FIG. 4.--_Dioon edule._]

[Illustration: FIG. 5.--_Bowenia spectabilis_: frond.]

[Illustration: FIG. 6.--_Macrozamia heteromera_. A, part of frond; B,

single pinna.]

Stem and leaf.

The stems of cycads are often described as unbranched; it is true that

in comparison with conifers, in which the numerous branches, springing

from the main stem, give a characteristic form to the tree, the

tuberous or columnar stem of the Cycadaceae constitutes a striking

distinguishing feature. Branching, however, occurs not infrequently:

in _Cycas_ the tall stem often produces several candelabra-like arms;

in _Zamia_ the main axis may break up near the base into several

cylindrical branches; in species of _Dioon_ (fig. 4) lateral branches

are occasionally produced. The South African _Encephalartos_

frequently produces several branches. Probably the oldest example of

this genus in cultivation is in the Botanic Garden of Amsterdam, its

age is considered by Professor de Vries to be about two thousand

years: although an accurate determination of age is impossible, there

is no doubt that many cycads grow very slowly and are remarkable for

longevity. The thick armour of petiole-bases enveloping the stem is a

characteristic Cycadean feature; in _Cycas_ the alternation of

scale-leaves and fronds is more clearly shown than in other cycads; in

_Encephalartos_, _Dioon_, &c., the persistent scale-leaf bases are

almost equal in size to those of the foliage-leaves, and there is no

regular alternation of zones such as characterizes some species of

_Cycas_. Another type of stem is illustrated by _Stangeria_ and

_Zamia_, also by a few forms of _Cycas_ (fig. 2), in which the fronds

fall off completely, leaving a comparatively smooth stem. The _Cyas_

type of frond, except as regards the presence of a midrib in each

pinna, characterizes the cycads generally, except _Bowenia_ and

_Stangeria_. In the monotypic genus _Bowenia_ the large fronds, borne

singly on the short and thick stem, are bi-pinnate (fig. 5); the

segments, which are broadly ovate or rhomboidal, have several forked

spreading veins, and resemble the large pinnules of some species of

_Adiantum_. In _Stangeria_, also a genus represented by one species

(_S. paradoxa_ of South Africa), the long and comparatively broad

pinnae, with an entire or irregularly incised margin, are very

fern-like, a circumstance which led Kunze to describe the plant in

1835 as a species of the fern _Lomaria_. In rare cases the pinnae of

cycads are lobed or branched: in _Dioon spinulosum_ (Central America)

the margin of the segments bears numerous spinous processes; in some

species of _Encephalartos_, e.g. _E. horridus_, the lamina is deeply

lobed; and in a species of the Australian genus _Macrozamia_, _M.

heteromera_, the narrow pinnae are dichotomously branched almost to

the base (fig. 6), and resemble the frond of some species of the fern

_Schizaea_, or the fossil genus _Baiera_ (Ginkgoales). An interesting

species of _Cycas_, _C. Micholitzii_, has recently been described by

Sir William Thiselton-Dyer from Annam, where it was collected by one

of Messrs Sanders & Son's collectors, in which the pinnae instead of

being of the usual simple type are dichotomously branched as in

_Macrozamia heteromera_. In _Ceratozamia_ the broad petiole-base is

characterized by the presence of two lateral spinous processes,

suggesting stipular appendages, comparable, on a reduced scale, with

the large stipules of the Marattiaceae among Ferns. The vernation

varies in different genera; in _Cycas_ the rachis is straight and the

pinnae circinately coiled (fig. 3); in _Encephalartos_, _Dioon_, &c.,

both rachis and segments are straight; in _Zamia_ the rachis is bent

or slightly coiled, bearing straight pinnae. The young leaves arise on

the stem-apex as conical protuberances with winged borders on which

the pinnae appear as rounded humps, usually in basipetal order; the

scale-leaves in their young condition resemble fronds, but the lamina

remains undeveloped. A feature of interest in connexion with the

phylogeny of cycads is the presence of long hairs clothing the

scale-leaves, and forming a cap on the summit of the stem-apex or

attached to the bases of petioles; on some fossil cycadean plants

these outgrowths have the form of scales, and are identical in

structure with the ramenta (paleae) of the majority of ferns.

Flower.

The male flowers of cycads are constructed on a uniform plan, and in

all cases consist of an axis bearing crowded, spirally disposed

sporophylls. These are often wedge-shaped and angular; in some cases

they consist of a short, thick stalk, terminating in a peltate

expansion, or prolonged upwards in the form of a triangular lamina.

