GYMNOSPERMS.
The white pine.
=607. General aspect of the white pine.=—The white pine (Pinus
strobus) is found in the Eastern United States. In favorable situations
in the forest it reaches a height of about 50 meters (about 160 feet),
and the trunk a diameter of over 1 meter. In well-formed trees the
trunk is straight and towering; the branches where the sunlight has
access and the trees are not crowded, or are young, reaching out in
graceful arms, form a pyramidal outline to the tree. In old and dense
forests the lower branches, because of lack of sunlight, have died
away, leaving tall, bare trunks for a considerable height.
=608. The long shoots of the pine.=—The branches are of two
kinds. Those which we readily recognize are the long branches, so
called because the growth in length each year is considerable. The
terminal bud of the long branches, as well as of the main stem,
continues each year the growth of the main branch or shoot; while the
lateral long branches arise each year from buds which are crowded close
together around the base of the terminal bud. The lateral long branches
of each year thus appear to be in a whorl. The distance between each
false whorl of branches, then, represents one year’s growth in length
of the main stem or long branch.
=609. The dwarf shoots of the pine.=—The dwarf branches are all
lateral on the long branches, or shoots. They are scattered over the
year’s growth, and each bears a cluster of five long, needle-shaped,
green leaves, which remain on the tree for several years. At the base
of the green leaves are a number of chaff-like scales, the previous bud
scales. While the dwarf branches thus bear green leaves, and scales,
the long branches bear only thin scale-like leaves which are not green.
=610. Spore-bearing leaves of the pine.=—The two kinds of
spore-bearing leaves of the pine, and their close relatives, are so
different from anything which we have yet studied, and are so unlike
the green leaves of the pine, that we would scarcely recognize them as
belonging to this category. Indeed there is great uncertainty regarding
their origin.
[Illustration: Fig. 339. Spray of white pine showing cluster of male
cones just before the scattering of the pollen.]
=611. Male cones, or male flowers.=—The male cones are borne in
clusters as shown in fig. 339. Each compact, nearly cylindrical, or
conical mass is termed a cone, or flower, and each arises in place of a
long lateral branch. One of these cones is shown considerably enlarged
in fig. 340. The central axis of each cone is a lateral branch, and
belongs to the stem series. The stem axis of the cone can be seen
in fig. 341. It is completely covered by stout, thick, scale-like
outgrowths. These scales are obovate in outline, and at the inner
angle of the upper end there are several rough, short spines. They
are attached by their inner lower angle, which forms a short stalk
or petiole, and continues through the inner face of the scale as
a “midrib.” What corresponds to the lamina of the scale-like leaf
bulges out on each side below and makes the bulk of the scale. These
prominences on the under side are the sporangia (microsporangia). There
are thus two sporangia on a sporophyll (microsporophyll). When the
spores (microspores), which here are usually called pollen grains, are
mature, each sporangium, or anther locule, splits down the middle as
shown in fig. 342, and the spores are set free.
[Illustration: Fig. 340. Staminate cone of white pine, with bud scales
removed on one side.]
[Illustration: Fig. 341. Section of staminate cone, showing sporangia.]
[Illustration: Fig. 342. Two sporophylls removed, showing opening of
sporangia.]
[Illustration: Fig. 343. Pollen grain of white pine.]
=612. Microspores of the pine, or pollen grains.=—A mature pollen
grain of the pine is shown in fig. 343. It is a queer-looking object,
possessing on two sides an air sac, formed by the upheaval of the outer
coat of the spore at these two points. When the pollen is mature, the
moisture dries out of the scale (or stamen, as it is often called here)
while it ripens. When a limb, bearing a cluster of male cones, is
jarred by the hand, or by currents of air, the split suddenly opens,
and a cloud of pollen bursts out from the numerous anther locules. The
pollen is thus borne on the wind and some of it falls on the female
flowers.
[Illustration: Fig. 344. White pine, branch with cluster of mature
cones shedding the seed. A few young cones four months old are shown on
branch at the left. Drawn from photograph.]
[Illustration: Fig. 345. Mature cone of white pine at time of
scattering of the seed, nearly natural size.]
