalartos_ or
_Macrozamia_ the secondary wood consists of several rather unevenly
concentric zones, while in some other genera it forms a continuous
mass as in conifers and normal dicotyledons. These concentric rings of
secondary xylem and phloem (fig. 9) afford a characteristic cycadean
feature. After the cambium has been active for some time producing
secondary xylem and phloem, the latter consisting of sieve-tubes,
phloem-parenchyma and frequently thick-walled fibres, a second cambium
is developed in the pericycle; this produces a second vascular zone,
which is in turn followed by a third cambium, and so on, until several
hollow cylinders are developed. It has been recently shown that
several cambium-zones may remain in a state of activity, so that the
formation of a new cambium does not necessarily mark a cessation of
growth in the more internal meristematic rings. It occasionally
happens that groups of xylem and phloem are developed internally to
some of the vascular rings; these are characterized by an inverse
orientation of the tissues, the xylem being centrifugal and the phloem
centripetal in its development. The broad cortical region, which
contains many secretory canals, is traversed by numerous vascular
bundles (fig. 9, c) some of which pursue a more or less vertical
course, and by frequent anastomoses with one another form a loose
reticulum of vascular strands; others are leaf-traces on their way
from the stele of the stem to the leaves. Most of these cortical
bundles are collateral in structure, but in some the xylem and phloem
are concentrically arranged; the secondary origin of these bundles
from procambium-strands was described by Mettenius in his classical
paper of 1860. During the increase in thickness of a cycadean stem
successive layers of cork-tissue are formed by phellogens in the
persistent bases of leaves (fig. 9, pd), which increase in size to
adapt themselves to the growth of the vascular zones. The leaf-traces
of cycads are remarkable both on account of their course and their
anatomy. In a transverse section of a stem (fig. 9) one sees some
vascular bundles following a horizontal or slightly oblique course in
the cortex, stretching for a longer or shorter distance in a direction
concentric with the woody cylinder. From each leaf-base two main
bundles spread right and left through the cortex of the stem (fig. 9,
lt), and as they curve gradually towards the vascular ring they
present the appearance of two rather flat ogee curves, usually spoken
of as the leaf-trace girdles (fig. 9, lt). The distal ends of these
girdles give off several branches, which traverse the petiole and
rachis as numerous collateral bundles. The complicated girdle-like
course is characteristic of the leaf-traces of most recent cycads, but
in some cases, e.g. in _Zamia floridana_, the traces are described by
Wieland in his recent monograph on American fossil cycads (_Carnegie
Institution Publications_, 1906) as possessing a more direct course
similar to that in Mesozoic genera. A leaf-trace, as it passes through
the cortex, has a collateral structure, the protoxylem being situated
at the inner edge of the xylem; when it reaches the leaf-base the
position of the spiral tracheids is gradually altered, and the endarch
arrangement (protoxylem internal) gives place to a mesarch structure
(protoxylem more or less central and not on the edge of the xylem
strand). In a bundle examined in the basal portion of a leaf the bulk
of the xylem is found to be centrifugal in position, but internally to
the protoxylem there is a group of centripetal tracheids; higher up in
the petiole the xylem is mainly centripetal, the centrifugal wood
being represented by a small arc of tracheids external to the
protoxylem and separated from it by a few parenchymatous elements.
Finally, in the pinnae of the frond the centrifugal xylem may
disappear, the protoxylem being now exarch in position and abutting on
the phloem. Similarly in the sporophylls of some cycads the bundles
are endarch near the base and mesarch near the distal end of the
stamen or carpel. The vascular system of cycadean seedlings presents
some features worthy of note; centripetal xylem occurs in the
cotyledonary bundles associated with transfusion-tracheids. The
bundles from the cotyledons pursue a direct course to the stele of the
main axis, and do not assume the girdle-form characteristic of the
adult plant. This is of interest from the point of view of the
comparison of recent cycads with extinct species (_Bennettites_), in
which the leaf-traces follow a much more direct course than in modern
cycads. The mesarch structure of the leaf-bundles is met with in a
less pronounced form in the flower peduncles of some cycads. This fact
is of importance as showing that the type of vascular structure, which
characterized the stems of many Palaeozoic genera, has not entirely
disappeared from the stems of modern cycads; but the mesarch bundle is
now confined to the leaves and peduncles. The roots of some cycads
resemble the stems in producing several cambium-rings; they possess 2
to 8 protoxylem-groups, and are characterized by a broad pericyclic
zone. A common phenomenon in cycads is the production of roots which
grow upwards (apogeotropic), and appear as coralline branched
structures above the level of the ground; some of the cortical cells
of these roots are hypertrophied, and contain numerous filaments of
blue-green Algae (Nostocaceae), which live as endoparasites in the
cell-cavities.

pd, Periderm in leaf-bases.
lt, Leaf-traces in cortex.
ph, Phloem.
x, Xylem.
m, Medullary bundles.
c, Cortical bundles.]