The sporangia (pollen-sacs), which occur on the under-side of the

stamens, are often arranged in more or less definite groups or sori,

interspersed with hairs (paraphyses); dehiscence takes place along a

line marked out by the occurrence of smaller and thinner-walled cells

bounded by larger and thicker-walled elements, which form a fairly

prominent cap-like "annulus" near the apex of the sporangium, not

unlike the annulus characteristic of the Schizaeaceae among ferns. The

sporangial wall, consisting of several layers of cells, encloses a

cavity containing numerous oval spores (pollen-grains). In structure a

cycadean sporangium recalls those of certain ferns (Marattiaceae,

Osmundaceae and Schizaeaceae), but in the development of the spores

there are certain peculiarities not met with among the Vascular

Cryptogams. With the exception of _Cycas_, the female flowers are also

in the form of cones, bearing numerous carpellary scales. In _Cycas

revoluta_ and _C. circinalis_ each leaf-like carpel may produce

several laterally attached ovules, but in _C. Normanbyana_ the carpel

is shorter and the ovules are reduced to two; this latter type brings

us nearer to the carpels of _Dioon_, in which the flower has the form

of a cone, and the distal end of the carpels is longer and more

leaf-like than in the other genera of the _Zamieae_, which are

characterized by shorter carpels with thick peltate heads bearing two

ovules on the morphologically lower surface. The cones of cycads

attain in some cases (e.g. _Encephalartos_) a considerable size,

reaching a length of more than a foot. Cases have been recorded (by

Thiselton-Dyer in _Encephalartos_ and by Wieland in _Zamia_) in which

the short carpellary cone-scales exhibit a foliaceous form. It is

interesting that no monstrous cycadean cone has been described in

which ovuliferous and staminate appendages are borne on the same axis:

in the Bennettitales (see PALAEOBOTANY: _Mesozoic_) flowers were

produced bearing on the same axis both androecium and gynoecium.

Microspores and megaspores.

The pollen-grains when mature consist of three cells, two small and

one large cell; the latter grows into the pollen-tube, as in the

Coniferales, and from one of the small cells two large ciliated

spermatozoids are eventually produced. A remarkable exception to this

rule has recently been recorded by Caldwell, who found that in

_Microcycas Calocoma_ the body-cells may be eight or even ten in

number and the sperm-cells twice as numerous. One of the most

important discoveries made during the latter part of the 19th century

was that by Ikeno, a Japanese botanist, who first demonstrated the

existence of motile male cells in the genus _Cycas_. Similar

spermatozoids were observed in some species of _Zamia_ by H. J.

Webber, and more recent work enables us to assume that all cycads

produce ciliated male gametes. Before following the growth of the

pollen-grain after pollination, we will briefly describe the structure

of a cycadean ovule. An ovule consists of a conical nucellus

surrounded by a single integument. At an early stage of development a

large cell makes its appearance in the central region of the nucellus;

this increases in size and eventually forms three cells; the lowest of

these grows vigorously and constitutes the megaspore (embryo-sac),

which ultimately absorbs the greater part of the nucellus. The

megaspore-nucleus divides repeatedly, and cells are produced from the

peripheral region inwards, which eventually fill the spore-cavity with

a homogeneous tissue (prothallus); some of the superficial cells at

the micropylar end of the megaspore increase in size and divide by a

tangential wall into two, an upper cell which gives rise to the short

two-celled neck of the archegonium, and a lower cell which develops

into a large egg-cell. Each megaspore may contain 2 to 6 archegonia.

