Friedrich Hebbel - "Agnes Bernauer"

Robert Neilson Stephens - "An Enemy To The King"

Victor Appleton - "Tom Swift and His Motor Cycle"

Edmond Holmes - "What Is and What Might Be"

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




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, x 3. _p_, pith. _c_, bark. The radiating lines
are medullary rays. _r_, resin ducts. _B_, part of the same, x 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, x 150. _D_, longitudinal section of a branch,
showing the form of the tracheids and the bordered pits upon their
walls. _m_, medullary ray, x 150. _E_, part of a sieve tube, x 300.
_F_, cross-section of a tracheid passing through two of the pits in
the wall (_p_), x 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, x 150. _H_,
cross-section of a resin duct, x 150. _I_, cross-section of a leaf,
x 20. _fb._ fibro-vascular bundle. _r_, resin duct. _J_, section of a
breathing pore, x 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 ([Male]), x 1/2. _B_, a similar
branch, with two young female flowers ([Female]), natural size. _C_, a
scale from a male flower, showing the two sporangia (_sp._); x 5. _D_,
a single ripe pollen spore (microspore), showing the vegetative cell
(_x_), x 150. _E_, a similar scale, from a female flower of the
Austrian pine, seen from within, x 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, x 10. _H_, the neck of the archegonium, x 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 (_Cycadeae_);
II., conifers (_Coniferae_); III., joint firs (_Gnetaceae_). 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._), x 1/4.
_B_, leaf of _Gingko_, x 1/2. _C_, branch of hemlock (_Tsuga_), with a
ripe cone, x 1. _D_, red cedar (_Juniperus_), x 1. _E_, _Arbor-vitae_
(_Thuja_), x 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-vitae_ (_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.




CHAPTER XV.

SPERMAPHYTES.


CLASS II.--ANGIOSPERMS.

The angiosperms include an enormous assemblage of plants, all those
ordinarily called "flowering plants" belonging here. There is almost
infinite variety shown in the form and structure of the tissues and
organs, this being particularly the case with the flowers. As already
stated, the ovules, instead of being borne on open carpels, are
enclosed in a cavity formed by a single closed carpel or several
united carpels. To the organ so formed the name "pistil" is usually
applied, and this is known as "simple" or "compound," as it is
composed of one or of two or more carpels. The leaves bearing the
pollen spores are also much modified, and form the so-called
"stamens." In addition to the spore-bearing leaves there are usually
other modified leaves surrounding them, these being often brilliantly
colored and rendering the flower very conspicuous. To these leaves
surrounding the sporophylls, the general name of "perianth" or
"perigone" is given. The perigone has a twofold purpose, serving both
to protect the sporophylls, and, at least in bright-colored flowers,
to attract insects which, as we shall see, are important agents in
transferring pollen from one flower to another.

When we compare the embryo sac (macrospore) of the angiosperms with
that of the gymnosperms a great difference is noticed, there being
much more difference than between the latter and the higher
pteridophytes. Unfortunately there are very few plants where the
structure of the embryo sac can be readily seen without very skilful
manipulation.

There are, however, a few plants in which the ovules are very small
and transparent, so that they may be mounted whole and examined
alive. The best plant for this purpose is probably the "Indian pipe"
or "ghost flower," a curious plant growing in rich woods, blossoming
in late summer. It is a parasite or saprophyte, and entirely
destitute of chlorophyll, being pure white throughout. It bears a
single nodding flower at the summit of the stem. (Another species
much like it, but having several brownish flowers, is shown in
Figure 115, _L_.)

If this plant can be had, the structure of the ovule and embryo sac
may be easily studied, by simply stripping away the tissue bearing
the numerous minute ovules, and mounting a few of them in water, or
water to which a little sugar has been added.

[Illustration: FIG. 79.--_A_, ripe ovule of _Monotropa uniflora_, in
optical section, x 100. _m_, micropyle. _e_, embryo sac. _B_, the
embryo sac, x 300. At the top is the egg apparatus, consisting of the
two synergidae (_s_), and the egg cell (_o_). In the centre is the
"endosperm nucleus" (_k_). At the bottom, the "antipodal cells" (_g_).]

The ovules are attached to a stalk, and each consists of about two
layers of colorless cells enclosing a central, large, oblong cell
(Fig. 79, _A_, _E_), the embryo sac or macrospore. If the ovule is
from a flower that has been open for some time, we shall find in the
centre of the embryo sac a large nucleus (_k_) (or possibly two
which afterward unite into one), and at each end three cells. Those
at the base (_g_) probably represent the prothallium, and those at
the upper end a very rudimentary archegonium, here generally called
the "egg apparatus."

Of the three cells of the "egg apparatus" the lower (_o_) one is the
egg cell; the others are called "synergidae." The structure of the
embryo sac and ovules is quite constant among the angiosperms, the
differences being mainly in the shape of the ovules, and the degree
to which its coverings or integuments are developed.

The pollen spores of many angiosperms will germinate very easily in
a solution of common sugar in water: about fifteen per cent of sugar
is the best. A very good plant for this purpose is the sweet pea,
whose pollen germinates very rapidly, especially in warm weather.
The spores may be sown in a little of the sugar solution in any
convenient vessel, or in a hanging drop suspended in a moist
chamber, as described for germinating the spores of the slime
moulds. The tube begins to develop within a few minutes after the
spores are placed in the solution, and within an hour or so will
have reached a considerable length. Each spore has two nuclei, but
they are less evident here than in some other forms (Fig. 79).

[Illustration: FIG. 80.--Germinating pollen spores of the sweet pea,
x 200.]

