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 (Celakovský) 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.
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 Anatomy. 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