During the growth of the ovum nourishment is supplied from the

contents of the cells immediately surrounding the egg-cell, as in the

development of the ovum of _Pinus_ and other conifers. Meanwhile the

tissue in the apical region of the nucellus has been undergoing

disorganization, which results in the formation of a pollen-chamber

(fig. 7, C) immediately above the megaspore. Pollination in cycads has

always been described as anemophilous, but according to recent

observations by Pearson on South African species it seems probable

that, at least in some cases, the pollen is conveyed to the ovules by

animal agency. The pollen-grains find their way between the

carpophylls, which at the time of pollination are slightly apart owing

to the elongation of the internodes of the flower-axis, and pass into

the pollen-chamber; the large cell of the pollen-grain grows out into

a tube (Pt), which penetrates the nucellar tissue and often branches

repeatedly; the pollen-grain itself, with the prothallus-cells,

projects freely into the pollen-chamber (fig. 7). The nucleus of the

outermost (second) small cell (fig. 7, G) divides, and one of the

daughter-nuclei passes out of the cell, and may enter the lowest

(first) small cell. The outermost cell, by the division of the

remaining nucleus, produces two large spermatozoids (fig. 8, a, a). In

_Microcycas_ 16 sperm-cells are produced. In the course of division

two bodies appear in the cytoplasm, and behave as centrosomes during

the karyokinesis; they gradually become threadlike and coil round each

daughter nucleus. This thread gives rise to a spiral ciliated band

lying in a depression on the body of each spermatozoid; the large

spermatozoids eventually escape from the pollen-tube, and are able to

perform ciliary movements in the watery liquid which occurs between

the thin papery remnant of nucellar tissue and the archegonial necks.

Before fertilization a neck-canal cell is formed by the division of

the ovum-nucleus. After the body of a spermatozoid has coalesced with

the egg-nucleus the latter divides repeatedly and forms a mass of

tissue which grows more vigorously in the lower part of the fertilized

ovum, and extends upwards towards the apex of the ovum as a peripheral

layer of parenchyma surrounding a central space. By further growth

this tissue gives rise to a proembryo, which consists, at the

micropylar end, of a sac; the tissue at the chalazal end grows into a

long and tangled suspensor, terminating in a mass of cells, which is

eventually differentiated into a radicle, plumule and two cotyledons.

In the ripe seed the integument assumes the form of a fleshy envelope,

succeeded internally by a hard woody shell, internal to which is a

thin papery membrane--the apical portion of the nucellus--which is

easily dissected out as a conical cap covering the apex of the

endosperm. A thorough examination of cycadean seeds has recently been

made by Miss Stopes, more particularly with a view to a comparison of

their vascular supply with that in Palaeozoic gymnospermous seeds

(_Flora_, 1904). The first leaves borne on the seedling axis are often

scale-like, and these are followed by two or more larger laminae,

which foreshadow the pinnae of the adult frond.

[Illustration: FIG. 7.--_Zamia._ Part of Ovule in longitudinal

section. (After Webber.)

P, Prothallus.

A, Archegonia.

N, Nucellus.

C, Pollen-chamber.

Pt, Pollen-tube.

Pg, Pollen-grain.

G, Generative cell (second cell of pollen-tube).]

[Illustration: FIG. 8.--_Zamia._ Proximal end of Pollen-tube, a, a,

Spermatozoids from G of fig. 7; Pg, pollen-grain; c, proximal cell

(first cell). (After Webber.)]

Anatomy.

Roots.

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.

[Illustration: FIG. 9.--_Macrozamia._ Diagrammatic transverse section

of part of Stem. (After Worsdell.)

pd, Periderm in leaf-bases.

lt, Leaf-traces in cortex.

ph, Phloem.

x, Xylem.

m, Medullary bundles.

c, Cortical bundles.]

[Illustration: FIG. 10.--_Ginkgo biloba._ Leaves.]

[Illustration: FIG. 11.--_Ginkgo adiantoides._ Fossil (Eocene) leaf

from the Island of Mull.]

[Illustration: FIG. 12.--_Ginkgo biloba._ A, Male flower; B, C, single

stamens; D, female flower.]

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.

[Illustration: FIG. 13.--_Ginkgo._ Apex of Ovule, and Pollen-grain.

(After Hirase.)

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.

[Illustration: FIG. 14.--_Ginkgo._ Abnormal female Flowers. A,

Peduncle; b, scaly bud; B, leaf bearing marginal ovule. (After

Fujii.)]

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.

[Illustration: (C and D after Worsdell.)

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).]

[Illustration: FIG. 16.--Abnormal Cone of _Pinus rigida_. (After

Masters.)]

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.

The abundance of petrified coniferous wood in rocks of various ages

has led many botanists to investigate the structure of modern genera

with a view to determining how far anatomical characters may be used

as evidence of generic distinctions. There are a few well-marked types

of wood which serve as convenient standards of comparison, but these

cannot be used except in a few cases to distinguish individual genera.

