La Motte Fouque, Friedrich Heinrich Karl, Freiherr de - "The Two Captains"

Washington Irving - "The Life and Voyages of Christopher Columbus (Vol. II)"

M. R. (Montague Rhodes) James - "A Thin Ghost and Others"

Henry Beam Piper - "Oomphel in the Sky"

Annual Bibliography of Commonwealth Literature 2007
This paper argues that discourses of love in Ghanaian market literature for youth offer a view into complex negotiations of agency and empowerment. Drawing on Deborah Durham's notion of youth as "social `shifters'" and Francis Nyamnjoh's conception of the "interconnectedness" of agency, I take Ghanaian market literature as one specific case of how African literature for youth foregrounds questions of continuity and change as African societies enter into increasingly complex global relations. In this literature for youth, received notions of love, often constructed out of impressions from American pop and hip hop music, carry new notions of agency that compete with existing "domesticated" forms. Authors like Ike Tandoh and Evelyn Tay employ discourses of love to offer youth alternative avenues for empowerment in a context of socio-economic disenfranchizement. In a creative process of "straddling", this writing both reveals and reproduces the contradictions that obtain in youth configurations of agency.

Elements of Structural and Systematic Botany

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




Up to this point, all the cells of the embryo are much alike, and
the embryo, like that of the bryophytes, is completely surrounded by
the enlarged base of the archegonium (compare Fig. 67, _A_, with
Fig. 55); but before the embryo breaks through the overlying cells a
differentiation of the tissues begins. In the axis of each of the
four divisions the cells divide lengthwise so as to form a
cylindrical mass of narrow cells, not unlike those in the stem of a
moss. Here, however, some of the cells undergo a further change; the
walls thicken in places, and the cells lose their contents, forming
a peculiar conducting tissue (tracheary tissue), found only in the
two highest sub-kingdoms. The whole central cylinder is called a
"fibro-vascular bundle," and in its perfect form, at least, is found
in no plants below the ferns, which are also the first to develop
true roots.

The young root and leaf now rapidly elongate, and burst through the
overlying cells, the former growing downward and becoming fastened in
the ground, the latter growing upward through the notch in the front
of the prothallium, and increasing rapidly in size (Fig. 67, _B_). The
leaf is more or less deeply cleft, and traversed by veins which are
continuations of the fibro-vascular bundle of the stalk, and
themselves fork once or twice. The surface of the leaf is covered with
a well-developed epidermis, and the cells occupying the space between
the veins contain numerous chloroplasts, so that the little plant is
now quite independent of the prothallium, which has hitherto supported
it. As soon as the fern is firmly established, the prothallium withers
away.

Comparing this now with the development of the sporogonium in the
bryophytes, it is evident that the young fern is the equivalent of the
sporogonium or spore fruit of the former, being, like it, the direct
product of the fertilized egg cell; and the prothallium represents the
moss or liverwort, upon which are borne the sexual organs. In the
fern, however, the sporogonium becomes entirely independent of the
sexual plant, and does not produce spores until it has reached a large
size, living many years. The sexual stage, on the other hand, is very
much reduced, as we have seen, being so small as to be ordinarily
completely overlooked; but its resemblance to the lower liverworts,
like _Riccia_, or the horned liverworts, is obvious. The terms
ooephyte (egg-bearing plant) and sporophyte (spore-bearing plant, or
sporogonium) are sometimes used to distinguish between the sexual
plant and the spore-bearing one produced from it.

The common maiden-hair fern (_Adiantum pedatum_) has been selected
here for studying the structure of the full-grown sporophyte, but
almost any other common fern will answer. The maiden-hair fern is
common in rich woods, and may be at once recognized by the form of its
leaves. These arise from a creeping, underground stem (Fig. 67, _C_),
which is covered with brownish scales, and each leaf consists of a
slender stalk, reddish brown or nearly black in color, which divides
into two equal branches at the top. Each of these main branches bears
a row of smaller ones on the outside, and these have a row of delicate
leaflets on each side (Fig. 67, _E_). The stem of the plant is
fastened to the ground by means of numerous stout roots. The youngest
of these, near the growing point of the stem, are unbranched, but the
older ones branch extensively (_C_).

