Elements of Structural and Systematic Botany

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

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[Illustration: FIG. 60.--_A_, _B_, young antheridia of _Funaria_,
optical section, x 150. _C_, two sperm cells of _Atrichum_. _D_,
spermatozoids of _Sphagnum_, x 600.]

Starch is composed of carbon, hydrogen, and oxygen, and so far as is
known is only produced by chlorophyll-bearing cells, under the
influence of light. The carbon used in the manufacture of starch is
taken from the atmosphere in the form of carbonic acid, so that
green plants serve to purify the atmosphere by the removal of this
substance, which is deleterious to animal life, while at the same
time the carbon, an essential part of all living matter, is combined
in such form as to make it available for the food of other
organisms.

The marginal cells of the leaf are narrow, and some of them
prolonged into teeth.

A cross-section of the stem (63, _E_) shows on the outside a single
row of epidermal cells, then larger chlorophyll-bearing cells, and
in the centre a group of very delicate, small, colorless cells,
which in longitudinal section are seen to be elongated, and similar
to those forming the midrib of the leaf. These cells probably serve
for conducting fluids, much as the similar but more perfectly
developed bundles of cells (fibro-vascular bundles) found in the
stems and leaves of the higher plants.

The root hairs, fastening the plant to the ground, are rows of
cells with brown walls and oblique partitions. They often merge
insensibly into the green filaments (protonema) already noticed.
These latter have usually colorless walls, and more numerous
chloroplasts, looking very much like a delicate specimen of
_Cladophora_ or some similar alga. If a sufficient number of these
filaments is examined, some of them will probably show young moss
plants growing from them (Fig. 63, _A_, _k_), and with a little
patience the leafy plant can be traced back to a little bud
originating as a branch of the filament. Its diameter is at first
scarcely greater than that of the filament, but a series of walls,
close together, are formed, so placed as to cut off a pyramidal cell
at the top, forming the apical cell of the young moss plant. This
apical cell has the form of a three-sided pyramid with the base
upward. From it are developed three series of cells, cut off in
succession from the three sides, and from these cells are derived
all the tissues of the plant which soon becomes of sufficient size
to be easily recognizable.

The protonemal filaments may be made to grow from almost any part of
the plant by keeping it moist, but grow most abundantly from the
base of the stem.

The sexual organs are much like those of the liverworts and are
borne at the apex of the stems.

The antheridia (Figs. 59, 60) are club-shaped bodies with a short
stalk. The upper part consists of a single layer of large
chlorophyll-bearing cells, enclosing a mass of very small, nearly
cubical, colorless, sperm cells each of which contains an
excessively small spermatozoid.

The young antheridium has an apical cell giving rise to two series
of segments (Fig. 60, _A_), which in the earlier stages are very
plainly marked.

When ripe the chlorophyll in the outer cells changes color, becoming
red, and if a few such antheridia from a plant that has been kept
rather dry for a day or two, are teased out in a drop of water, they
will quickly open at the apex, the whole mass of sperm cells being
discharged at once.

Among the antheridia are borne peculiar hairs (Fig. 59, _p_) tipped
by a large globular cell.

[Illustration: FIG. 61.--_A_, _B_, young; _C_, nearly ripe archegonium
of _Funaria_, optical section, x 150. _D_, upper part of the neck of
_C_, seen from without, showing how it is twisted. _E_, base of a ripe
archegonium. _F_, open apex of the same, x 150. _o_, egg cell. _b_,
ventral canal cell.]

Owing to their small size the spermatozoids are difficult to see
satisfactorily and other mosses (_e.g._ peat mosses, Figure 64, the
hairy cap moss, Figure 65, _I_), are preferable where obtainable.
The spermatozoids of a peat moss are shown in Figure 60, _D_. Like
all of the bryophytes they have but two cilia.

The archegonia (Fig. 61) should be looked for in the younger plants
in the neighborhood of those that bear capsules. Like the antheridia
they occur in groups. They closely resemble those of the liverworts,
but the neck is longer and twisted and the base more massive.
Usually but a single one of the group is fertilized.

[Illustration: FIG. 62.--_A_, young embryo of _Funaria_, still
enclosed within the base of the archegonium, x 300. _B_, an older
embryo freed from the archegonium, x 150. _a_, the apical cell.]