The genus _Pinus_ serves as an illustration of wood of a distinct type

characterized by the absence of xylem-parenchyma, except such as is

associated with the numerous resin-canals that occur abundantly in the

wood, cortex and medullary rays; the medullary rays are composed of

parenchyma and of horizontal tracheids with irregular ingrowths from

their walls. In a radial section of a pine stem each ray is seen to

consist in the median part of a few rows of parenchymatous cells with

large oval simple pits in their walls, accompanied above and below by

horizontal tracheids with bordered pits. The pits in the radial walls

of the ordinary xylem-tracheids occur in a single row or in a double

row, of which the pits are not in contact, and those of the two rows

are placed on the same level. The medullary rays usually consist of a

single tier of cells, but in the _Pinus_ type of wood broader

medullary rays also occur and are traversed by horizontal

resin-canals. In the wood of _Cypressus_, _Cedrus_, _Abies_ and

several other genera, parenchymatous cells occur in association with

the xylem-tracheids and take the place of the resin-canals of other

types. In the Araucarian type of wood (_Araucaria_ and _Agathis_) the

bordered pits, which occur in two or three rows on the radial walls of

the tracheids, are in mutual contact and polygonal in shape, the pits

of the different rows are alternate and not on the same level; in this

type of wood the annual rings are often much less distinct than in

_Cupressus_, _Pinus_ and other genera. In _Taxus_, _Torreya_

(California and the Far East) and _Cephalotaxus_ the absence of

resin-canals and the presence of spiral thickening-bands on the

tracheids constitute well-marked characteristics. An examination of

the wood of branches, stems and roots of the same species or

individual usually reveals a fairly wide variation in some of the

characters, such as the abundance and size of the medullary rays, the

size and arrangement of pits, the presence of

wood-parenchyma--characters to which undue importance has often been

attached in systematic anatomical work. The phloem consists of

sieve-tubes, with pitted areas on the lateral as well as on the

inclined terminal walls, phloem-parenchyma and, in some genera,

fibres. In the _Abietineae_ the phloem consists of parenchyma and

sieve-tubes only, but in most other forms tangential rows of fibres

occur in regular alternation with the parenchyma and sieve-tubes. The

characteristic companion-cells of Angiosperms are represented by

phloem-parenchyma cells with albuminous contents; other parenchymatous

elements of the bast contain starch or crystals of calcium oxalate.

When tracheids occur in the medullary rays of the xylem these are

replaced in the phloem-region by irregular parenchymatous cells known

as albuminous cells. Resin-canals, which occur abundantly in the

xylem, phloem or cortex, are not found in the wood of the yew.

_Cephalotaxus (Taxeae)_ is also peculiar in having resin-canals in the

pith (cf. _Ginkgo_). One form of _Cephalotaxus_ is characterized by

the presence of short tracheids in the pith, in shape like ordinary

parenchyma, but in the possession of bordered pits and lignified walls

agreeing with ordinary xylem-tracheids; it is probable that these

short tracheids serve as reservoirs for storing rather than for

conducting water. The vascular bundle entering the stem from a leaf

with a single vein passes by a more or less direct course into the

central cylinder of the stem, and does not assume the girdle-like

form characteristic of the cycadean leaf-trace. In species of which

the leaves have more than one vein (e.g. _Araucaria imbricata_, &c.)

the leaf-trace leaves the stele of the stem as a single bundle which

splits up into several strands in its course through the cortex. In

the wood of some conifers, e.g. _Araucaria_, the leaf-traces persist

for a considerable time, perhaps indefinitely, and may be seen in

tangential sections of the wood of old stems. The leaf-trace in the

Coniferales is simple in its course through the stem, differing in

this respect from the double leaf-trace of _Ginkgo_. A detailed

account of the anatomical characters of conifers has been published by

Professor D. P. Penhallow of Montreal and Dr. Gothan of Berlin which

will be found useful for diagnostic purposes. The characters of leaves

most useful for diagnostic purposes are the position of the stomata,

the presence and arrangement of resin-canals, the structure of the

mesophyll and vascular bundles. The presence of hypodermal fibres is

another feature worthy of note, but the occurrence of these elements

is too closely connected with external conditions to be of much

systematic value. A pine needle grown in continuous light differs from

one grown under ordinary conditions in the absence of hypodermal

fibres, in the absence of the characteristic infoldings of the

mesophyll cell-walls, in the smaller size of the resin-canals, &c. The

endodermis in _Pinus_, _Picea_ and many other genera is usually a

well-defined layer of cells enclosing the vascular bundles, and

separated from them by a tissue consisting in part of ordinary

parenchyma and to some extent of isodiametric tracheids; but this

tissue, usually spoken of as the pericycle, is in direct continuity

with other stem-tissues as well as the pericycle. The occurrence of

short tracheids in close proximity to the veins is a characteristic of

coniferous leaves; these elements assume two distinct forms--(1) the

short isodiametric tracheids (transfusion-tracheids) closely

associated with the veins; (2) longer tracheids extending across the

mesophyll at right angles to the veins, and no doubt functioning as

representatives of lateral veins. It has been suggested that

transfusion-tracheids represent, in part at least, the centripetal

xylem, which forms a distinctive feature of cycadean leaf-bundles;