On breaking the stem across, it is seen to be dark-colored, except in
the centre, which is traversed by a woody cylinder (fibro-vascular
bundle) of a lighter color. This is sometimes circular in sections,
sometimes horse-shoe shaped. Where the stem branches, the bundle of
the branch may be traced back to where it joins that of the main stem.

A thin cross-section of the stem shows, when magnified, three
regions. First, an outer row of cells, often absent in the older
portions; this is the epidermis. Second, within the epidermis are
several rows of cells similar to the epidermal cells, but somewhat
larger, and like them having dark-brown walls. These merge gradually
into larger cells, with thicker golden brown walls (Fig. 67, _I_).
The latter, if sufficiently magnified, show distinct striation of
the walls, which are often penetrated by deep narrow depressions or
"pits." This thick-walled tissue is called "stony tissue"
(schlerenchyma). All the cells contain numerous granules, which the
iodine test shows to be starch. All of this second region lying
between the epidermis and the fibro-vascular bundle is known as the
ground tissue. The third region (fibro-vascular) is, as we have seen
without the microscope, circular or horse-shoe shaped. It is sharply
separated from the ground tissue by a row of small cells, called the
"bundle sheath." The cross-section of the bundle of the leaf stalk
resembles, almost exactly, that of the stem; and, as it is much
easier to cut, it is to be preferred in studying the arrangement of
the tissues of the bundle (Fig. 67, _G_). Within the bundle sheath
(_sh._) there are two well-marked regions, a central band (_x_) of
large empty cells, with somewhat angular outlines, and distinctly
separated walls; and an outer portion (_y_) filling up the space
between these central cells and the bundle sheath. The central
tissue (_x_) is called the woody tissue (xylem); the outer, the bast
(phloem). The latter is composed of smaller cells of variable form,
and with softer walls than the wood cells.

A longitudinal section of either the stem or leaf stalk shows that
all the cells are decidedly elongated, especially those of the
fibro-vascular bundle. The xylem (Fig. 68, _C_, _x_) is made up
principally of large empty cells, with pointed ends, whose walls are
marked with closely set, narrow, transverse pits, giving them the
appearance of little ladders, whence they are called "scalariform,"
or ladder-shaped markings. These empty cells are known as
"tracheids," and tissue composed of such empty cells, "tracheary
tissue." Besides the tracheids, there are a few small cells with
oblique ends, and with some granular contents.

The phloem is composed of cells similar to the latter, but there may
also be found, especially in the stem, other larger ones (Fig. 67,
_J_), whose walls are marked with shallow depressions, whose bottoms
are finely pitted. These are the so-called "sieve tubes."

For microscopical examination, either fresh or alcoholic material
may be used, the sections being mounted in water. Potash will be
found useful in rendering opaque sections transparent.

The leaves, when young, are coiled up (Fig. 67, _C_), owing to growth
in the earlier stages being greater on the lower than on the upper
side. As the leaf unfolds, the stalk straightens, and the upper
portion (blade) becomes flat.

The general structure of the leaf stalk may be understood by making a
series of cross-sections at different heights, and examining them with
a hand lens. The arrangement is essentially the same as in the stem.
The epidermis and immediately underlying ground tissue are
dark-colored, but the inner ground tissue is light-colored, and much
softer than the corresponding part of the stem; and some of the outer
cells show a greenish color, due to the presence of chlorophyll.

The section of the fibro-vascular bundle differs at different heights.
Near the base of the stalk (Fig. _D_ i) it is horseshoe-shaped; but,
if examined higher up, it is found to divide (II, III), one part going
to each of the main branches of the leaf. These secondary bundles
divide further, forming the veins of the leaflets.