GINKGOALES.--This class-designation has been recently proposed to give
emphasis to the isolated position of the genus _Ginkgo_ (_Salisburia_)
among the Gymnosperms. _Ginkgo biloba_, the maidenhair tree, has
usually been placed by botanists in the Taxeae in the neighbourhood of
the yew (_Taxus_), but the proposal by Eichler in 1852 to institute a
special family, the _Salisburieae_, indicated a recognition of the
existence of special characteristics which distinguish the genus from
other members of the Coniferae. The discovery by the Japanese botanist
Hirase of the development of ciliated spermatozoids in the pollen-tube
of _Ginkgo_, in place of the non-motile male cells of typical
conifers, served as a cogent argument in favour of separating the
genus from the Coniferales and placing it in a class of its own. In
1712 Kaempfer published a drawing of a Japanese tree, which he
described under the name _Ginkgo_; this term was adopted in 1771 by
Linnaeus, who spoke of Kaempfer's plant as _Ginkgo biloba_. In 1797
Smith proposed to use the name _Salisburia adiantifolia_ in preference
to the "uncouth" genus _Ginkgo_ and "incorrect" specific term
_biloba_. Both names are still in common use. On account of the
resemblance of the leaves to those of some species of _Adiantum_, the
appellation maidenhair tree has long been given to _Ginkgo biloba_.
_Ginkgo_ is of special interest on account of its isolated position
among existing plants, its restricted geographical distribution, and
its great antiquity (see PALAEOBOTANY: _Mesozoic_). This solitary
survivor of an ancient stock is almost extinct, but a few old and
presumably wild trees are recorded by travellers in parts of China.
_Ginkgo_ is common as a sacred tree in the gardens of temples in the
Far East, and often cultivated in North America and Europe. _Ginkgo
biloba_, which may reach a height of over 30 metres, forms a tree of
pyramidal shape with a smooth grey bark. The leaves (figs. 10 and 11)
have a long, slender petiole terminating in a fan-shaped lamina, which
may be entire, divided by a median incision into two wedge-shaped
lobes, or subdivided into several narrow segments. The venation is
like that of many ferns, e.g. _Adiantum_; the lowest vein in each half
of the lamina follows a course parallel to the edge, and gives off
numerous branches, which fork repeatedly as they spread in a palmate
manner towards the leaf margin. The foliage-leaves occur either
scattered on long shoots of unlimited growth, or at the apex of short
shoots (spurs), which may eventually elongate into long shoots.

p, Pollen-tube (proximal end).
c, Pollen-chamber.
e, Upward prolongation of megaspore.
a, Archegonia.
Pg, Pollen-grain.
Ex, Exine.]

Flowers.

The flowers are dioecious. The male flowers (fig. 12), borne in the
axil of scale-leaves, consist of a stalked central axis bearing
loosely disposed stamens; each stamen consists of a slender filament
terminating in a small apical scale, which bears usually two, but not
infrequently three or four pollen-sacs (fig. 12, C). The axis of the
flower is a shoot bearing leaves in the form of stamens. A mature
pollen-grain contains a prothallus of 3 to 5 cells (Fig. 13, Pg); the
exine extends over two-thirds of the circumference, leaving a thin
portion of the wall, which on collapsing produces a longitudinal
groove similar to the median depression on the pollen-grain of a
cycad. The ordinary type of female flower has the form of a long,
naked peduncle bearing a single ovule on either side of the apex (fig.
12), the base of each being enclosed by a small, collar-like rim, the
nature of which has been variously interpreted. A young ovule consists
of a conical nucellus surrounded by a single integument terminating as
a two-lipped micropyle. A large pollen-chamber occupies the apex of
the nucellus; immediately below this, two or more archegonia (fig. 13,
a) are developed in the upper region of the megaspore, each consisting
of a large egg-cell surmounted by two neck-cells and a canal-cell
which is cut off shortly before fertilization. After the entrance of
the pollen-grain the pollen-chamber becomes roofed over by a blunt
protuberance of nucellar tissue. The megaspore (embryo-sac) continues
to grow after pollination until the greater part of the nucellus is
gradually destroyed; it also gives rise to a vertical outgrowth, which
projects from the apex of the megaspore as a short, thick column (fig.
13, e) supporting the remains of the nucellar tissue which forms the
roof of the pollen-chamber (fig. 13, c). Surrounding the pitted wall
of the ovum there is a definite layer of large cells, no doubt
representing a tapetum, which, as in cycads and conifers, plays an
important part in nourishing the growing egg-cell. The endosperm
detached from a large _Ginkgo_ ovule after fertilization bears a close
resemblance to that of a cycad; the apex is occupied by a depression,
on the floor of which two small holes mark the position of the
archegonia, and the outgrowth from the megaspore apex projects from
the centre as a short peg. After pollination the pollen-tube grows
into the nucellar tissue, as in cycads, and the pollen-grain itself
(fig. 13, Pg) hangs down into the pollen-chamber; two large spirally
ciliated spermatozoids are produced, their manner of development
agreeing very closely with that of the corresponding cells in _Cycas_
and _Zamia_. After fertilization the ovum-nucleus divides and
cell-formation proceeds rapidly, especially in the lower part of the
ovum, in which the cotyledon and axis of the embryo are
differentiated; the long, tangled suspensor of the cycadean embryo is
not found in _Ginkgo_. It is often stated that fertilization occurs
after the ovules have fallen, but it has been demonstrated by Hirase
that this occurs while the ovules are still attached to the tree. The
ripe seed, which grows as large as a rather small plum, is enclosed by
a thick, fleshy envelope covering a hard woody shell with two or
rarely three longitudinal keels. A papery remnant of nucellus lines
the inner face of the woody shell, and, as in cycadean seeds, the
apical portion is readily separated as a cap covering the summit of
the endosperm.

Anatomy.

