SUB-KINGDOM VI.
SPERMAPHYTES: PHÆNOGAMS.
The last and highest great division of the vegetable kingdom has been
named _Spermaphyta_, "seed plants," from the fact that the structures
known as seeds are peculiar to them. They are also commonly called
flowering plants, though this name might be also appropriately given
to certain of the higher pteridophytes.
In the seed plants the macrosporangia remain attached to the parent
plant, in nearly all cases, until the archegonia are fertilized and
the embryo plant formed. The outer walls of the sporangium now become
hard, and the whole falls off as a seed.
In the higher spermaphytes the spore-bearing leaves (sporophylls)
become much modified, and receive special names, those bearing the
microspores being commonly known as stamens; those bearing the
macrospores, carpels or carpophylls. The macrosporangia are also
ordinarily known as "ovules," a name given before it was known that
these were the same as the macrosporangia of the higher pteridophytes.
In addition to the spore-bearing leaves, those surrounding them may be
much changed in form and brilliantly colored, forming, with the
enclosed sporophylls, the "flower" of the higher spermaphytes.
As might be expected, the tissues of the higher spermaphytes are the
most highly developed of all plants, though some of them are very
simple. The plants vary extremely in size, the smallest being little
floating plants, less than a millimetre in diameter, while others are
gigantic trees, a hundred metres and more in height.
There are two classes of the spermaphytes: I., the Gymnosperms, or
naked-seeded ones, in which the ovules (macrosporangia) are borne upon
open carpophylls; and II., Angiosperms, covered-seeded plants, in
which the carpophylls form a closed cavity (ovary) containing the
ovules.
CLASS I.--GYMNOSPERMS (_Gymnospermæ_).
The most familiar of these plants are the common evergreen trees
(conifers), pines, spruces, cedars, etc. A careful study of one of
these will give a good idea of the most important characteristics of
the class, and one of the best for this purpose is the Scotch pine
(_Pinus sylvestris_), which, though a native of Europe, is not
infrequently met with in cultivation in America. If this species
cannot be had by the student, other pines, or indeed almost any other
conifer, will answer. The Scotch pine is a tree of moderate size,
symmetrical in growth when young, with a central main shaft, and
circles of branches at regular intervals; but as it grows older its
growth becomes irregular, and the crown is divided into several main
branches.[10] The trunk and branches are covered with a rough, scaly
bark of a reddish brown color, where it is exposed by the scaling off
of the outer layers. Covering the younger branches, but becoming
thinner on the older ones, are numerous needle-shaped leaves. These
are in pairs, and the base of each pair is surrounded by several dry,
blackish scales. Each pair of leaves is really attached to a very
short side branch, but this is so short as to make the leaves appear
to grow directly from the main branch. Each leaf is about ten
centimetres in length and two millimetres broad. Where the leaves are
in contact they are flattened, but the outer side is rounded, so that
a cross-section is nearly semicircular in outline. With a lens it is
seen that there are five longitudinal lines upon the surface of the
leaf, and careful examination shows rows of small dots corresponding
to these. These dots are the breathing pores. If a cross-section is
even slightly magnified it shows three distinct parts,--a whitish
outer border, a bright green zone, and a central oval, colorless area,
in which, with a little care, may be seen the sections of two
fibro-vascular bundles. In the green zone are sometimes to be seen
colorless spots, sections of resin ducts, containing the resin so
characteristic of the tissues of the conifers.
[10] In most conifers the symmetrical form of the young tree is
maintained as long as the tree lives.
The general structure of the stem may be understood by making a series
of cross-sections through branches of different ages. In all, three
regions are distinguishable; viz., an outer region (bark or cortex)
(Fig. 76, _A_, _c_), composed in part of green cells, and, if the
section has been made with a sharp knife, showing a circle of little
openings, from each of which oozes a clear drop of resin. These are
large resin ducts (_r_). The centre is occupied by a soft white tissue
(pith), and the space between the pith and bark is filled by a mass of
woody tissue. Traversing the wood are numerous radiating lines, some
of which run from the bark to the pith, others only part way. These
are called the medullary rays. While in sections from branches of any
age these three regions are recognizable, their relative size varies
extremely. In a section of a twig of the present year the bark and
pith make up a considerable part of the section; but as older branches
are examined, we find a rapid increase in the quantity of wood, while
the thickness of the bark increases but slowly, and the pith scarcely
at all. In the wood, too, each year's growth is marked by a distinct
ring (_A_ i, ii). As the branches grow in diameter the outer bark
becomes split and irregular, and portions die, becoming brown and
hard.