The leaflets (_E_, _F_) are one-sided, the principal vein running
close to the lower edge, and the others branching from it, and forking
as they approach the upper margin, which is deeply lobed, the lobes
being again divided into teeth. The leaflets are very thin and
delicate, with extremely smooth surface, which sheds water perfectly.
If the plant is a large one, some of the leaves will probably bear
spores. The spore-bearing leaves are at once distinguished by having
the middle of each lobe of the leaflets folded over upon the lower
side (_F_). On lifting one of these flaps, numerous little rounded
bodies (spore cases) are seen, whitish when young, but becoming brown
as they ripen. If a leaf with ripe spore cases is placed upon a piece
of paper, as it dries the spores are discharged, covering the paper
with the spores, which look like fine brown powder.

[Illustration: FIG. 68.--_A_, vertical section of the leaf of the
maiden-hair fern, which has cut across a vein (_f.b._), x 150. _B_,
surface view of the epidermis from the lower surface of a leaf. _f_,
vein. _p_, breathing pore, x 150. _C_, longitudinal section of the
fibro-vascular bundle of the leaf stalk, showing tracheids with
ladder-shaped markings, x 150. _D_, longitudinal section through the
tip of a root, x 150. _a_, apical cell. _Pl._ young fibro-vascular
bundle. _Pb._ young ground tissue. _E_, cross-section of the root,
through the region of the apical cell (_a_), x 150. _F_, cross-section
through a full-grown root, x 25. _r_, root hairs. _G_, the
fibro-vascular bundle of the same, x 150.]

A microscopical examination of the leaf stalk shows the tissues to
be almost exactly like those of the stem, except the inner ground
tissue, whose cells are thin-walled and colorless (soft tissue or
"parenchyma") instead of stony tissue. The structure of the blade of
the leaf, however, shows a number of peculiarities. Stripping off a
little of the epidermis with a needle, or shaving off a thin slice
with a razor, it may be examined in water, removing the air if
necessary with alcohol. It is composed of a single layer of cells,
of very irregular outline, except where it overlies a vein (Fig. 68,
_B_, _f_). Here the cells are long and narrow, with heavy walls. The
epidermal cells contain numerous chloroplasts, and on the under
surface of the leaf breathing pores (_stomata_, sing. _stoma_), not
unlike those on the capsules of some of the bryophytes. Each
breathing pore consists of two special crescent-shaped epidermal
cells (guard cells), enclosing a central opening or pore
communicating with an air space below. They arise from cells of the
young epidermis that divide by a longitudinal wall, that separates
in the middle, leaving the space between.

[Illustration: FIG. 69.--_A_, mother cell of the sporangium of the
maiden-hair fern, x 300. _B_, young sporangium, surface view, x 150:
i, from the side; ii, from above. _C-E_, successive stages in the
development of the sporangium seen in optical section, x 150. _F_,
nearly ripe sporangium, x 50: i, from in front; ii, from the side.
_an._ ring. _st._ point of opening. _G_, group of four spores, x 150.
_H_, a single spore, x 300.]

By holding a leaflet between two pieces of pith, and using a very
sharp razor, cross-sections can be made. Such a section is shown in
Fig. 68, _A_. The epidermis (_e_) bounds the upper and lower
surfaces, and if a vein (_f.b._) is cut across its structure is
found to be like that of the fibro-vascular bundle of the leaf
stalk, but much simplified.

The ground tissue of the leaf is composed of very loose, thin-walled
cells, containing numerous chloroplasts. Between them are large and
numerous intercellular spaces, filled with air, and communicating
with the breathing pores. These are the principal assimilating cells
of the plant; _i.e._ they are principally concerned in the
absorption and decomposition of carbonic acid from the atmosphere,
and the manufacture of starch.

The spore cases, or sporangia (Fig. 69), are at first little papillae
(_A_), arising from the epidermal cells, from which they are early
cut off by a cross-wall. In the upper cell several walls next arise,
forming a short stalk, composed of three rows of cells, and an upper
nearly spherical cell--the sporangium proper. The latter now divides
by four walls (_B_, _C_, i-iv), into a central tetrahedral cell, and
four outer ones. The central cell, whose contents are much denser
than the outer ones, divides again by walls parallel to those first
formed, so that the young sporangium now consists of a central cell,
surrounded by two outer layers of cells. From the central cell a
group of cells is formed by further divisions (_D_), which finally
become entirely separated from each other. The outer cells of the
spore case divide only by walls, at right angles to their outer
surface, so that the wall is never more than two cells thick. Later,
the inner of these two layers becomes disorganized, so that the
central mass of cells floats free in the cavity of the sporangium,
which is now surrounded by but a single layer of cells (_E_).