The morphology of the female flowers has been variously interpreted by
botanists; the peduncle bearing the ovules has been described as
homologous with the petiole of a foliage-leaf and as a
shoot-structure, the collar-like envelope at the base of the ovules
being referred to as a second integument or arillus, or as the
representative of a carpel. The evidence afforded by normal and
abnormal flowers appears to be in favour of the following
interpretation: The peduncle is a shoot bearing two or more carpels.
Each ovule is enclosed at the base by an envelope or collar homologous
with the lamina of a leaf; the fleshy and hard coats of the nucellus
constitute a single integument. The stalk of an ovule, considerably
reduced in normal flowers and much larger in some abnormal flowers, is
homologous with a leaf-stalk, with which it agrees in the structure
and number of vascular bundles. The facts on which this description is
based are derived partly from anatomical evidence, and in part from an
account given by a Japanese botanist, Fujii, of several abnormal
female flowers; in some cases the collar at the base of an ovule,
often described as an arillus, is found to pass gradually into the
lamina of a leaf bearing marginal ovules (fig. 14, B). The occurrence
of more than two ovules on one peduncle is by no means rare; a
particularly striking example is described by Fujii, in which an
unusually thick peduncle bearing several stalked ovules terminates in
a scaly bud (fig. 14, A, b). The frequent occurrence of more than two
pollen-sacs and the equally common occurrence of additional ovules
have been regarded by some authors as evidence in favour of the view
that ancestral types normally possessed a greater number of these
organs than are usually found in the recent species. This view
receives support from fossil evidence. Close to the apex of a shoot
the vascular bundles of a leaf make their appearance as double
strands, and the leaf-traces in the upper part of a shoot have the
form of distinct bundles, which in the older part of the shoot form a
continuous ring. Each double leaf-trace passes through four internodes
before becoming a part of the stele; the double nature of the trace is
a characteristic feature. Secretory sacs occur abundantly in the
leaf-lamina, where they appear as short lines between the veins; they
are abundant also in the cortex and pith of the shoot, in the fleshy
integument of the ovule, and elsewhere. The secondary wood of the
shoot and root conforms in the main to the coniferous type; in the
short shoots the greater breadth of the medullary rays in the more
internal part of the xylem recalls the cycadean type. The secondary
phloem contains numerous thick-walled fibres, parenchymatous cells,
and large sieve-tubes with plates on the radial walls; swollen
parenchymatous cells containing crystals are commonly met with in the
cortex, pith and medullary-ray tissues. The wood consists of
tracheids, with circular bordered pits on their radial walls, and in
the late summer wood pits are unusually abundant on the tangential
walls. A point of anatomical interest is the occurrence in the
vascular bundles of the cotyledons, scale-leaves, and elsewhere of a
few centripetally developed tracheids, which give to the xylem-strands
a mesarch structure such as characterizes the foliar bundles of
cycads. The root is diarch in structure, but additional
protoxylem-strands may be present at the base of the main root; the
pericycle consists of several layers of cells.

Geological history.

This is not the place to discuss in detail the past history of
_Ginkgo_ (see PALAEOBOTANY: _Mesozoic_). Among Palaeozoic genera there
are some which bear a close resemblance to the recent type in the form
of the leaves; and petrified Palaeozoic seeds, almost identical with
those of the maidenhair tree, have been described from French and
English localities. During the Triassic and Jurassic periods the genus
_Baiera_--no doubt a representative of the Ginkgoales--was widely
spread throughout Europe and in other regions; _Ginkgo_ itself occurs
abundantly in Mesozoic and Tertiary rocks, and was a common plant in
the Arctic regions as elsewhere during the Jurassic and Lower
Cretaceous periods. Some unusually perfect _Ginkgo_ leaves have been
found in the Eocene leaf-beds between the lava-flows exposed in the
cliffs of Mull (fig. 11). From an evolutionary point of view, it is of
interest to note the occurrence of filicinean and cycadean characters
in the maidenhair tree. The leaves at once invite a comparison with
ferns; the numerous long hairs which form a delicate woolly covering
on young leaves recall the hairs of certain ferns, but agree more
closely with the long filamentous hairs of recent cycads. The
spermatozoids constitute the most striking link with both cycads and
ferns. The structure of the seed, the presence of two neck-cells in
the archegonia, the late development of the embryo, the
partially-fused cotyledons and certain anatomical characters, are
features common to _Ginkgo_ and the cycads. The maidenhair tree is one
of the most interesting survivals from the past; it represents a type
which, in the Palaeozoic era, may have been merged into the extinct
class Cordaitales. Through the succeeding ages the Ginkgoales were
represented by numerous forms, which gradually became more restricted
in their distribution and fewer in number during the Cretaceous and
Tertiary periods, terminating at the present day in one solitary
survivor.

CONIFERALES.--Trees and shrubs characterized by a copious branching of
the stem and frequently by a regular pyramidal form. Leaves simple,
small, linear or short and scale-like, usually persisting for more
than one year. Flowers monoecious or dioecious, unisexual, without a
perianth, often in the form of cones, but never terminal on the main
stem.

External features.