these short tracheids form conspicuous groups laterally attached to

the veins in _Cunninghamia_, abundantly represented in a similar

position in the leaves of _Sequoia_, and scattered through the

so-called pericycle in _Pinus_, _Picea_, &c. It is of interest to note

the occurrence of precisely similar elements in the mesophyll of

_Lepidodendron_ leaves. An anatomical peculiarity in the veins of

_Pinus_ and several other genera is the continuity of the medullary

rays, which extend as continuous plates from one end of the leaf to

the other. The mesophyll of _Pinus_ and _Cedrus_ is characterized by

its homogeneous character and by the presence of infoldings of the

cell-walls. In many leaves, e.g. _Abies_, _Tsuga_, _Larix_, &c., the

mesophyll is heterogeneous, consisting of palisade and spongy

parenchyma. In the leaves of _Araucaria imbricata_, in which

palisade-tissue occurs in both the upper and lower part of the

mesophyll, the resin-canals are placed between the veins; in some

species of _Podocarpus_ (sect. _Nageia_) a canal occurs below each

vein; in _Tsuga_, _Torreya_, _Cephalotaxus_, _Sequoia_, &c., a single

canal occurs below the midrib; in _Larix_, _Abies_, &c., two canals

run through the leaf parallel to the margins. The stomata are

frequently arranged in rows, their position being marked by two white

bands of wax on the leaf-surface.

Distribution.

The chief home of the Coniferales is in the northern hemisphere, where

certain species occasionally extend into the Arctic circle and

penetrate beyond the northern limit of dicotyledonous trees. Wide

areas are often exclusively occupied by conifers, which give the

landscape a sombre aspect, suggesting a comparison with the forest

vegetation of the Coal period. South of the tree-limit a belt of

conifers stretches across north Europe, Siberia and Canada. In

northern Europe this belt is characterized by such species as _Picea

excelsa_ (spruce), which extends south to the mountains of the

Mediterranean region; _Pinus sylvestris_ (Scottish fir), reaching from

the far north to western Spain, Persia and Asia Minor; _Juniperus

communis_, &c. In north Siberia _Pinus Cembra_ (Cembra or Arolla Pine)