Each of the central cells divides into four spores, precisely as in
the bryophytes. The young spores (_G_, _H_) are nearly colorless and
are tetrahedral (like a three-sided pyramid) in form. As they ripen,
chlorophyll is formed in them, and some oil. The wall becomes
differentiated into three layers, the outer opaque and brown, the
two inner more delicate and colorless.

Running around the outside of the ripe spore case is a single row of
cells (_an._), differing from the others in shape, and having their
inner walls thickened. Near the bottom, two (sometimes four) of
these cells are wider than the others, and their walls are more
strongly thickened. It is at this place (_st._) that the spore case
opens. When the ripe sporangium becomes dry, the ring of thickened
cells (_an._) contracts more strongly than the others, and acts like
a spring pulling the sporangium open and shaking out the spores,
which germinate readily under favorable conditions, and form after a
time the sexual plants (prothallia).

The roots of the sporophyte arise in large numbers, the youngest being
always nearest the growing point of the stem or larger roots (Fig. 67,
_C_). The growing roots are pointed at the end which is also
light-colored, the older parts becoming dark brown. A cross-section of
the older portions shows a dark-brown ground tissue with a central,
light-colored, circular, fibro-vascular bundle (Fig. 68, _F_). Growing
from its outer surface are numerous brown root hairs (_r_).

When magnified the walls of all the outer cells (epidermis and
ground tissue) are found to be dark-colored but not very thick, and
the cells are usually filled with starch. There is a bundle sheath
of much-flattened cells separating the fibro-vascular bundle from
the ground tissue. The bundle (Fig. 68, _G_) shows a band of
tracheary tissue in the centre surrounded by colorless cells, all
about alike.

All of the organs of the fern grow from a definite apical cell, but
it is difficult to study except in the root.

Selecting a fresh, pretty large root, a series of thin longitudinal
sections should be made either holding the root directly in the
fingers or placing it between pieces of pith. In order to avoid
drying of the sections, as is indeed true in cutting any delicate
tissue, it is a good plan to wet the blade of the razor. If the
section has passed through the apex, it will show the structure
shown in Figure 68, _D_. The apical cell (_a_) is large and
distinct, irregularly triangular in outline. It is really a
triangular pyramid (tetrahedron) with the base upward, which is
shown by making a series of cross-sections through the root tip, and
comparing them with the longitudinal sections. The cross-section of
the apical cell (Fig. _L_) appears also triangular, showing all its
faces to be triangles. Regular series of segments are cut off in
succession from each of the four faces of the apical cell. These
segments undergo regular divisions also, so that very early a
differentiation of the tissues is evident, and the three tissue
systems (epidermal, ground, and fibro-vascular) may be traced
almost to the apex of the root (68, _D_). From the outer series of
segments is derived the peculiar structure (root cap) covering the
delicate growing point and protecting it from injury.

The apices of the stem and leaves, being otherwise protected,
develop segments only from the sides of the apical cell, the outer
face never having segments cut off from it.




CHAPTER XIII.

CLASSIFICATION OF THE PTERIDOPHYTES.


There are three well-marked classes of the Pteridophytes: the ferns
(_Filicinae_); horse-tails (_Equisetinae_); and the club mosses
(_Lycopodinae_).


CLASS I.--FERNS (_Filicinae_).

The ferns constitute by far the greater number of pteridophytes, and
their general structure corresponds with that of the maiden-hair fern
described. There are three orders, of which two, the true ferns
(_Filices_) and the adder-tongues (_Ophioglossaceae_), are represented
in the United States. A third order, intermediate in some respects
between these two, and called the ringless ferns (_Marattiaceae_), has
no representatives within our territory.

The classification is at present based largely upon the characters of
the sporophyte, the sexual plants being still very imperfectly known
in many forms.