The plants usually included in the Coniferae constitute a less
homogeneous class than the Cycadaceae. Some authors use the term
Coniferae in a restricted sense as including those genera which have
the female flowers in the form of cones, the other genera,
characterized by flowers of a different type, being placed in the
Taxaceae, and often spoken of as Taxads. In order to avoid confusion
in the use of the term Coniferae, we may adopt as a class-designation
the name Coniferales, including both the Coniferae--using the term in
a restricted sense--and the Taxaceae. The most striking characteristic
of the majority of the Coniferales is the regular manner of the
monopodial branching and the pyramidal shape. _Araucaria imbricata_,
the Monkey-puzzle tree, _A. excelsa_, the Norfolk Island pine, many
pines and firs, cedars and other genera illustrate the pyramidal form.
The mammoth redwood tree of California, _Sequoia (Wellingtonia)
gigantea_, which represents the tallest Gymnosperm, is a good example
of the regular tapering main stem and narrow pyramidal form. The
cypresses afford instances of tall and narrow trees similar in habit
to Lombardy poplars. The common cypress (_Cupressus sempervirens_), as
found wild in the mountains of Crete and Cyprus, is characterized by
long and spreading branches, which give it a cedar-like habit. A
pendulous or weeping habit is assumed by some conifers, e.g. _Picea
excelsa_ var. _virgata_ represents a form in which the main branches
attain a considerable horizontal extension, and trail themselves like
snakes along the ground. Certain species of _Pinus_, the yews
(_Taxus_) and some other genera grow as bushes, which in place of a
main mast-like stem possess several repeatedly-branched leading
shoots. The unfavourable conditions in Arctic regions have produced a
dwarf form, in which the main shoots grow close to the ground.
Artificially induced dwarfed plants of _Pinus_, _Cupressus_,
_Sciadopitys_ (umbrella pine) and other genera are commonly cultivated
by the Japanese. The dying off of older branches and the vigorous
growth of shoots nearer the apex of the stem produce a form of tree
illustrated by the stone pine of the Mediterranean region (_Pinus
Pinea_), which Turner has rendered familiar in his "Childe Harold's
Pilgrimage" and other pictures of Italian scenery. Conifers are not
infrequently seen in which a lateral branch has bent sharply upwards
to take the place of the injured main trunk. An upward tendency of all
the main lateral branches, known as fastigiation, is common in some
species, producing well-marked varieties, e.g. _Cephalotaxus
pedunculata_ var. _fastigiata_; this fastigiate habit may arise as a
sport on a tree with spreading branches. Another departure from the
normal is that in which the juvenile or seedling form of shoot
persists in the adult tree; the numerous coniferous plants known as
species of _Retinospora_ are examples of this. The name _Retinospora_,
therefore, does not stand for a true genus, but denotes persistent
young forms of _Juniperus_, _Thuja_, _Cupressus_, &c., in which the
small scaly leaves of ordinary species are replaced by the slender,
needle-like leaves, which stand out more or less at right angles from
the branches. The flat branchlets of _Cupressus_, _Thuja_ (arbor
vitae), _Thujopsis dolabrata_ (Japanese arbor vitae) are
characteristic of certain types of conifers; in some cases the
horizontal extension of the branches induces a dorsiventral structure.
A characteristic feature of the genus _Agathis (Dammara)_ the Kauri
pine of New Zealand, is the deciduous habit of the branches; these
become detached from the main trunk leaving a well-defined
absciss-surface, which appears as a depressed circular scar on the
stem. A new genus of conifers, _Taiwania_, has recently been described
from the island of Formosa; it is said to agree in habit with the
Japanese _Cryptomeria_, but the cones appear to have a structure which
distinguishes them from those of any other genus.

Leaves.