=613. Form of the mature female cone.=—A cluster of the
white pine cones is shown in fig. 344. These are mature, and the scales
have spread as they do when mature and becoming dry, in order that the
seeds may be set at liberty. The general outline of the cone is
lanceolate, or long oval, and somewhat curved. It measures about
10-15_cm_ long. If we remove one of the scales, just as they are
beginning to spread, or before the seeds have scattered, we shall find
the seeds attached to the upper surface at the lower end. There are
two seeds on each scale, one at each lower angle. They are ovate in
outline, and shaped somewhat like a biconvex lens. At this time the
seeds easily fall away, and may be freed by jarring the cone. As the
seed is detached from the scale a strip of tissue from the latter is
peeled off. This forms a “wing” for the seed. It is attached to one end
and is shaped something like a knife blade. On the back of the scale is
a small appendage known as the cover scale.
[Illustration: Fig. 346. Sterile scale. Seeds undeveloped.]
[Illustration: Fig. 347. Scale with well-developed seeds.]
[Illustration: Fig. 348. Seeds have split off from scale.]
[Illustration: Fig. 349. Back of scale with small cover scale.]
[Illustration: Fig. 350. Winged seed free from scale.
Figs. 346-350.—White pine showing details of mature scales and seed.]
[Illustration: Fig. 351. Female cones of the pine at time of
pollination, about natural size.]
=614. Formation of the female pine cone.=—The female flowers
begin their development rather late in the spring of the year. They
are formed from terminal buds of the higher branches of the tree. In
this way the cone may terminate the main shoot of a branch, or of the
lateral shoots in a whorl. After growth has proceeded for some time in
the spring, the terminal portion begins to assume the appearance of a
young female cone or flower. These young female cones, at about the
time that the pollen is escaping from the anthers, are long ovate,
measuring about 6-10 _mm_ long. They stand upright as shown in fig. 351.
=615. Form of a “scale” of the female flower.=—If we remove one
of the scales from the cone at this stage we can better study it in
detail. It is flattened, and oval in outline, with a stout “rib,” if
it may be so called, running through the middle line and terminating
in a point. The scale is in two parts as shown in fig. 354, which is
a view of the under side. The small “outgrowth” which appears as an
appendage is the cover scale, for while it is smaller in the pine than
the other portion, in some of the relatives of the pine it is larger
than its mate, and being on the outside, covers it. (The inner scale is
sometimes called the ovuliferous scale, because it bears the ovules.)
[Illustration: Fig. 352. Section of female cone of white pine, showing
young ovules (macrosporangia) at base of the ovuliferous scales.]
[Illustration: Fig. 353. Scale of white pine with the two ovules at
base of ovuliferous scale.]
[Illustration: Fig. 354. Scale of white pine seen from the outside,
showing the cover scale.]
=616. Ovules, or macrosporangia, of the pine.=—At each of the
lower angles of the scale is a curious oval body with two curved,
forceps-like processes at the lower and smaller end. These are the
macrosporangia, or, as they are called in the higher plants, the
ovules. These ovules, as we see, are in the positions of the seeds on
the mature cones. In fact the wall of the ovule forms the outer coat of
the seed, as we will later see.
[Illustration: Fig. 355. Branch of white pine showing young female
cones at time of pollination on the ends of the branches, and
one-year-old cones below, near the time of fertilization.]
=617. Pollination.=—At the time when the pollen is mature the
female cones are still erect on the branches, and the scales, which
during the earlier stages of growth were closely pressed against one
another around the axis, are now spread apart. As the clouds of pollen
burst from the clusters of the male cones, some of it is wafted by
the wind to the female cones. It is here caught in the open scales,
and rolls down to their bases, where some of it falls between these
forceps-like processes at the lower end of the ovule. At this time the
ovule has exuded a drop of a sticky fluid in this depression between
the curved processes at its lower end. The pollen sticks to this, and
later, as this viscid substance dries up, it pulls the pollen close up
in the depression against the lower end of the ovule. This depression
is thus known as the _pollen chamber_.
=618.= Now the open scales on the young female cone close up
again so tightly that water from rains is excluded. What is also very
curious, the cones, which up to this time have been standing erect, so
that the open scale could catch the pollen, now turn so that they hang
downward. This more certainly excludes the rains, since the overlapping
of the scales forms a shingled surface. Quantities of resin are also
formed in the scales, which exudes and makes the cone practically
impervious to water.
=619.= The female cone now slowly grows during the summer and
autumn, increasing but little in size during this time. During the
winter it rests, that is, ceases to grow. With the coming of spring,
growth commences again and at an accelerated rate. The increase in size
is more rapid. The cone reaches maturity in September. We thus see that
nearly eighteen months elapse from the beginning of the female flower
to the maturity of the cone, and about fifteen months from the time
that pollination takes place.