The adder-tongues (_Ophioglossaceae_) are mostly plants of rather small
size, ranging from about ten to fifty centimetres in height. There are
two genera in the United States, the true adder-tongues
(_Ophioglossum_) and the grape ferns (_Botrychium_). They send up but
one leaf each year, and this in fruiting specimens (Fig. 70, _A_) is
divided into two portions, the spore bearing (_x_) and the green
vegetative part. In _Botrychium_ the leaves are more or less deeply
divided, and the sporangia distinct (Fig. 71, _B_). In _Ophioglossum_
the sterile division of the leaf is usually smooth and undivided, and
the spore-bearing division forms a sort of spike, and the sporangia
are much less distinct. The sporangia in both differ essentially from
those of the true ferns in not being derived from a single epidermal
cell, but are developed in part from the ground tissue of the leaf.

[Illustration: FIG. 70.--Forms of ferns. _A_, grape fern
(_Botrychium_), x 1/2. _x_, fertile part of the leaf. _B_, sporangia of
_Botrychium_, x 3. _C_, flowering fern (_Osmunda_). _x_, spore-bearing
leaflets, x 1/2. _D_, a sporangium of _Osmunda_, x 25. _r_, ring. _E_,
_Polypodium_, x 1. _F_, brake (_Pteris_), x 1. _G_, shield fern
(_Aspidium_), x 2. _H_, spleen-wort (_Asplenium_), x 2. _I_, ostrich
fern (_Onoclea_), x 1. _J_, the same, with the incurved edges of the
leaflet partially raised so as to show the masses of sporangia
beneath, x 2.]

In the true ferns (_Filices_), the sporangia resemble those already
described, arising in all (unless possibly _Osmunda_) from a single
epidermal cell.

One group, the water ferns (_Rhizocarpeae_), produce two kinds of
spores, large and small. The former produce male, the latter female
prothallia. In both cases the prothallium is small, and often scarcely
protrudes beyond the spore, and may be reduced to a single archegonium
or antheridium (Fig. 71, _B_, _C_) with only one or two cells
representing the vegetative cells of the prothallium (_v_). The water
ferns are all aquatic or semi-aquatic plants, few in number and scarce
or local in their distribution. The commonest are those of the genus
_Marsilia_ (Fig. 71, _A_), looking like a four-leaved clover. Others
(_Salvinia_, _Azolla_) are floating forms (Fig. 71, _D_).

[Illustration: FIG. 71.--_A_, _Marsilia_, one of the _Rhizocarpeae_
(after Underwood). _sp._ the "fruits" containing the sporangia. _B_, a
small spore of _Pilularia_, with the ripe antheridium protruding,
x 180. _C_, male prothallium removed from the spore, x 180. _D_,
_Azolla_ (after Sprague), x 1.]

Of the true ferns there are a number of families distinguished mainly
by the position of the sporangia, as well as by some differences in
their structure. Of our common ferns, those differing most widely from
the types are the flowering ferns (_Osmunda_), shown in Figure 70,
_C_, _D_. In these the sporangia are large and the ring (_r_)
rudimentary. The leaflets bearing the sporangia are more or less
contracted and covered completely with the sporangia, sometimes all
the leaflets of the spore-bearing leaf being thus changed, sometimes
only a few of them, as in the species figured.

Our other common ferns have the sporangia in groups (_sori_, sing.
_sorus_) on the backs of the leaves. These sori are of different shape
in different genera, and are usually protected by a delicate
membranous covering (indusium). Illustrations of some of the commonest
genera are shown in Figure 70, _E_, _J_.


CLASS II.--HORSE-TAILS (_Equisetinae_).

The second class of the pteridophytes includes the horse-tails
(_Equisetinae_) of which all living forms belong to a single genus
(_Equisetum_). Formerly they were much more numerous than at present,
remains of many different forms being especially abundant in the coal
formations.