With a few exceptions conifers are evergreen, and retain the leaves
for several years (10 years in _Araucaria imbricata_, 8 to 10 in
_Picea excelsa_, 5 in _Taxus baccata_; in _Pinus_ the needles usually
fall in October of their third year). The larch (_Larix_) sheds its
leaves in the autumn, in the Chinese larch (_Pseudolarix Kaempferi_)
the leaves turn a bright yellow colour before falling. In the swamp
cypress (_Taxodium distichum_) the tree assumes a rich brown colour in
the autumn, and sheds its leaves together with the branchlets which
bear them; deciduous branches occur also in some other species, e.g.
_Sequoia sempervirens_ (redwood), _Thuja occidentalis_, &c. The leaves
of conifers are characterized by their small size, e.g. the
needle-form represented by _Pinus_, _Cedrus_, _Larix_, &c., the linear
flat or angular leaves, appressed to the branches, of _Thuja_,
_Cupressus_, _Libocedrus_, &c. The flat and comparatively broad leaves
of _Araucaria imbricata_, _A. Bidwillii_, and some species of the
southern genus _Podocarpus_ are traversed by several parallel veins,
as are also the still larger leaves of _Agathis_, which may reach a
length of several inches. In addition to the foliage-leaves several
genera also possess scale-leaves of various kinds, represented by
bud-scales in _Pinus_, _Picea_, &c., which frequently persist for a
time at the base of a young shoot which has pushed its way through the
yielding cap of protecting scales, while in some conifers the
bud-scales adhere together, and after being torn near the base are
carried up by the growing axis as a thin brown cap. The cypresses,
araucarias and some other genera have no true bud-scales; in some
species, e.g. _Araucaria Bidwillii_, the occurrence of small
foliage-leaves, which have functioned as bud-scales, at intervals on
the shoots affords a measure of seasonal growth. The occurrence of
long and short shoots is a characteristic feature of many conifers. In
_Pinus_ the needles occur in pairs, or in clusters of 3 or 5 at the
apex of a small and inconspicuous short shoot of limited growth
(spur), which is enclosed at its base by a few scale-leaves, and borne
on a branch of unlimited growth in the axil of a scale-leaf. In the
Californian _Pinus monophylla_ each spur bears usually one needle, but
two are not uncommon; it would seem that rudiments of two needles are
always produced, but, as a rule, only one develops into a needle. In
_Sciadopitys_ similar spurs occur, each bearing a single needle,
which in its grooved surface and in the possession of a double
vascular bundle bears traces of an origin from two needle-leaves. A
peculiarity of these leaves is the inverse orientation of the vascular
tissue; each of the two veins has its phloem next the upper and the
xylem towards the lower surface of the leaf; this unusual position of
the xylem and phloem may be explained by regarding the needle of
_Sciadopitys_ as being composed of a pair of leaves borne on a short
axillary shoot and fused by their margins (fig. 15, A). Long and short
shoots occur also in _Cedrus_ and _Larix_, but in these genera the
spurs are longer and stouter, and are not shed with the leaves; this
kind of short shoot, by accelerated apical growth, often passes into
the condition of a long shoot on which the leaves are scattered and
separated by comparatively long internodes, instead of being crowded
into tufts such as are borne on the ends of the spurs. In the genus
_Phyllocladus_ (New Zealand, &c.) there are no green foliage-leaves,
but in their place flattened branches (phylloclades) borne in the
axils of small scale-leaves. The cotyledons are often two in number,
but sometimes (e.g. _Pinus_) as many as fifteen; these leaves are
usually succeeded by foliage-leaves in the form of delicate spreading
needles, and these primordial leaves are followed, sooner or later, by
the adult type of leaf, except in Retinosporas, which retain the
juvenile foliage. In addition to the first foliage-leaves and the
adult type of leaf, there are often produced leaves which are
intermediate both in shape and structure between the seedling and
adult foliage. Dimorphism or heterophylly is fairly common. One of the
best known examples is the Chinese juniper (_Juniperus chinensis_), in
which branches with spinous leaves, longer and more spreading than the
ordinary adult leaf, are often found associated with the normal type
of branch. In some cases, e.g. _Sequoia sempervirens_, the fertile
branches bear leaves which are less spreading than those on the
vegetative shoots. Certain species of the southern hemisphere genus
_Dacrydium_ afford particularly striking instances of heterophylly,
e.g. _D. Kirkii_ of New Zealand, in which some branches bear small and
appressed leaves, while in others the leaves are much longer and more
spreading. A well-known fossil conifer from Triassic strata--_Voltzia
heterophylla_--also illustrates a marked dissimilarity in the leaves
of the same shoot. The variation in leaf-form and the tendency of
leaves to arrange themselves in various ways on different branches of