[Illustration: Fig. 356.
Macrosporangium of pine (ovule). _int_, integument; _n_, nucellus; _m_,
macrospore; _pc_, pollen chamber; _pg_, pollen grain; _an_, axile row;
_spt_, spongy tissue. (After Ferguson.)]
=620. Female prothallium of the pine.=—To study this we must
make careful longitudinal sections through the ovule (better made with
the aid of a microtome). Such a section is shown in fig. 358. The
outer layer of tissue, which at the upper end (point where the scale
is attached to the axis of the cone) stands free, is the ovular coat,
or _integument_. Within this integument, near the upper end, there is
a cone-shaped mass of tissue. This mass of tissue is the _nucellus_,
or the _macrosporangium_ proper. In the lower part of the nucellus in
fig. 356 can be seen a rounded mass of “spongy tissue” (_spt_), which
is a special nourishing tissue of the nucellus, or sporangium, around
the macrospore. Within this can be seen an axile row of three cells
(_an: m_). The lowest one, which is larger than the other two, is the
_macrospore_. Sometimes there are four of these cells in the axile row.
This axile row of three or four cells is formed by the two successive
divisions of a mother cell in the nucellus. So it would appear that
these three or four cells are all spores.
Only one of them, however, the lower one, develops; the others are
disorganized and disappear. The nucleus of the macrospore now divides
several times to form several free nuclei in the now enlarging cavity,
much as the nucleus of the macrospore in Selaginella and Isoetes
divides within the spore. The development thus far takes place during
the first summer, and now with the approach of winter the very young
female prothallium goes into rest about the stage shown in fig. 358.
The conical portion of the nucellus which lies above is the nucellar
cap.
[Illustration: Fig. 357.
Pollen grains of pine. One of them germinating. _p_¹ and _p_², the two
disintegrated prothallial cells, = sterile part of male gametophyte;
_a.c._, central cell of antheridium; _v.n._, vegetative nucleus or tube
nucleus of the single-wall cell of antheridium; _s.g._, starch grains.
(After Ferguson.)]
[Illustration: Fig. 358.
Section of ovule of white pine. _int_, integument; _pc_, pollen
chamber; _pt_, pollen tube; _n_, nucleus; _m_, macrospore cavity.]
=621. Male prothallia.=—By the time the pollen is mature the
male prothallium is already partly formed. In fig. 343 we can see two
well-formed cells. Two other cells are formed earlier, but they become
so flattened that it is difficult to make them out when the pollen
grain is mature. These are shown in fig. 357, _p_¹ and _p_², and they
are the only sterile cells of the male prothallium in the pines. The
large cell is the antheridium wall, its nucleus _v.n._ in fig. 357. The
smaller cell, _a.c._, is the central cell of the antheridium. During
the summer and autumn the male prothallium makes some farther growth,
but this is slow. The larger cell, called the vegetative cell or tube
cell, which is in reality the wall of the antheridium, elongates by
the formation of a tube, forming a sac, known as the pollen tube. It
is either simple or branched. It grows down into the tissue of the
nucellus, and at a stage represented in fig. 358, winter overtakes it
and it rests. At this time the central cell has divided into two cells,
and the vegetative nucleus is in the pollen tube.
[Illustration: Fig. 359.
Section of nucellus and endosperm of white pine. The inner layer
of cells of the integument shown just outside of nucellus; _arch_,
archegonium; _en_, egg nucleus. In the nucellar cap are shown three
pollen tubes. _vn_, vegetative nucleus or tube nucleus; _stc_, stalk
cell; _spn_, sperm nuclei, the larger one in advance is the one which
unites with the egg nucleus. The archegonia are in the endosperm or
female gametophyte. (After Ferguson.)]
=622. The endosperm.=—In the following spring growth of all these
parts continues. The nuclei in the macrospore divide to form more, and
eventually cell walls are formed between them making a distinct tissue,
known as the _endosperm_. This endosperm continues to grow until a
large part of the nucellus is consumed for food.
[Illustration: Fig. 360.
Last division of the egg in the white pine cutting off the ventral
canal cell at the apex of the archegonium. _End_, endosperm; _Arch_,
archegonium.]