The tree has a very perfect root system, but different from that of
any pteridophytes. The first root of the embryo persists as the main
or "tap" root of the full-grown tree, and from it branch off the
secondary roots, which in turn give rise to others.
The sporangia are borne on special scale-like leaves, and arranged
very much as in certain pteridophytes, notably the club mosses; but
instead of large and small spores being produced near together, the
two kinds are borne on special branches, or even on distinct trees
(_e.g._ red cedar). In the Scotch pine the microspores are ripe about
the end of May. The leaves bearing them are aggregated in small cones
("flowers"), crowded about the base of a growing shoot terminating the
branches (Fig. 77, _A_ ♂). The individual leaves (sporophylls) are
nearly triangular in shape, and attached by the smaller end. On the
lower side of each are borne two sporangia (pollen sacs) (_C_, _sp._),
opening by a longitudinal slit, and filled with innumerable yellow
microspores (pollen spores), which fall out as a shower of yellow dust
if the branch is shaken.
The macrosporangia (ovules) are borne on similar leaves, known as
carpels, and, like the pollen sacs, borne in pairs, but on the upper
side of the sporophyll instead of the lower. The female flowers appear
when the pollen is ripe. The leaves of which they are composed are
thicker than those of the male flowers, and of a pinkish color. At the
base on the upper side are borne the two ovules (macrosporangia)
(Fig. 77, _E_, _o_), and running through the centre is a ridge that
ends in a little spine or point.
The ovule-bearing leaf has on the back a scale with fringed edge (_F_,
_sc._), quite conspicuous when the flower is young, but scarcely to be
detected in the older cone. From the female flower is developed the
cone (Fig. 75, _A_), but the process is a slow one, occupying two
years. Shortly after the pollen is shed, the female flowers, which are
at first upright, bend downward, and assume a brownish color, growing
considerably in size for a short time, and then ceasing to grow for
several months.
[Illustration: FIG. 75.--Scotch pine (_Pinus sylvestris_). _A_, a ripe
cone, × ½. _B_, a year-old cone, × 1. _C_, longitudinal section of
_B_. _D_, a single scale of _B_, showing the sporangia (ovules) (_o_),
× 2. _E_, a scale from a ripe cone, with the seeds (_s_), × ½. _F_,
longitudinal section of a ripe seed, × 3. _em._ the embryo. _G_, a
germinating seed, × 2. _r_, the primary root. _H_, longitudinal
section through _G_, showing the first leaves of the young plant still
surrounded by the endosperm, × 4. _I_, an older plant with the leaves
(_l_) withdrawing from the seed coats, × 4. _J_, upper part of a young
plant, showing the circle of primary leaves (cotyledons), × 1. _K_,
section of the same, × 2. _b_, the terminal bud. _L_, cross-section of
the stem of the young plant, × 25. _fb._ a fibro-vascular bundle. _M_,
cross-section of the root, × 25. _x_, wood. _ph._ bast, of the
fibro-vascular bundle.]
In Figure 75, _B_, is shown such a flower as it appears in the winter
and early spring following. The leaves are thick and fleshy, closely
pressed together, as is seen by dividing the flower lengthwise, and
each leaf ends in a long point (_D_). The ovules are still very small.
As the growth of the tree is resumed in the spring, the flower (cone)
increases rapidly in size and becomes decidedly green in color, the
ovules increasing also very much in size. If a scale from such a cone
is examined about the first of June, the ovules will probably be
nearly full-grown, oval, whitish bodies two to three millimetres in
length. A careful longitudinal section of the scale through the ovule
will show the general structure. Such a section is shown in Figure 77,
_G_. Comparing this with the sporangia of the pteridophytes, the first
difference that strikes us is the presence of an outer coat or
integument (_in._), which is absent in the latter. The single
macrospore (_sp._) is very large and does not lie free in the cavity
of the sporangium, but is in close contact with its wall. It is filled
with a colorless tissue, the prothallium, and if mature, with care it
is possible to see, even with a hand lens, two or more denser oval
bodies (_ar._), the egg cells of the archegonia, which here are very
large. The integument is not entirely closed at the top, but leaves a
little opening through which the pollen spores entered when the flower
was first formed.
