Elements of Structural and Systematic Botany

D >> Douglas Houghton Campbell >> Elements of Structural and Systematic Botany

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[9] A vessel differs from a tracheid in being composed of several
cells placed end to end, the partitions being wholly or partially
absorbed, so as to throw the cells into close communication.

The epidermis is composed of elongated cells whose walls present a
peculiar beaded appearance, due to the deposition of flint within
them. The breathing pores are arranged in vertical lines, and
resemble in general appearance those of the ferns, though differing
in some minor details. Like the other epidermal cells the guard
cells have heavy deposits of flint, which here are in the form of
thick transverse bars.

The spore cases have thin walls whose cells, shortly before
maturity, develop thickenings upon their walls, which have to do
with the opening of the spore case. The spores (_H_, _I_) are round
cells containing much chlorophyll and provided with four peculiar
appendages called elaters. The elaters are extremely sensitive to
changes in moisture, coiling up tightly when moistened (_I_), but
quickly springing out again when dry (_H_). By dusting a few dry
spores upon a slide, and putting it under the microscope without any
water, the movement may be easily examined. Lightly breathing upon
them will cause the elaters to contract, but in a moment, as soon as
the moisture of the breath has evaporated, they will uncoil with a
quick jerk, causing the spores to move about considerably.

The fresh spores begin to germinate within about twenty-four hours,
and the early stages, which closely resemble those of the ferns, may
be easily followed by sowing the spores in water. With care it is
possible to get the mature prothallia, which should be treated as
described for the fern prothallia. Under favorable conditions, the
first antheridia are ripe in about five weeks; the archegonia, which
are borne on separate plants, a few weeks later. The antheridia
(Fig. 72, _J_, _an._) are larger than those of the ferns, and the
spermatozoids (_K_) are thicker and with fewer coils, but otherwise
much like fern spermatozoids.

The archegonia have a shorter neck than those of the ferns, and the
neck is straight.

Both male and female prothallia are much branched and very irregular
in shape.

There are a number of common species of _Equisetum_. Some of them,
like the common scouring rush (_E. hiemale_), are unbranched, and the
spores borne at the top of ordinary green branches; others have all
the stems branching like the sterile stems of the field horse-tail,
but produce a spore-bearing cone at the top of some of them.

CLASS III.--THE CLUB MOSSES (_Lycopodinae_).

The last class of the pteridophytes includes the ground pines, club
mosses, etc., and among cultivated plants numerous species of the
smaller club mosses (_Selaginella_).

Two orders are generally recognized, although there is some doubt as
to the relationship of the members of the second order. The first
order, the larger club mosses (_Lycopodiaceae_) is represented in the
northern states by a single genus (_Lycopodium_), of which the common
ground pine (_L. dendroideum_) (Fig. 73) is a familiar species. The
plant grows in the evergreen forests of the northern United States as
well as in the mountains further south, and in the larger northern
cities is often sold in large quantities at the holidays for
decorating. It sends up from a creeping, woody, subterranean stem,
numerous smaller stems which branch extensively, and are thickly set
with small moss-like leaves, the whole looking much like a little
tree. At the ends of some of the branches are small cones (_A_, _x_,
_B_) composed of closely overlapping, scale-like leaves, much as in a
fir cone. Near the base, on the inner surface of each of these scales,
is a kidney-shaped capsule (_C_, _sp._) opening by a cleft along the
upper edge and filled with a mass of fine yellow powder. These
capsules are the spore cases.

The bases of the upright stems are almost bare, but become covered
with leaves higher up. The leaves are in shape like those of a moss,
but are thicker. The spore-bearing leaves are broader and when
slightly magnified show a toothed margin.

The stem is traversed by a central fibro-vascular cylinder that
separates easily from the surrounding tissue, owing to the rupture of
the cells of the bundle sheath, this being particularly frequent in
dried specimens. When slightly magnified the arrangement of the
tissues may be seen (Fig. 73, _E_). Within the epidermis is a mass of
ground tissue of firm, woody texture surrounding the central oval or
circular fibro-vascular cylinder. This shows a number of white bars
(xylem) surrounded by a more delicate tissue (phloem).

On magnifying the section more strongly, the cells of the ground
tissue (_G_) are seen to be oval in outline, with thick striated
walls and small intercellular spaces. Examined in longitudinal
sections they are long and pointed, belonging to the class of cells
known as "fibres."

[Illustration: FIG. 73.--_A_, a club moss (_Lycopodium_), x 1/3. _x_,
cone. _r_, root. _B_, a cone, x 1. _C_, single scale with sporangium
(_sp._). _D_, spores: i, from above; ii, from below, x 325. _E_, cross
section of stem, x 8. _f.b._ fibro-vascular bundle. _F_, portion of
the fibro-vascular bundle, x 150. _G_, cells of the ground tissue,
x 150.]

The xylem (_F_, _xy._) of the fibro-vascular bundle is composed of
tracheids, much like those of the ferns; the phloem is composed of
narrow cells, pretty much all alike.