To study the first division of the embryo, it is usually necessary
to render the archegonium transparent, which may be done by using a
little caustic potash; or letting it lie for a few hours in dilute
glycerine will sometimes suffice. If potash is used it must be
thoroughly washed away, by drawing pure water under the cover glass
with a bit of blotting paper, until every trace of the potash is
removed. The first wall in the embryo is nearly at right angles to
the axis of the archegonium and divides the egg cell into nearly
equal parts. This is followed by nearly vertical walls in each cell
(Fig. 62, _A_). Very soon a two-sided apical cell (Fig. 62, _B_,
_a_) is formed in the upper half of the embryo, which persists until
the embryo has reached a considerable size. As in the liverworts the
young embryo is completely covered by the growing archegonium wall.

The embryo may be readily removed from the archegonium by adding a
little potash to the water in which it is lying, allowing it to
remain for a few moments and pressing gently upon the cover glass
with a needle. In this way it can be easily forced out of the
archegonium, and then by thoroughly washing away the potash,
neutralizing if necessary with a little acetic acid, very beautiful
preparations may be made. If desired, these may be mounted
permanently in glycerine which, however, must be added very
gradually to avoid shrinking the cells.

[Illustration: FIG. 63.--_A_, protonema of _Funaria_, with a bud
(_k_), x 50. _B_, outline of a leaf, showing also the thickened
midrib, x 12. _C_, cells of the leaf, x 300. _n_, nucleus. _D_,
chlorophyll granules undergoing division, x 300. _E_, cross-section of
the stem, x 50.]

For some time the embryo has a nearly cylindrical form, but as it
approaches maturity the differentiation into stalk and capsule
becomes apparent. The latter increases rapidly in diameter, assuming
gradually the oval shape of the full-grown capsule. A longitudinal
section of the nearly ripe capsule (Fig. 58, _G_) shows two distinct
portions; an outer wall of two layers of cells, and an inner mass of
cells in some of which the spores are produced. This inner mass of
cells is continuous with the upper part of the capsule, but
connected with the side walls and bottom by means of slender,
branching filaments of chlorophyll-bearing cells.

The spores arise from a single layer of cells near the outside of
the inner mass of cells (_G_, _sp._). These cells (_H_, _sp._) are
filled with glistening, granular protoplasm; have a large and
distinct nucleus, and no chlorophyll. They finally become entirely
separated and each one gives rise to four spores which closely
resemble those of the liverworts but are smaller.

Near the base of the capsule, on the outside, are formed breathing
pores (Fig. 58, _F_) quite similar to those of the higher plants.

If the spores are kept in water for a few days they will germinate,
bursting the outer brown coat, and the contents protruding through
the opening surrounded by the colorless inner spore membrane. The
protuberance grows rapidly in length and soon becomes separated from
the body of the spore by a wall, and lengthening, more and more,
gives rise to a green filament like those we found attached to the
base of the full-grown plant, and like those giving rise to buds
that develop into leafy plants.

CLASSIFICATION OF THE MOSSES.

The mosses may be divided into four orders: I. The peat mosses
(_Sphagnaceae_); II. _Andreaeaceae_; III. _Phascaceae_; IV. The common
mosses (_Bryaceae_).

[Illustration: FIG. 64.--_A_, a peat moss (_Sphagnum_), x 1/2. _B_, a
sporogonium of the same, x 3. _C_, a portion of a leaf, x 150. The
narrow, chlorophyll-bearing cells form meshes, enclosing the large,
colorless empty cells, whose walls are marked with thickened bars, and
contain round openings (_o_).]

The peat mosses (Fig. 64) are large pale-green mosses, growing often
in enormous masses, forming the foundation of peat-bogs. They are of a
peculiar spongy texture, very light when dry, and capable of absorbing
a great amount of water. They branch (Fig. 64, _A_), the branches
being closely crowded at the top, where the stems continue to grow,
dying away below.

[Illustration: FIG. 65.--Forms of mosses. _A_, plant of _Phascum_,
x 3. _B_, fruiting plant of _Atrichum_, x 2. _C_, young capsule of
hairy-cap moss (_Polytrichum_), covered by the large, hairy calyptra.
_D_, capsules of _Bartramia_: i, with; ii, without the calyptra. _E_,
upper part of a male plant of _Atrichum_, showing the flower, x 2.
_F_, a male plant of _Mnium_, x 4. _G_, pine-tree moss (_Clemacium_),
x 1. _H_, _Hypnum_, x 1. _I_, ripe capsules of hairy-cap moss: i,
with; ii, without calyptra.]