the same plant are features which it is important to bear in mind in
the identification of fossil conifers. In this connexion we may note
the striking resemblance between some of the New Zealand Alpine
Veronicas, e.g. _Veronica Hectori_, _V. cupressoides_, &c. (also
_Polycladus cupressinus_, a Composite), and some of the cypresses and
other conifers with small appressed leaves. The long linear leaves of
some species of _Podocarpus_, in which the lamina is traversed by a
single vein, recall the pinnae of Cycas; the branches of some
Dacrydiums and other forms closely resemble those of lycopods; these
superficial resemblances, both between different genera of conifers
and between conifers and other plants, coupled with the usual
occurrence of fossil coniferous twigs without cones attached to them,
render the determination of extinct types a very unsatisfactory and
frequently an impossible task.

Flowers.

A typical male flower consists of a central axis bearing numerous
spirally-arranged sporophylls (stamens), each of which consists of a
slender stalk (filament) terminating distally in a more or less
prominent knob or triangular scale, and bearing two or more
pollen-sacs (microsporangia) on its lower surface. The pollen-grains
of some genera (e.g. _Pinus_) are furnished with bladder-like
extensions of the outer wall, which serve as aids to wind-dispersal.
The stamens of _Araucaria_ and _Agathis_ are peculiar in bearing
several long, and narrow free pollen-sacs; these may be compared with
the sporangiophores of the horsetails (_Equisetum_); in _Taxus_ (yew)
the filament is attached to the centre of a large circular distal
expansion, which bears several pollen-sacs on its under surface. In
the conifers proper the female reproductive organs have the form of
cones, which may be styled flowers or inflorescences according to
different interpretations of their morphology. In the Taxaceae the
flowers have a simpler structure. The female flowers of the
_Abietineae_ may be taken as representing a common type. A pine cone
reaches maturity in two years; a single year suffices for the full
development in _Larix_ and several other genera. The axis of the cone
bears numerous spirally disposed flat scales (cone-scales), each of
which, if examined in a young cone, is found to be double, and to
consist of a lower and an upper portion. The latter is a thin flat
scale bearing a median ridge or keel (e.g. _Abies_), on each side of
which is situated an inverted ovule, consisting of a nucellus
surrounded by a single integument. As the cone grows in size and
becomes woody the lower half of the cone-scale, which we may call the
carpellary scale, may remain small, and is so far outgrown by the
upper half (seminiferous scale) that it is hardly recognizable in the
mature cone. In many species of _Abies_ (e.g. _Abies pectinata_, &c.)
the ripe cone differs from those of _Pinus_, _Picea_ and _Cedrus_ in
the large size of the carpellary scales, which project as conspicuous
thin appendages beyond the distal margins of the broader and more
woody seminiferous scales; the long carpellary scale is a prominent
feature also in the cone of the Douglas pine (_Pseudotsuga
Douglasii_). The female flowers (cones) vary considerably in size; the
largest are the more or less spherical cones of _Araucaria_--a single
cone of _A. imbricata_ may produce as many as 300 seeds, one seed to
each fertile cone-scale--and the long pendent cones, 1 to 2 ft. in
length, of the sugar pine of California (_Pinus Lambertiana_) and
other species. Smaller cones, less than an inch long, occur in the
larch, _Athrotaxis_ (Tasmania), _Fitzroya_ (Patagonia and Tasmania),
&c. In the _Taxodieae_ and _Araucarieae_ the cones are similar in
appearance to those of the _Abietineae_, but they differ in the fact
that the scales appear to be single, even in the young condition; each
cone-scale in a genus of the _Taxodiinae_ (_Sequoia_, &c.) bears
several seeds, while in the _Araucariinae_ (_Araucaria_ and _Agathis_)
each scale has one seed. The _Cupressineae_ have cones composed of a
few scales arranged in alternate whorls; each scale bears two or more
seeds, and shows no external sign of being composed of two distinct
portions. In the junipers the scales become fleshy as the seeds ripen,
and the individual scales fuse together in the form of a berry. The
female flowers of the Taxaceae assume another form; in _Microcachrys_
(Tasmania) the reproductive structures are spirally disposed, and form
small globular cones made up of red fleshy scales, to each of which is
attached a single ovule enclosed by an integument and partially
invested by an arillus; in _Dacrydium_ the carpellary leaves are very
similar to the foliage leaves--each bears one ovule with two
integuments, the outer of which constitutes an arillus. Finally in the
yew, as a type of the family Taxeae, the ovules occur singly at the
apex of a lateral branch, enclosed when ripe by a conspicuous red or
yellow fleshy arillus, which serves as an attraction to animals, and
thus aids in the dispersal of the seeds.