[Illustration: FIG. 72.--_A_, spore-bearing stem of the field
horse-tail (_Equisetum_), x 1. _x_, the spore-bearing cone. _B_,
sterile stem of the same, x 1/2. _C_, underground stem, with tubers
(_o_), x 1/2. _D_, cross-section of an aerial stem, x 5. _f.b._
fibro-vascular bundle. _E_, a single fibro-vascular bundle, x 150.
_tr._ vessels. _F_, a single leaf from the cone, x 5. _G_, the same
cut lengthwise, through a spore sac (_sp._), x 5. _H_, a spore, x 50.
_I_, the same, moistened so that the elaters are coiled up, x 150.
_J_, a male prothallium, x 50. _an._ an antheridium. _K_,
spermatozoids, x 300.]

One of the commonest forms is the field horse-tail (_Equisetum
arvense_), a very abundant and widely distributed species. It grows in
low, moist ground, and is often found in great abundance growing in
the sand or gravel used as "ballast" for railway tracks.

The plant sends up branches of two kinds from a creeping underground
stem that may reach a length of a metre or more. This stem (Fig. 72,
_C_) is distinctly jointed, bearing at each joint a toothed sheath,
best seen in the younger portions, as they are apt to be destroyed in
the older parts. Sometimes attached to this are small tubers (_o_)
which are much-shortened branches and under favorable circumstances
give rise to new stems. They have a hard, brown rind, and are composed
within mainly of a firm, white tissue, filled with starch.

The surface of the stem is marked with furrows, and a section across
it shows that corresponding to these are as many large air spaces that
traverse the stem from joint to joint. From the joints numerous roots,
quite like those of the ferns, arise.

If the stem is dug up in the late fall or winter, numerous short
branches of a lighter color will be found growing from the joints.
These later grow up above ground into branches of two sorts. Those
produced first (Fig. 72, _A_), in April or May, are stouter than the
others, and nearly destitute of chlorophyll. They are usually twenty
to thirty centimetres in height, of a light reddish brown color, and,
like all the stems, distinctly jointed. The sheaths about the joints
(_L_) are much larger than in the others, and have from ten to twelve
large black teeth at the top. These sheaths are the leaves. At the top
of the branch the joints are very close together, and the leaves of
different form, and closely set so as to form a compact cone (_x_).

A cross-section of the stem (_D_) shows much the same structure as the
underground stem, but the number of air spaces is larger, and in
addition there is a large central cavity. The fibro-vascular bundles
(_f.b._) are arranged in a circle, alternating with the air channels,
and each one has running through it a small air passage.

The cone at the top of the branch is made up of closely set,
shield-shaped leaves, which are mostly six-sided, on account of the
pressure. These leaves (_F_, _G_) have short stalks, and are arranged
in circles about the stem. Each one has a number of spore cases
hanging down from the edge, and opening by a cleft on the inner side
(_G_, _sp._). They are filled with a mass of greenish spores that
shake out at the slightest jar when ripe.

The sterile branches (_B_) are more slender than the spore-bearing
ones, and the sheaths shorter. Surrounding the joints, apparently just
below the sheaths, but really breaking through their bases, are
circles of slender branches resembling the main branch, but more
slender. The sterile branches grow to a height of forty to fifty
centimetres, and from their bushy form the popular name of the plant,
"horse-tail," is taken. The surface of the plant is hard and rough,
due to the presence of great quantities of flint in the epidermis,--a
peculiarity common to all the species.

The stem is mainly composed of large, thin-walled cells, becoming
smaller as they approach the epidermis. The outer cells of the
ground tissue in the green branches contain chlorophyll, and the
walls of some of them are thickened. The fibro-vascular bundles
differ entirely from those of the ferns. Each bundle is nearly
triangular in section (_E_), with the point inward, and the inner
end occupied by a large air space. The tracheary tissue is only
slightly developed, being represented by a few vessels[9] (_tr._) at
the outer angles of the bundle, and one or two smaller ones close to
the air channel. The rest of the bundle is made up of nearly
uniform, rather thin-walled, colorless cells, some of which,
however, are larger, and have perforated cross-walls, representing
the sieve tubes of the fern bundle. There is no individual bundle
sheath, but the whole circle of bundles has a common outer sheath.

Copyright (c) 2007. topboookz.com. All rights reserved.