After the archegonia are fertilized the outer parts of the ovule
become hard and brown, and serve to protect the embryo plant, which
reaches a considerable size before the sporangium falls off. As the
walls of the ovule harden, the carpel or leaf bearing it undergoes a
similar change, becoming extremely hard and woody, and as each one
ends in a sharp spine, and they are tightly packed together, it is
almost impossible to separate them. The ripe cone (Fig. 75, _A_)
remains closed during the winter, but in the spring, about the time
the flowers are mature, the scales open spontaneously and discharge
the ripened ovules, now called seeds. Each seed (_E_, _s_) is
surrounded by a membranous envelope derived from the scale to which it
is attached, which becomes easily separated from the seed. The opening
of the cones is caused by drying, and if a number of ripe cones are
gathered in the winter or early spring, and allowed to dry in an
ordinary room, they will in a day or two open, often with a sharp,
crackling sound, and scatter the ripe seeds.
A section of a ripe seed (_F_) shows the embryo (_em._) surrounded by
a dense, white, starch-bearing tissue derived from the prothallium
cells, and called the "endosperm." This fills up the whole seed which
is surrounded by the hardened shell derived from the integument and
wall of the ovule. The embryo is elongated with a circle of small
leaves at the end away from the opening of the ovule toward which is
directed the root of the embryo.
The seed may remain unchanged for months, or even years, without
losing its vitality, but if the proper conditions are provided, the
embryo will develop into a new plant. To follow the further growth of
the embryo, the ripe seeds should be planted in good soil and kept
moderately warm and moist. At the end of a week or two some of the
seeds will probably have sprouted. The seed absorbs water, and the
protoplasm of the embryo renews its activity, beginning to feed upon
the nourishing substances in the cells of the endosperm. The embryo
rapidly increases in length, and the root pushes out of the seed
growing rapidly downward and fastening itself in the soil (_G_, _r_).
Cutting the seed lengthwise we find that the leaves have increased
much in length and become green (one of the few cases where
chlorophyll is formed in the absence of light). As these leaves
(called "cotyledons" or seed leaves) increase in length, they
gradually withdraw from the seed whose contents they have exhausted,
and the young plant enters upon an independent existence.
The young plant has a circle of leaves, about six in number,
surrounding a bud which is the growing point of the stem, and in many
conifers persists as long as the stem grows (Fig. 75, _K_, _b_). A
cross-section of the young stem shows about six separate
fibro-vascular bundles arranged in a circle (_S_, _fb._). The root
shows a central fibro-vascular cylinder surrounded by a dark-colored
ground tissue. Growing from its surface are numerous root hairs
(Fig. 75, _M_).
For examining the microscopic structure of the pine, fresh material
is for most purposes to be preferred, but alcoholic material will
answer, and as the alcohol hardens the resin, it is for that reason
preferable.
Cross-sections of the leaf, when sufficiently magnified, show that
the outer colorless border of the section is composed of two parts:
the epidermis of a single row of regular cells with very thick outer
walls, and irregular groups of cells lying below them. These latter
have thick walls appearing silvery and clearer than the epidermal
cells. They vary a good deal, in some leaves being reduced to a
single row, in others forming very conspicuous groups of some size.
The green tissue of the leaf is much more compact than in the fern
we examined, and the cells are more nearly round and the
intercellular spaces smaller. The chloroplasts are numerous and
nearly round in shape.
Scattered through the green tissue are several resin passages (_r_),
each surrounded by a circle of colorless, thick-walled cells, like
those under the epidermis. At intervals in the latter are
openings--breathing pores--(Fig. 76, _J_), below each of which is an
intercellular space (_i_). They are in structure like those of the
ferns, but the walls of the guard cells are much thickened like the
other epidermal cells.
Each leaf is traversed by two fibro-vascular bundles of entirely
different structure from those of the ferns. Each is divided into
two nearly equal parts, the wood (_x_) lying toward the inner, flat
side of the leaf, the bast (_T_) toward the outer, convex side. This
type of bundle, called "collateral," is the common form found in the
stems and leaves of seed plants. The cells of the wood or xylem are
rather larger than those of the bast or phloem, and have thicker
walls than any of the phloem cells, except the outermost ones which
are thick-walled fibres like those under the epidermis. Lying
between the bundles are comparatively large colorless cells, and
surrounding the whole central area is a single line of cells that
separates it sharply from the surrounding green tissue.