The spores (_D_) are destitute of chlorophyll and have upon the
outside a network of ridges, except on one side where three straight
lines converge, the spore being slightly flattened between them.

Almost nothing is known of the prothallia of our native species.

The second order (_Ligulatae_) is represented by two very distinct
families: the smaller club mosses (_Selaginelleae_) and the quill-worts
(_Isoeteae_). Of the former the majority are tropical, but are common
in greenhouses where they are prized for their delicate moss-like
foliage (Fig. 74, _A_).

[Illustration: FIG. 74.--_A_, one of the smaller club mosses
(_Selaginella_). _sp._ spore-bearing branch, x 2. _B_, part of a stem,
sending down naked rooting branches (_r_), x 1. _C_, longitudinal
section of a spike, with a single macrosporangium at the base; the
others, microsporangia, x 3. _D_, a scale and microsporangium, x 5.
_E_, young microsporangium, x 150. The shaded cells are the spore
mother cells. _F_, a young macrospore, x 150. _G_, section of the
stem, x 50. _H_, a single fibro-vascular bundle, x 150. _I_, vertical
section of the female prothallium of _Selaginella_, x 50. _ar._
archegonium. _J_, section of an open archegonium, x 300. _o_, the egg
cell. _K_, microspore, with the contained male prothallium, x 300.
_x_, vegetative cell. _sp._ sperm cells. _L_, young plant, with the
attached macrospore, x 6. _r_, the first root. _l_, the first leaves.]

The leaves in most species are like those of the larger club mosses,
but more delicate. They are arranged in four rows on the upper side of
the stem, two being larger than the others. The smaller branches grow
out sideways so that the whole branch appears flattened, reminding one
of the habit of the higher liverworts. Special leafless branches (_B_,
_r_) often grow downward from the lower side of the main branches, and
on touching the ground develop roots which fork regularly.

The sporangia are much like those of the ground pines, and produced
singly at the bases of scale leaves arranged in a spike or cone (_A_,
_sp._), but two kinds of spores, large and small, are formed. In the
species figured the lower sporangium produces four large spores
(macrospores); the others, numerous small spores (microspores).

Even before the spores are ripe the development of the prothallium
begins, and this is significant, as it shows an undoubted
relationship between these plants and the lowest of the seed plants,
as we shall see when we study that group.

If ripe spores can be obtained by sowing them upon moist earth, the
young plants will appear in about a month. The microspore (Fig. 74,
_K_) produces a prothallium not unlike that of some of the water
ferns, there being a single vegetative cell (_x_), and the rest of
the prothallium forming a single antheridium. The spermatozoids are
excessively small, and resemble those of the bryophytes.

The macrospore divides into two cells, a large lower one, and a
smaller upper one. The latter gives rise to a flat disc of cells
producing a number of small archegonia of simple structure (Fig. 74,
_I_, _J_). The lower cell produces later a tissue that serves to
nourish the young embryo.

The development of the embryo recalls in some particulars that of
the seed plants, and this in connection with the peculiarities of
the sporangia warrants us in regarding the _Ligulatae_ as the highest
of existing pteridophytes, and to a certain extent connecting them
with the lowest of the spermaphytes.

Resembling the smaller club mosses in their development, but differing
in some important points, are the quill-worts (_Isoeteae_). They are
mostly aquatic forms, growing partially or completely submerged, and
look like grasses or rushes. They vary from a few centimetres to half
a metre in height. The stem is very short, and the long cylindrical
leaves closely crowded together. The leaves which are narrow above are
widely expanded and overlapping at the base. The spores are of two
kinds, as in _Selaginella_, but the macrosporangia contain numerous
macrospores. The very large sporangia (_M_, _sp._) are in cavities at
the bases of the leaves, and above each sporangium is a little pointed
outgrowth (ligula), which is also found in the leaves of
_Selaginella_. The quill-worts are not common plants, and owing to
their habits of growth and resemblance to other plants, are likely to
be overlooked unless careful search is made.

CHAPTER XIV.

SUB-KINGDOM VI.

SPERMAPHYTES: PHAENOGAMS.

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 (_Gymnospermae_).

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_ [Male]). 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, x 1/2. _B_, a year-old cone, x 1. _C_, longitudinal section of
_B_. _D_, a single scale of _B_, showing the sporangia (ovules) (_o_),
x 2. _E_, a scale from a ripe cone, with the seeds (_s_), x 1/2. _F_,
longitudinal section of a ripe seed, x 3. _em._ the embryo. _G_, a
germinating seed, x 2. _r_, the primary root. _H_, longitudinal
section through _G_, showing the first leaves of the young plant still
surrounded by the endosperm, x 4. _I_, an older plant with the leaves
(_l_) withdrawing from the seed coats, x 4. _J_, upper part of a young
plant, showing the circle of primary leaves (cotyledons), x 1. _K_,
section of the same, x 2. _b_, the terminal bud. _L_, cross-section of
the stem of the young plant, x 25. _fb._ a fibro-vascular bundle. _M_,
cross-section of the root, x 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.

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