=623. Female prothallium and archegonia.=—The endosperm is the
female prothallium. This is very evident from the fact that several
archegonia are developed in it usually on the side toward the pollen
chamber. The archegonia are sexual organs, and since the sexual organs
are developed on the gametophyte, therefore, the endosperm is the
female gametophyte, or prothallium. In fig. 359 are represented two
archegonia in the endosperm and the pollen tubes are growing down
through the nucellus. The archegonia are quite large, the wall is a
sheath or jacket of cells which encloses the very large egg which has a
large nucleus in the center.
=624. Pollen tube and sperm cells.=—While the endosperm (female
prothallium) and archegonia are developing the pollen tube continues
its growth down through the nucellar cap, as shown in fig. 359. At
the same time the two cells which were formed in the pollen grain
(antheridium) from the central cell move down into the tube. One of
these is the “generative” cell, or “body” cell, and the other is called
the stalk cell, though it is more properly a sterile half of the
central cell. The nucleus of the generative cell, about the time the
archegonium is mature, divides to form two nuclei, which are the sperm
nuclei, and the one in advance is the larger, though it is much smaller
than the egg nucleus.
=625. Fertilization.=—Very soon after the archegonia are mature
(early in June in the northern United States) the pollen tube grows
through into the archegonium and empties the two sperm nuclei, the
vegetative nucleus and the stalk cell, into the protoplasm of the large
egg. The larger of the two sperm nuclei at once comes in contact with
the very large egg nucleus and sinks down into a depression of the
same, as shown in fig. 361. These two nuclei, in the pines, do not fuse
into a resting nucleus, but at once organize the nuclear figure for the
first division of the embryo. Two nuclei are thus formed, and these
divide to form four nuclei which sink to the bottom of the archegonium
and there organize the embryo which pushes its way into the endosperm
from which it derives its food (fig. 362).
[Illustration: Fig. 361.
Archegonium of white pine at stage of fertilization, _en_, egg nucleus;
_spn_, sperm nucleus in conjugation with it; _nb_, nutritive bodies in
cytoplasm of large egg; _cpt_, cavity of pollen tube; _vn_, vegetative
nucleus or tube nucleus; _stc_, stalk cell; _spn_, second sperm
nucleus: _pr_, portion of prothallium or endosperm; _sg_, starch grains
in pollen tube. The sheath of jacket cells of the archegonium is not
shown. (After Ferguson.)]
=626. Homology of the parts of the female cone.=—Opinions are
divided as to the homology of the parts of the female cone of the pine.
Some consider the entire cone to be homologous with a flower of the
angiosperms. The entire scale according to this view is a carpel, or
sporophyll, which is divided into the cover scale and the ovuliferous
scale. This division of the sporophyll is considered similar to that
which we have in isoetes, where the sporophyll has a ligule above the
sporangium, or as in ophioglossum, where the leaf is divided into a
fertile and a sterile portion.
Others believe that the ovuliferous scale is composed of two leaves
situated laterally and consolidated representing a shoot in the axis of
the bract. There is some support for this in the fact that in certain
abnormal cones which show proliferation a short axis appears in the
axil of the bract and bears lateral leaves, and in some cases all
gradations are present between these lateral leaves on the axis and
their consolidation into an ovuliferous scale. In the normal condition
of the ovuliferous scale the axis has disappeared and the shoot is
represented only by the consolidated leaves, which would represent then
the macrosporophylls (or carpels) each bearing one macrosporangium
(ovule).
[Illustration: Fig. 362.
Pine seed, section of, _sc_, seed coat; _n_, remains of nucellus;
_end_, endosperm (= female gametophyte); _emb_, embryo = young
sporophyte. Seed coat and nucellus = remains of old sporophyte.]
[Illustration: Fig. 363. Embryo of white pine removed from seed, showing
several cotyledons.]
[Illustration: Fig. 364. Pine seedling just emerging from the ground.]
One of the most interesting and plausible views is that of Celakovsky.
He believes that the axial shoot is reduced to two ovules, that the
ovules have two integuments, but the outer integument of each has
become proliferated into scales which are consolidated. In this
proliferation of the outer integument it is thrown off from the ovule
so that it only remains attached to one side and the larger part of the
ovule is thus left with only one integument. This view is supported
by the fact that in gingko, for example (another gymnosperm), the
outer integument (the “collar”) sometimes proliferates into a leaf.
Celakovsky’s view is, therefore, not very different from the second one
mentioned above.
[Illustration: Fig. 365. White pine seedling casting seed coats.]