has a wide range; also _Abies sibirica_ (Siberian silver fir), _Larix

sibirica_ and _Juniperus Sabina_ (savin). In the North American area

_Picea alba_, _P. nigra_, _Larix americana_, _Abies balsamea_ (balsam

fir), _Tsuga canadensis_ (hemlock spruce), _Pinus Strobus_ (Weymouth

pine), _Thuja occidentalis_ (white cedar), _Taxus canadensis_ are

characteristic species. In the Mediterranean region occur _Cupressus

sempervirens_, _Pinus Pinea_ (stone pine), species of juniper, _Cedrus

atlantica_, _C. Libani_, _Callitris quadrivalvis_, _Pinus montana_,

&c. Several conifers of economic importance are abundant on the

Atlantic side of North America--_Juniperus virginiana_ (red cedar,

used in the manufacture of lead pencils, and extending as far south as

Florida), _Taxodium distichum_ (swamp cypress), _Pinus rigida_ (pitch

pine), _P. mitis_ (yellow pine), _P. taeda_, _P. palustris_, &c. On

the west side of the American continent conifers play a still more

striking role; among them are _Chamaecyparis nutkaensis_, _Picea

sitchensis_, _Libocedrus decurrens_, _Pseudotsuga Douglasii_ (Douglas

fir), _Sequoia sempervirens_, _S. gigantea_ (the only two surviving

species of this generic type are now confined to a few localities in

California, but were formerly widely spread in Europe and elsewhere),

_Pinus Coulteri_, _P. Lambertiana_, &c. Farther south, a few

representatives of such genera as _Abies_, _Cupressus_, _Pinus_ and

juniper are found in the Mexican Highlands, tropical America and the

West Indies. In the far East conifers are richly represented; among

them occur _Pinus densiflora_, _Cryptomeria japonica_, _Cephalotaxus_,

species of _Abies_, _Larix_, _Thujopsis_, _Sciadopitys verticillata_,

_Pseudolarix Kaempferi_, &c. In the Himalaya occur _Cedrus deodara_,

_Taxus_, species of _Cupressus_, _Pinus excelsa_, _Abies Webbiana_,

&c. The continent of Africa is singularly poor in conifers. _Cedrus

atlantica_, a variety of _Abies Pinsapo_, _Juniperus thurifera_,

_Callitris quadrivalvis_, occur in the north-west region, which may be

regarded as the southern limit of the Mediterranean region. The

greater part of Africa north of the equator is without any

representatives of the conifers; _Juniperus procera_ flourishes in

Somaliland and on the mountains of Abyssinia; a species of

_Podocarpus_ occurs on the Cameroon mountains, and _P. milanjiana_ is

widely distributed in east tropical Africa. _Widdringtonia Whytei_, a

species closely allied to _W. juniperoides_ of the Cedarberg mountains

of Cape Colony, is recorded from Nyassaland and from N.E. Rhodesia;

while a third species, _W. cupressoides_, occurs in Cape Colony.

_Podocarpus elongata_ and _P. Thunbergii_ (yellow wood) form the

principal timber trees in the belt of forest which stretches from the

coast mountains of Cape Colony to the north-east of the Transvaal.

_Libocedrus tetragona_, _Fitzroya patagonica_, _Araucaria

brasiliensis_, _A. imbricata_, _Saxegothaea_ and others are met with

in the Andes and other regions in South America. _Athrotaxis_ and

_Microcachrys_ are characteristic Australian types. _Phyllocladus_

occurs also in New Zealand, and species of _Dacrydium_, _Araucaria_,

_Agathis_ and _Podocarpus_ are represented in Australia, New Zealand

and the Malay regions.

GNETALES.--These are trees or shrubs with simple leaves. The flowers

are dioecious, rarely monoecious, provided with one or two perianths.

The wood is characterized by the presence of vessels in addition to

tracheids. There are no resin-canals. The three existing genera,

usually spoken of as members of the Gnetales, differ from one another

more than is consistent with their inclusion in a single family; we

may therefore better express their diverse characters by regarding

them as types of three separate families--(1) _Ephedroideae_, genus

_Ephedra_; (2) _Welwitschioideae_, genus _Welwitschia_; (3)

_Gnetoideae_, genus _Gnetum_. Our knowledge of the Gnetales leaves

much to be desired, but such facts as we possess would seem to

indicate that this group is of special importance as foreshadowing,

more than any other Gymnosperms, the Angiospermous type. In the more

heterogeneous structure of the wood and in the possession of true

vessels the Gnetales agree closely with the higher flowering plants.

It is of interest to note that the leaves of _Gnetum_, while typically

Dicotyledonous in appearance, possess a Gymnospermous character in the

continuous and plate-like medullary rays of their vascular bundles.

The presence of a perianth is a feature suggestive of an approach to

the floral structure of Angiosperms; the prolongation of the

integument furnishes the flowers with a substitute for a stigma and

style. The genus _Ephedra_, with its prothallus and archegonia, which

are similar to those of other Gymnosperms, may be safely regarded as

the most primitive of the Gnetales. In _Welwitschia_ also the

megaspore is filled with prothallus-tissue, but single egg-cells take

the place of archegonia. In certain species of _Gnetum_ described by

Karsten the megaspore contains a peripheral layer of protoplasm, in

which scattered nuclei represent the female reproductive cells; in

_Gnetum Gnemon_ a similar state of things exists in the upper half of

the megaspore, while the lower half agrees with the megaspore of

_Welwitschia_ in being full of prothallus-tissue, which serves merely

as a reservoir of food. Lotsy has described the occurrence of special

cells at the apex of the prothallus of _Gnetum Gnemon_, which he

regards as imperfect archegonia (fig. 17, C, a); he suggests they may

represent vestigial structures pointing back to some ancestral form

beyond the limits of the present group. The Gnetales probably had a

separate origin from the other Gymnosperms; they carry us nearer to

the Angiosperms, but we have as yet no satisfactory evidence that they

represent a stage in the direct line of Angiospermic evolution. It is

not improbable that the three genera of this ancient phylum survive as

types of a blindly-ending branch of the Gymnosperms; but be that as it

may, it is in the Gnetales more than in any other Gymnosperms that we

find features which help us to obtain a dim prospect of the lines

along which the Angiosperms may have been evolved.