The upper part of the pistil is variously modified, having either
little papillae which hold the pollen spores, or are viscid. In either
case the spores germinate when placed upon this receptive part
(stigma) of the pistil, and send their tubes down through the tissues
of the pistil until they reach the ovules, which are fertilized much
as in the gymnosperms.

The effect of fertilization extends beyond the ovule, the ovary and
often other parts of the flower being affected, enlarging and often
becoming bright-colored and juicy, forming the various fruits of the
angiosperms. These fruits when ripe may be either dry, as in the case
of grains of various kinds, beans, peas, etc.; or the ripe fruit may
be juicy, serving in this way to attract animals of many kinds which
feed on the juicy pulp, and leave the hard seeds uninjured, thus
helping to distribute them. Common examples of these fleshy fruits are
offered by the berries of many plants; apples, melons, cherries, etc.,
are also familiar examples.

The seeds differ a good deal both in regard to size and the degree to
which the embryo is developed at the time the seed ripens.


CLASSIFICATION OF THE ANGIOSPERMS.

The angiosperms are divided into two sub-classes: I. _Monocotyledons_
and II. _Dicotyledons_.

The monocotyledons comprise many familiar plants, both ornamental and
useful. They have for the most part elongated, smooth-edged leaves
with parallel veins, and the parts of the flower are in threes in the
majority of them. As their name indicates, there is but one cotyledon
or seed leaf, and the leaves from the first are alternate. As a rule
the embryo is very small and surrounded by abundant endosperm.

The most thoroughly typical members of the sub-class are the lilies
and their relatives. The one selected for special study here, the
yellow adder-tongue, is very common in the spring; but if not
accessible, almost any liliaceous plant will answer. Of garden
flowers, the tulip, hyacinth, narcissus, or one of the common lilies
may be used; of wild flowers, the various species of _Trillium_
(Fig. 83, _A_) are common and easily studied forms, but the leaves are
not of the type common to most monocotyledons.

The yellow adder-tongue (_Erythronium americanum_) (Fig. 81) is one of
the commonest and widespread of wild flowers, blossoming in the
northern states from about the middle of April till the middle of May.
Most of the plants found will not be in flower, and these send up but
a single, oblong, pointed leaf. The flowering plant has two similar
leaves, one of which is usually larger than the other. They seem to
come directly from the ground, but closer examination shows that they
are attached to a stem of considerable length entirely buried in the
ground. This arises from a small bulb (_B_) to whose base numerous
roots (_r_) are attached. Rising from between the leaves is a slender,
leafless stalk bearing a single, nodding flower at the top.

The leaves are perfectly smooth, dull purplish red on the lower side,
and pale green with purplish blotches above. The epidermis may be very
easily removed, and is perfectly colorless. Examined closely,
longitudinal rows of whitish spots may be detected: these are the
breathing pores.

[Illustration: FIG. 81.--_A_, plant of the yellow adder-tongue
(_Erythronium americanum_), x 1/3. _B_, the bulb of the same, x 1/2. _r_,
roots. _C_, section of _B_. _st._ the base of the stem bearing the
bulb for next year (_b_) at its base. _D_, a single petal and stamen,
x 1/2. _f_, the filament. _an._ anther. _E_, the gynoecium (pistil), x 1.
_o_, ovary. _st._ style. _z_, stigma. _F_, a full-grown fruit, x 1/2.
_G_, section of a full-grown macrosporangium (ovule), x 25: i, ii, the
two integuments. _sp._ macrospore (embryo sac). _H_, cross-section of
the ripe anther, x 12. _I_, a single pollen spore, x 150, showing the
two nuclei (_n_, _n'_). _J_, a ripe seed, x 2. _K_, the same, in
longitudinal section. _em._ the embryo. _L_, cross-section of the
stem, x 12. _fb._ fibro-vascular bundle. _M_, diagram of the flower.]

A cross-section of the stem shows numerous whitish areas scattered
through it. These are the fibro-vascular bundles which in the
monocotyledons are of a simple type. The bulb is composed of thick
scales, which are modified leaves, and on cutting it lengthwise, we
shall probably find the young bulb of next year (Fig. _C_, _b_)
already forming inside it, the young bulb arising as a bud at the
base of the stem of the present year.

The flower is made up of five circles of very much modified leaves,
three leaves in each set. The two outer circles are much alike, but
the three outermost leaves are slightly narrower and strongly tinged
with red on the back, completely concealing the three inner ones
before the flower expands. The latter are pure yellow, except for a
ridge along the back, and a few red specks near the base inside. These
six leaves constitute the perigone of the flower; the three outer are
called sepals, the inner ones petals.

The next two circles are composed of the sporophylls bearing the
pollen spores.[12] These are the stamens, and taken collectively are
known as the "_Androecium_." Each leaf or stamen consists of two
distinct portions, a delicate stalk or "filament" (_D_, _f_), and the
upper spore-bearing part, the "anther" (_an._). The anther in the
freshly opened flower has a smooth, red surface; but shortly after,
the flower opens, splits along each side, and discharges the pollen
spores. A section across the anther shows it to be composed of four
sporangia or pollen sacs attached to a common central axis
("connective") (Fig. _H_).

[12] The three outer stamens are shorter than the inner set.

The central circle of leaves, the carpels (collectively the
"gynoecium") are completely united to form a compound pistil (Fig. 81,
_E_). This shows three distinct portions, the ovule-bearing portion
below (_o_), the "ovary," a stalk above (_st._), the "style," and the
receptive portion (_z_) at the top, the "stigma." Both stigma and
ovary show plainly their compound nature, the former being divided
into three lobes, the latter completely divided into three chambers,
as well as being flattened at the sides with a more or less decided
seam at the three angles. The ovules, which are quite large, are
arranged in two rows in each chamber of the ovary, attached to the
central column ("placenta").

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