The sexual organs are rarely met with, but should be looked for late
in autumn or early spring. The antheridial branches are often
bright-colored, red or yellow, so as to be very conspicuous. The
capsules, which are not often found, are larger than in most of the
common mosses, and quite destitute of a stalk, the apparent stalk
being a prolongation of the axis of the plant in the top of which the
base of the sporogonium is imbedded. The capsule is nearly globular,
opening by a lid at the top (Fig. 64, _B_).

A microscopical examination of the leaves, which are quite destitute
of a midrib, shows them to be composed of a network of narrow
chlorophyll-bearing cells surrounding much larger empty ones whose
walls are marked with transverse thickenings, and perforated here
and there with large, round holes (Fig. 64, _C_). It is to the
presence of these empty cells that the plant owes its peculiar
spongy texture, the growing plants being fairly saturated with
water.

The _Andreaeaceae_ are very small, and not at all common. The capsule
splits into four valves, something like a liverwort.

The _Phascaceae_ are small mosses growing on the ground or low down on
the trunks of trees, etc. They differ principally from the common
mosses in having the capsule open irregularly and not by a lid. The
commonest forms belong to the genus _Phascum_ (Fig. 65, _A_).

The vast majority of the mosses the student is likely to meet with
belong to the last order, and agree in the main with the one
described. Some of the commoner forms are shown in Figure 65.

CHAPTER XII.

SUB-KINGDOM V.

PTERIDOPHYTES.

If we compare the structure of the sporogonium of a moss or liverwort
with the plant bearing the sexual organs, we find that its tissues are
better differentiated, and that it is on the whole a more complex
structure than the plant that bears it. It, however, remains attached
to the parent plant, deriving its nourishment in part through the
"foot" by means of which it is attached to the plant.

In the Pteridophytes, however, we find that the sporogonium becomes
very much more developed, and finally becomes entirely detached from
the sexual plant, developing in most cases roots that fasten it to the
ground, after which it may live for many years, and reach a very large
size.

The sexual plant, which is here called the "prothallium," is of very
simple structure, resembling the lower liverworts usually, and never
reaches more than about a centimetre in diameter, and is often much
smaller than this.

The common ferns are the types of the sub-kingdom, and a careful study
of any of these will illustrate the principal peculiarities of the
group. The whole plant, as we know it, is really nothing but the
sporogonium, originating from the egg cell in exactly the same way as
the moss sporogonium, and like it gives rise to spores which are
formed upon the leaves.

The spores may be collected by placing the spore-bearing leaves on
sheets of paper and letting them dry, when the ripe spores will be
discharged covering the paper as a fine, brown powder. If these are
sown on fine, rather closely packed earth, and kept moist and covered
with glass so as to prevent evaporation, within a week or two a fine,
green, moss-like growth will make its appearance, and by the end of
five or six weeks, if the weather is warm, little, flat, heart-shaped
plants of a dark-green color may be seen. These look like small
liverworts, and are the sexual plants (prothallia) of our ferns
(Fig. 66, _F_). Removing one of these carefully, we find on the lower
side numerous fine hairs like those on the lower surface of the
liverworts, which fasten it firmly to the ground. By and by, if our
culture has been successful, we may find attached to some of the
larger of these, little fern plants growing from the under side of the
prothallia, and attached to the ground by a delicate root. As the
little plant becomes larger the prothallium dies, leaving it attached
to the ground as an independent plant, which after a time bears the
spores.

[Illustration: FIG. 66.--_A_, spore of the ostrich fern (_Onoclea_),
with the outer coat removed. _B_, germinating spore, x 150. _C_, young
prothallium, x 50. _r_, root hair. _sp._ spore membrane. _D_, _E_,
older prothallia. _a_, apical cell, x 150. _F_, a female prothallium,
seen from below, x 12. _ar._ archegonia. _G_, _H_, young archegonia,
in optical section, x 150. _o_, central cell. _b_, ventral canal cell.
_c_, upper canal cell. _I_, a ripe archegonium in the act of opening,
x 150. _o_, egg cell. _J_, a male prothallium, x 50. _an._ antheridia.
_K_, _L_, young antheridia, in optical section, x 300. _M_, ripe
antheridium, x 300. _sp._ sperm cells. _N_, _O_, antheridia that have
partially discharged their contents, x 300. _P_, spermatozoids, killed
with iodine, x 500. _v_, vesicle attached to the hinder end.]