In longitudinal sections, the cells, except of the mesophyll (green
tissue) are much elongated. The mesophyll cells, however, are short
and the intercellular spaces much more evident than in the
cross-section. The colorless cells have frequently rounded
depressions or pits upon their walls, and in the fibro-vascular
bundle the difference between the two portions becomes more obvious.
The wood is distinguished by the presence of vessels with close,
spiral or ring-shaped thickenings, while in the phloem are found
sieve tubes, not unlike those in the ferns.
The fibro-vascular bundles of the stem of the seedling plant show a
structure quite similar to that of the leaf, but very soon a
difference is manifested. Between the two parts of the bundle the
cells continue to divide and add constantly to the size of the
bundle, and at the same time the bundles become connected by a line
of similar growing cells, so that very early we find a ring of
growing cells extending completely around the stem. As the cells in
this ring increase in number, owing to their rapid division, those
on the borders of the ring lose the power of dividing, and gradually
assume the character of the cells on which they border (Fig. 76,
_B_, _cam._). The growth on the inside of the ring is more rapid
than on the outer border, and the ring continues comparatively near
the surface of the stem (Fig. 76, _A_, _cam._). The spaces between
the bundles do not increase materially in breadth, and as the
bundles increase in size become in comparison very small, appearing
in older stems as mere lines between the solid masses of wood that
make up the inner portion of the bundles. These are the primary
medullary rays, and connect the pith in the centre of the stem with
the bark. Later, similar plates of cells are formed, often only a
single cell thick, and appearing when seen in cross-section as a
single row of elongated cells (_C_, _m_).
As the stem increases in diameter the bundles become broader and
broader toward the outside, and taper to a point toward the centre,
appearing wedge-shaped, the inner ends projecting into the pith. The
outer limits of the bundles are not nearly so distinct, and it is
not easy to tell when the phloem of the bundles ends and the ground
tissue of the bark begins.
A careful examination of a cross-section of the bark shows first, if
taken from a branch not more than two or three years old, the
epidermis composed of cells not unlike those of the leaf, but whose
walls are usually browner. Underneath are cells with brownish walls,
and often more or less dry and dead. These cells give the brown
color to the bark, and later both epidermis and outer ground tissue
become entirely dead and disappear. The bulk of the ground tissue is
made up of rather large, loose cells, the outer ones containing a
good deal of chlorophyll. Here and there are large resin ducts
(Fig. 76, _H_), appearing in cross-section as oval openings
surrounded by several concentric rows of cells, the innermost
smaller and with denser contents. These secrete the resin that fills
the duct and oozes out when the stem is cut. All of the cells of the
bark contain more or less starch.
The phloem, when strongly magnified, is seen to be made up of cells
arranged in nearly regular radiating rows. Their walls are not very
thick and the cells are usually somewhat flattened in a radial
direction.
Some of the cells are larger than the others, and these are found to
be, when examined in longitudinal section, sieve tubes (Fig. 76,
_E_) with numerous lateral sieve plates quite similar to those found
in the stems of ferns.
[Illustration: FIG. 76.--Scotch pine. _A_, cross-section of a
two-year-old branch, × 3. _p_, pith. _c_, bark. The radiating lines
are medullary rays. _r_, resin ducts. _B_, part of the same, × 150.
_cam._ cambium cells. _x_, tracheids. _C_, cross-section of a
two-year-old branch at the point where the two growth rings join: _I_,
the cells of the first year's growth; _II_, those of the second year.
_m_, a medullary ray, × 150. _D_, longitudinal section of a branch,
showing the form of the tracheids and the bordered pits upon their
walls. _m_, medullary ray, × 150. _E_, part of a sieve tube, × 300.
_F_, cross-section of a tracheid passing through two of the pits in
the wall (_p_), × 300. _G_, longitudinal section of a branch, at right
angles to the medullary rays (_m_). At _y_, the section has passed
through the wall of a tracheid, bearing a row of pits, × 150. _H_,
cross-section of a resin duct, × 150. _I_, cross-section of a leaf,
× 20. _fb._ fibro-vascular bundle. _r_, resin duct. _J_, section of a
breathing pore, × 150. _i_, the air space below it.]