Morphology of female flower.

It is important to draw attention to some structural features
exhibited by certain cone-scales, in which there is no external sign
indicative of the presence of a carpellary and a seminiferous scale.
In _Araucaria Cookii_ and some allied species each scale has a small
pointed projection from its upper face near the distal end; the scales
of _Cunninghamia_ (China) are characterized by a somewhat ragged
membranous projection extending across the upper face between the
seeds and the distal end of the scale; in the scales of _Athrotaxis_
(Tasmania) a prominent rounded ridge occupies a corresponding
position. These projections and ridges may be homologous with the
seminiferous scale of the pines, firs, cedars, &c. The simplest
interpretation of the cone of the _Abietineae_ is that which regards
it as a flower consisting of an axis bearing several open carpels,
which in the adult cone may be large and prominent or very small, the
scale bearing the ovules being regarded as a placental outgrowth from
the flat and open carpel. In _Araucaria_ the cone-scale is regarded as
consisting of a flat carpel, of which the placenta has not grown out
into the scale-like structure. The seminiferous scale of _Pinus_, &c.,
is also spoken of sometimes as a ligular outgrowth from the carpellary
leaf. Robert Brown was the first to give a clear description of the
morphology of the Abietineous cone in which carpels bear naked ovules;
he recognized gymnospermy as an important distinguishing feature in
conifers as well as in cycads. Another view is to regard the cone as
an inflorescence, each carpellary scale being a bract bearing in its
axil a shoot the axis of which has not been developed; the
seminiferous scale is believed to represent either a single leaf or a
fused pair of leaves belonging to the partially suppressed axillary
shoot. In 1869 van Tieghem laid stress on anatomical evidence as a key
to the morphology of the cone-scales; he drew attention to the fact
that the collateral vascular bundles of the seminiferous scale are
inversely orientated as compared with those of the carpellary scale;
in the latter the xylem of each bundle is next the upper surface,
while in the seminiferous scale the phloem occupies that position. The
conclusion drawn from this was that the seminiferous scale (fig. 15,
B, Sc) is the first and only leaf of an axillary shoot (b) borne on
that side of the shoot, the axis of which is suppressed, opposite the
subtending bract (fig. 15, A, B, C, Br). Another view is to apply to
the seminiferous scale an explanation similar to that suggested by von
Mohl in the case of the double needle of _Sciadopitys_, and to
consider the seed-bearing scale as being made up of a pair of leaves
(fig. 15, A, a, a) of an axillary shoot (b) fused into one by their
posterior margins (fig. 15, A). The latter view receives support from
abnormal cones in which carpellary scales subtend axillary shoots, of
which the first two leaves (fig. 15, C, l^1, l^1) are often harder and
browner than the others; forms have been described transitional
between axillary shoots, in which the leaves are separate, and others
in which two of the leaves are more or less completely fused. In a
young cone the seminiferous scale appears as a hump of tissue at the
base or in the axil of the carpellary scale, but Celakovsky, a strong
supporter of the axillary-bud theory, attaches little or no importance
to this kind of evidence, regarding the present manner of development
as being merely an example of a short cut adopted in the course of
evolution, and replacing the original production of a branch in the
axil of each carpellary scale. Eichler, one of the chief supporters of
the simpler view, does not recognize in the inverse orientation of the
vascular bundles an argument in support of the axillary-bud theory,
but points out that the seminiferous scale, being an outgrowth from
the surface of the carpellary scale, would, like outgrowths from an
ordinary leaf, naturally have its bundles inversely orientated. In
such cone-scales as show little or no external indication of being
double in origin, e.g. _Araucaria_ (fig. 15, D) Sequoia, &c., there
are always two sets of bundles; the upper set, having the phloem
uppermost, as in the seminiferous scale of _Abies_ or _Pinus_, are
regarded as belonging to the outgrowth from the carpellary scale and
specially developed to supply the ovules. Monstrous cones are fairly
common; these in some instances lend support to the axillary-bud
theory, and it has been said that this theory owes its existence to
evidence furnished by abnormal cones. It is difficult to estimate the
value of abnormalities as evidence bearing on morphological
interpretation; the chief danger lies perhaps in attaching undue
weight to them, but there is also a risk of minimizing their
importance. Monstrosities at least demonstrate possible lines of
development, but when the abnormal forms of growth in various
directions are fairly evenly balanced, trustworthy deductions become
difficult. The occurrence of buds in the axils of carpellary scales
may, however, simply mean that buds, which are usually undeveloped in
the axils of sporophylls, occasionally afford evidence of their
existence. Some monstrous cones lend no support to the axillary-bud
theory. In _Larix_ the axis of the cone often continues its growth;
similarly in _Cephalotaxus_ the cones are often proliferous. (In rare
cases the proliferated portion produces male flowers in the
leaf-axils.) In _Larix_ the carpellary scale may become leafy, and the
seminiferous scale may disappear. Androgynous cones may be produced,
as in the cone of _Pinus rigida_ (fig. 16), in which the lower part
bears stamens and the upper portion carpellary and seminiferous
scales. An interesting case has been figured by Masters, in which
scales of a cone of _Cupressus Lawsoniana_ bear ovules on the upper
surface and stamens on the lower face. One argument that has been
adduced in support of the axillary bud theory is derived from the
Palaeozoic type _Cordaites_, in which each ovule occurs on an axis
borne in the axil of a bract. The whole question is still unsolved,
and perhaps insoluble. It may be that the interpretation of the female
cone of the _Abietineae_ as an inflorescence, which finds favour with
many botanists, cannot be applied to the cones of _Agathis_ and
_Araucaria_. Without expressing any decided opinion as to the
morphology of the double cone-scale of the _Abietineae_, preference
may be felt in favour of regarding the cone-scale of the _Araucarieae_
as a simple carpellary leaf bearing a single ovule. A discussion of
this question may be found in a paper on the _Araucarieae_ by Seward
and Ford, published in the Transactions of the Royal Society of London
(1906). _Cordaites_ is an extinct type which in certain respects
resembles _Ginkgo_, cycads and the _Araucarieae_, but its agreement
with true conifers is probably too remote to justify our attributing
much weight to the bearing of the morphology of its female flowers on
the interpretation of that of the Coniferae. The greater simplicity of
the Eichler theory may prejudice us in its favour; but, on the other
hand, the arguments advanced in favour of the axillary-bud theories
are perhaps not sufficiently cogent to lead us to accept an
explanation based chiefly on the uncertain evidence of monstrosities.