In choosing spores for germination it is best to select those of large
size and containing abundant chlorophyll, as they germinate more
readily. Especially favorable for this purpose are the spores of the
ostrich fern (_Onoclea struthiopteris_) (Fig. 70, _I_, _J_), or the
sensitive fern (_O. sensibilis_). Another common and readily grown
species is the lady fern (_Asplenium filixfoemina_) (Fig. 70, _H_). The
spores of most ferns retain their vitality for many months, and hence
can be kept dry until wanted.

The first stages of germination may be readily seen by sowing the
spores in water, where, under favorable circumstances, they will
begin to grow within three or four days. The outer, dry, brown coat
of the spore is first ruptured, and often completely thrown off by
the swelling of the spore contents. Below this is a second colorless
membrane which is also ruptured, but remains attached to the spore.
Through the orifice in the second coat, the inner delicate membrane
protrudes in the form of a nearly colorless papilla which rapidly
elongates and becomes separated from the body of the spore by a
partition, constituting the first root hair (Fig. 66, _B_, _C_,
_r_). The body of the spore containing most of the chlorophyll
elongates more slowly, and divides by a series of transverse walls
so as to form a short row of cells, resembling in structure some of
the simpler algae (_C_).

In order to follow the development further, spores must be sown upon
earth, as they do not develop normally in water beyond this stage.

In studying plants grown on earth, they should be carefully removed
and washed in a drop of water so as to remove, as far as possible,
any adherent particles, and then may be mounted in water for
microscopic examination.

In most cases, after three or four cross-walls are formed, two walls
arise in the end cell so inclined as to enclose a wedge-shaped cell
(_a_) from which are cut off two series of segments by walls
directed alternately right and left (Fig. 66, _D_, _E_, _a_), the
apical cell growing to its original dimensions after each pair of
segments is cut off. The segments divide by vertical walls in
various directions so that the young plant rapidly assumes the form
of a flat plate of cells attached to the ground by root hairs
developed from the lower surfaces of the cells, and sometimes from
the marginal ones. As the division walls are all vertical, the plant
is nowhere more than one cell thick. The marginal cells of the young
segments divide more rapidly than the inner ones, and soon project
beyond the apical cell which thus comes to lie at the bottom of a
cleft in the front of the plant which in consequence becomes
heart-shaped (_E_, _F_). Sooner or later the apical cell ceases to
form regular segments and becomes indistinguishable from the other
cells.

In the ostrich fern and lady fern the plants are dioecious. The male
plants (Fig. 66, _J_) are very small, often barely visible to the
naked eye, and when growing thickly form dense, moss-like patches.
They are variable in form, some irregularly shaped, others simple
rows of cells, and some have the heart shape of the larger plants.

The female plants (Fig. 66, _F_) are always comparatively large and
regularly heart-shaped, occasionally reaching a diameter of nearly or
quite one centimetre, so that they are easily recognizable without
microscopical examination.

All the cells of the plant except the root hairs contain large and
distinct chloroplasts much like those in the leaves of the moss, and
like them usually to be found in process of division.

The archegonia arise from cells of the lower surface, just behind
the notch in front (Fig. 66, _F_, _ar._). Previous to their
formation the cells at this point divide by walls parallel to the
surface of the plant, so as to form several layers of cells, and
from the lowest layer of cells the archegonia arise. They resemble
those of the liverworts but are shorter, and the lower part is
completely sunk within the tissues of the plant (Fig. 66, _G_, _I_).
They arise as single surface cells, this first dividing into three
by walls parallel to the outer surface. The lower cell undergoes one
or two divisions, but undergoes no further change; the second cell
(_C_, _o_), becomes the egg cell, and from it is cut off another
cell (_c_), the canal cell of the neck; the uppermost of the three
becomes the neck. There are four rows of neck cells, the two forward
ones being longer than the others, so that the neck is bent
backward. In the full-grown archegonium, there are two canal cells,
the lower one (_H_, _b_) called the ventral canal cell, being
smaller than the other.