The growing tissue (cambium), separating the phloem from the wood,
is made up of cells quite like those of the phloem, into which they
insensibly merge, except that their walls are much thinner, as is
always the case with rapidly growing cells. These cells (_B_,
_cam._) are arranged in radial rows and divide, mainly by walls, at
right angles to the radii of the stem. If we examine the inner side
of the ring, the change the cells undergo is more marked. They
become of nearly equal diameter in all directions, and the walls
become woody, showing at the same time distinct stratification (_B_,
_x_).
On examining the xylem, where two growth rings are in contact, the
reason of the sharply marked line seen when the stem is examined
with the naked eye is obvious. On the inner side of this line (_I_),
the wood cells are comparatively small and much flattened, while the
walls are quite as heavy as those of the much larger cells (_II_)
lying on the outer side of the line. The small cells show the point
where growth ceased at the end of the season, the cells becoming
smaller as growth was feebler. The following year when growth
commenced again, the first wood cells formed by the cambium were
much larger, as growth is most vigorous at this time, and the wood
formed of these larger cells is softer and lighter colored than that
formed of the smaller cells of the autumn growth.
The wood is mainly composed of tracheids, there being no vessels
formed except the first year. These tracheids are characterized by
the presence of peculiar pits upon their walls, best seen when thin
longitudinal sections are made in a radial direction. These pits
(Fig. 76, _D_, _p_) appear in this view as double circles, but if
cut across, as often happens in a cross-section of the stem, or in a
longitudinal section at right angles to the radius (tangential),
they are seen to be in shape something like an inverted saucer with
a hole through the bottom. They are formed in pairs, one on each
side of the wall of adjacent tracheids, and are separated by a very
delicate membrane (_F_, _p_, _G_, _y_). These "bordered" pits are
very characteristic of the wood of all conifers.
The structure of the root is best studied in the seedling plant, or
in a rootlet of an older one. The general plan of the root is much
like that of the pteridophytes. The fibro-vascular bundle (Fig. 75,
_M_, _fb._) is of the so-called radial type, there being three xylem
masses (_x_) alternating with as many phloem masses (_ph._) in the
root of the seedling. This regularity becomes destroyed as the root
grows older by the formation of a cambium ring, something like that
in the stem.
The development of the sporangia is on the whole much like that of
the club mosses, and will not be examined here in detail. The
microspores (pollen spores) are formed in groups of four in
precisely the same way as the spores of the bryophytes and
pteridophytes, and by collecting the male flowers as they begin to
appear in the spring, and crushing the sporangia in water, the
process of division may be seen. For more careful examination they
may be crushed in a mixture of water and acetic acid, to which is
added a little gentian violet. This mixture fixes and stains the
nuclei of the spores, and very instructive preparations may thus be
made.[11]
[11] See the last chapter for details.
[Illustration: FIG. 77.--Scotch pine (except _E_ and _F_). _A_, end of
a branch bearing a cluster of male flowers (♂), × ½. _B_, a similar
branch, with two young female flowers (♀), natural size. _C_, a scale
from a male flower, showing the two sporangia (_sp._); × 5. _D_, a
single ripe pollen spore (microspore), showing the vegetative cell
(_x_), × 150. _E_, a similar scale, from a female flower of the
Austrian pine, seen from within, × 4. _o_, the sporangium (ovule).
_F_, the same, seen from the back, showing the scale (_sc._) attached
to the back. _G_, longitudinal section through a full-grown ovule of
the Scotch pine. _p_, a pollen spore sending down its tube to the
archegonia (_ar._). _sp._ the prothallium (endosperm), filling up the
embryo sac, × 10. _H_, the neck of the archegonium, × 150.]
The ripe pollen spores (Fig. 77, _D_) are oval cells provided with
a double wall, the outer one giving rise to two peculiar
bladder-like appendages (_z_). Like the microspores of the smaller
club mosses, a small cell is cut off from the body of the spore
(_x_). These pollen spores are carried by the wind to the ovules,
where they germinate.
The wall of the ripe sporangium or pollen sac is composed of a
single layer of cells in most places, and these cells are provided
with thickened ridges which have to do with opening the pollen sac.