FIG. 15.--Diagrammatic treatment of:

A, Double needle of _Sciadopitys_ (a, a, leaves; b, shoot; Br,
bract).

B, seminiferous scale as leaf of axillary shoot (b, shoot; Sc,
seminiferous scale; Br, bract).

C, seminiferous scale as fused pair of leaves (l^1, l^2, 1^3,
first, second and third leaves; b, shoot; Br, bract),

D, cone-scale of _Araucaria_ (n, nucellus; i, integument; x,
xylem).]

Micro-spores and megaspores.

A pollen-grain when first formed from its mother-cell consists of a
single cell; in this condition it may be carried to the nucellus of
the ovule (e.g. _Taxus_, _Cupressus_, &c.), or more usually (_Pinus_,
_Larix_, &c.) it reaches maturity before the dehiscence of the
microsporangium. The nucleus of the microspore divides and gives rise
to a small cell within the large cell, a second small cell is then
produced; this is the structure of the ripe pollen-grain in some
conifers (_Taxus_, &c.). The large cell grows out as a pollen-tube;
the second of the two small cells (body-cell) wanders into the tube,
followed by the nucleus of the first small cell (stalk-cell). In
_Taxus_ the body-cell eventually divides into two, in which the
products of division are of unequal size, the larger constituting the
male generative cell, which fuses with the nucleus of the egg-cell. In
_Juniperus_ the products of division of the body-cell are equal, and
both function as male generative cells. In the _Abietineae_
cell-formation in the pollen-grain is carried farther. Three small
cells occur inside the cavity of the microspore; two of them collapse
and the third divides into two, forming a stalk-cell and a larger
body-cell. The latter ultimately divides in the apex of the
pollen-tube into two non-motile generative cells. Evidence has lately
been adduced of the existence of numerous nuclei in the pollen-tubes
of the _Araucarieae_, and it seems probable that in this as in several
other respects this family is distinguished from other members of the
Coniferales. The precise method of fertilization in the Scots Pine was
followed by V. H. Blackman, who also succeeded in showing that the
nuclei of the sporophyte generation contain twice as many chromosomes
as the nuclei of the gametophyte. Other observers have in recent years
demonstrated a similar relation in other genera between the number of
chromosomes in the nuclei of the two generations. The ovule is usually
surrounded by one integument, which projects beyond the tip of the
nucellus as a wide-open lobed funnel, which at the time of pollination
folds inwards, and so assists in bringing the pollen-grains on to the
nucellus. In some conifers (e.g. _Taxus_, _Cephalotaxus_, _Dacrydium_,
&c.) the ordinary integument is partially enclosed by an arillus or
second integument. It is held by some botanists (Celakovsky) that the
seminiferous scale of the _Abietineae_ is homologous with the arillus
or second integument of the Taxaceae, but this view is too strained to
gain general acceptance. In _Araucaria_ and _Saxegothaea_ the nucellus
itself projects beyond the open micropyle and receives the
pollen-grains direct. During the growth of the cell which forms the
megaspore the greater part of the nucellus is absorbed, except the
apical portion, which persists as a cone above the megaspore; the
partial disorganization of some of the cells in the centre of the
nucellar cone forms an irregular cavity, which may be compared with
the larger pollen-chamber of _Ginkgo_ and the cycads. In each ovule
one megaspore comes to maturity, but, exceptionally, two may be
present (e.g. _Pinus sylvestris_). It has been shown by Lawson that in
_Sequoia sempervirens_ (_Annals of Botany_, 1904) and by other workers
in the genera that several megaspores may attain a fairly large size
in one prothallus. The megaspore becomes filled with tissue
(prothallus), and from some of the superficial cells archegonia are
produced, usually three to five in number, but in rare cases ten to
twenty or even sixty may be present. In the genus _Sequoia_ there may
be as many as sixty archegonia (Arnoldi and Lawson) in one megaspore;
these occur either separately or in some parts of the prothallus they
may form groups as in the _Cupressineae_; they are scattered through
the prothallus instead of being confined to the apical region as in
the majority of conifers. Similarly in the _Araucarieae_ and in
_Widdringtonia_ the archegonia are numerous and scattered and often
sunk in the prothallus tissue. In _Libocedrus decurrens_
(Cupressineae) Lawson describes the archegonia as varying in number
from 6 to 24 (_Annals of Botany_ xxi., 1907). An archegonium consists
of a large oval egg-cell surmounted by a short neck composed of one or
more tiers of cells, six to eight cells in each tier. Before
fertilization the nucleus of the egg-cell divides and cuts off a
ventral canal-cell; this cell may represent a second egg-cell. The
egg-cells of the archegonia may be in lateral contact (e.g.
_Cupressineae_) or separated from one another by a few cells of the
prothallus, each ovum being immediately surrounded by a layer of cells
distinguished by their granular contents and large nuclei. During the
development of the egg-cell, food material is transferred from these
cells through the pitted wall of the ovum. The tissue at the apex of
the megaspore grows slightly above the level of the archegonia, so
that the latter come to lie in a shallow depression. In the process of
fertilization the two male generative nuclei, accompanied by the
pollen-tube nucleus and that of the stalk-cell, pass through an open
pit at the apex of the pollen-tube into the protoplasm of the ovum.
After fertilization the nucleus of the egg divides, the first stages
of karyokinesis being apparent even before complete fusion of the male
and female nuclei has occurred. The result of this is the production
of four nuclei, which eventually take up a position at the bottom of
the ovum and become separated from one another by vertical cell-walls;
these nuclei divide again, and finally three tiers of cells are
produced, four in each tier. In the _Abietineae_ the cells of the
middle tier elongate and push the lowest tier deeper into the
endosperm; the cells of the bottom tier may remain in lateral contact
and produce together one embryo, or they may separate (_Pinus_,
_Juniperus_, &c.) and form four potential embryos. The ripe albuminous
seed contains a single embryo with two or more cotyledons. The seeds
of many conifers are provided with large thin wings, consisting in
some genera (e.g. _Pinus_) of the upper cell-layers of the
seminiferous scale, which have become detached and, in some cases,
adhere loosely to the seed as a thin membrane; the loose attachment
may be of use to the seeds when they are blown against the branches of
trees, in enabling them to fall away from the wing and drop to the
ground. The seeds of some genera depend on animals for dispersal, the
carpellary scale (_Microcachrys_) or the outer integument being
brightly coloured and attractive. In some _Abietineae_ (e.g. _Pinus_
and _Picea_)--in which the cone-scales persist for some time after the
seeds are ripe--the cones hang down and so facilitate the fall of the
seeds; in _Cedrus_, _Araucaria_ and _Abies_ the scales become detached
and fall with the seeds, leaving the bare vertical axis of the cone on
the tree. In all cases, except some species of _Araucaria_ (sect.
_Colymbea_) the germination is epigean. The seedling plants of some
Conifers (e.g. _Araucaria imbricata_) are characterized by a
carrot-shaped hypocotyl, which doubtless serves as a food-reservoir.

Anatomy.

The roots of many conifers possess a narrow band of primary
xylem-tracheids with a group of narrow spiral protoxylem-elements at
each end (diarch). A striking feature in the roots of several genera,
excluding the _Abietineae_, is the occurrence of thick and somewhat
irregular bands of thickening on the cell-walls of the cortical layer
next to the endodermis. These bands, which may serve to strengthen the
central cylinder, have been compared with the netting surrounding the
delicate wall of an inflated balloon. It is not always easy to
distinguish a root from a stem; in some cases (e.g. _Sequoia_) the
primary tetrarch structure is easily identified in the centre of an
old root, but in other cases the primary elements are very difficult
to recognize. The sudden termination of the secondary tracheids
against the pith-cells may afford evidence of root-structure as
distinct from stem-structure, in which the radial rows of secondary
tracheids pass into the irregularly-arranged primary elements next the
pith. The annual rings in a root are often less clearly marked than in
the stem, and the xylem-elements are frequently larger and thinner.
The primary vascular bundles in a young conifer stem are collateral,
and, like those of a Dicotyledon, they are arranged in a circle round
a central pith and enclosed by a common endodermis. It is in the
nature of the secondary xylem that the Coniferales are most readily
distinguished from the Dicotyledons and Cycadaceae; the wood is
homogeneous in structure, consisting almost entirely of tracheids with
circular or polygonal bordered pits on the radial walls, more
particularly in the late summer wood. In many genera xylem-parenchyma
is present, but never in great abundance. A few Dicotyledons, e.g.
_Drimys_ (Magnoliaceae) closely resemble conifers in the homogeneous
character of the wood, but in most cases the presence of large spring
vessels, wood-fibres and abundant parenchyma affords an obvious
distinguishing feature.