We have already examined in some detail the structure of the
macrosporangium or ovule. In the full-grown ovule the macrospore,
which in the seed plants is generally known as the "embryo sac," is
completely filled with the prothallium or "endosperm." In the upper
part of the prothallium several large archegonia are formed in much
the same way as in the pteridophytes. The egg cell is very large,
and appears of a yellowish color, and filled with large drops that
give it a peculiar aspect. There is a large nucleus, but it is not
always readily distinguished from the other contents of the egg
cell. The neck of the archegonium is quite long, but does not
project above the surface of the prothallium (Fig. 77, _H_).
The pollen spores are produced in great numbers, and many of them fall
upon the female flowers, which when ready for pollination have the
scales somewhat separated. The pollen spores now sift down to the base
of the scales, and finally reach the opening of the ovule, where they
germinate. No spermatozoids are produced, the seed plants differing in
this respect from all pteridophytes. The pollen spore bursts its
outer coat, and sends out a tube which penetrates for some distance
into the tissue of the ovule, acting very much as a parasitic fungus
would do, and growing at the expense of the tissue through which it
grows. After a time growth ceases, and is not resumed until the
development of the female prothallium and archegonia is nearly
complete, which does not occur until more than a year from the time
the pollen spore first reaches the ovule. Finally the pollen tube
penetrates down to and through the open neck of the archegonium, until
it comes in contact with the egg cell. These stages can only be seen
by careful sections through a number of ripe ovules, but the track of
the pollen tube is usually easy to follow, as the cells along it are
often brown and apparently dead (Fig. 77, _G_).
CLASSIFICATION OF THE GYMNOSPERMS.
There are three classes of the gymnosperms: I., cycads (_Cycadeæ_);
II., conifers (_Coniferæ_); III., joint firs (_Gnetaceæ_). All of the
gymnosperms of the northern United States belong to the second order,
but representatives of the others are found in the southern and
southwestern states.
The cycads are palm-like forms having a single trunk crowned by a
circle of compound leaves. Several species are grown for ornament in
conservatories, and a few species occur native in Florida, but
otherwise do not occur within our limits.
[Illustration: FIG. 78.--Illustrations of gymnosperms. _A_, fruiting
leaf of a cycad (_Cycas_), with macrosporangia (ovules) (_ov._), × ¼.
_B_, leaf of _Gingko_, × ½. _C_, branch of hemlock (_Tsuga_), with a
ripe cone, × 1. _D_, red cedar (_Juniperus_), × 1. _E_, _Arbor-vitæ_
(_Thuja_), × 1.]
The spore-bearing leaves usually form cones, recalling somewhat in
structure those of the horse-tails, but one of the commonest
cultivated species (_Cycas revoluta_) bears the ovules, which are very
large, upon leaves that are in shape much like the ordinary ones
(Fig. 78, _A_).
Of the conifers, there are numerous familiar forms, including all our
common evergreen trees. There are two sub-orders,--the true conifers
and the yews. In the latter there is no true cone, but the ovules are
borne singly at the end of a branch, and the seed in the yew (_Taxus_)
is surrounded by a bright red, fleshy integument. One species of yew,
a low, straggling shrub, occurs sparingly in the northern states, and
is the only representative of the group at the north. The European yew
and the curious Japanese _Gingko_ (Fig. 78, _B_) are sometimes met
with in cultivation.
Of the true conifers, there are a number of families, based on
peculiarities in the leaves and cones. Some have needle-shaped leaves
and dry cones like the firs, spruces, hemlock (Fig. 78, _C_). Others
have flattened, scale-like leaves, and more or less fleshy cones, like
the red cedar (Fig. 78, _D_) and _Arbor-vitæ_ (_E_).
A few of the conifers, such as the tamarack or larch (_Larix_) and
cypress (_Taxodium_), lose their leaves in the autumn, and are not,
therefore, properly "evergreen."
The conifers include some of the most valuable as well as the largest
of trees. Their timber, especially that of some of the pines, is
particularly valuable, and the resin of some of them is also of much
commercial importance. Here belong the giant red-woods (_Sequoia_) of
California, the largest of all American trees.
The joint firs are comparatively small plants, rarely if ever reaching
the dimensions of trees. They are found in various parts of the world,
but are few in number, and not at all likely to be met with by the
ordinary student. Their flowers are rather more highly differentiated
than those of the other gymnosperms, and are said to show some
approach in structure to those of the angiosperms.