Shortly before the archegonium opens, the canal cells become
disorganized in the same way as in the bryophytes, and the
protoplasm of the central cell contracts to form the egg cell which
shows a large, central nucleus, and in favorable cases, a clear
space at the top called the "receptive spot," as it is here that the
spermatozoid enters. When ripe, if placed in water, the neck cells
become very much distended and finally open widely at the top, the
upper ones not infrequently being detached, and the remains of the
neck cells are forced out (Fig. 66, _I_).

The antheridia (Fig. 66. _J_, _M_) arise as simple hemispherical
cells, in which two walls are formed (_K_ I, II), the lower
funnel-shaped, the upper hemispherical and meeting the lower one so
as to enclose a central cell (shaded in the figure), from which the
sperm cells arise. Finally, a ring-shaped wall (_L_ iii) is formed,
cutting off a sort of cap cell, so that the antheridium at this
stage consists of a central cell, surrounded by three other cells,
the two lower ring-shaped, the upper disc-shaped. The central cell,
which contains dense, glistening protoplasm, is destitute of
chlorophyll, but the outer cells have a few small chloroplasts. The
former divides repeatedly, until a mass of about thirty-two sperm
cells is formed, each giving rise to a large spirally-coiled
spermatozoid. When ripe, the mass of sperm cells crowds so upon the
outer cells as to render them almost invisible, and as they ripen
they separate by a partial dissolving of the division walls. When
brought into water, the outer cells of the antheridium swell
strongly, and the matter derived from the dissolved walls of the
sperm cells also absorbs water, so that finally the pressure becomes
so great that the wall of the antheridium breaks, and the sperm
cells are forced out by the swelling up of the wall cells (_N_,
_O_). After lying a few moments in the water, the wall of each sperm
cell becomes completely dissolved, and the spermatozoids are
released, and swim rapidly away with a twisting movement. They may
be killed with a little iodine, when each is seen to be a somewhat
flattened band, coiled several times. At the forward end, the coils
are smaller, and there are numerous very long and delicate cilia. At
the hinder end may generally be seen a delicate sac (_P_, _v_),
containing a few small granules, some of which usually show the
reaction of starch, turning blue when iodine is applied.

In studying the development of the antheridia, it is only necessary
to mount the plants in water and examine them directly; but the
study of the archegonia requires careful longitudinal sections of
the prothallium. To make these, the prothallium should be placed
between small pieces of pith, and the razor must be very sharp. It
may be necessary to use a little potash to make the sections
transparent enough to see the structure, but this must be used
cautiously on account of the great delicacy of the tissues.

If a plant with ripe archegonia is placed in a drop of water, with
the lower surface uppermost, and at the same time male plants are
put with it, and the whole covered with a cover glass, the
archegonia and antheridia will open simultaneously; and, if examined
with the microscope, we shall see the spermatozoids collect about
the open archegonia, to which they are attracted by the substance
forced out when it opens. With a little patience, one or more may be
seen to enter the open neck through which it forces itself, by a
slow twisting movement, down to the egg cell. In order to make the
experiment successful, the plants should be allowed to become a
little dry, care being taken that no water is poured over them for a
day or two beforehand.

The first divisions of the fertilized egg cell resemble those in the
moss embryo, except that the first wall is parallel with the
archegonium axis, instead of at right angles to it. Very soon,
however, the embryo becomes very different, four growing points
being established instead of the single one found in the moss
embryo. The two growing points on the side of the embryo nearest the
archegonium neck grow faster than the others, one of these
outstripping the other, and soon becoming recognizable as the first
leaf of the embryo (Fig. 67, _A_, _L_). The other (_r_) is peculiar,
in having its growing point covered by several layers of cells, cut
off from its outer face, a peculiarity which we shall find is
characteristic of the roots of all the higher plants, and, indeed,
this is the first root of the young fern. Of the other two growing
points, the one next the leaf grows slowly, forming a blunt cone
(_st._), and is the apex of the stem. The other (_f_) has no
definite form, and serves merely as an organ of absorption, by means
of which nourishment is supplied to the embryo from the prothallium;
it is known as the foot.

[Illustration: FIG. 67.--_A_, embryo of the ostrich fern just before
breaking through the prothallium, x 50. _st._ apex of stem. _l_, first
leaf. _r_, first root. _ar._ neck of the archegonium. _B_, young
plant, still attached to the prothallium (_pr._). _C_, underground
stem of the maiden-hair fern (_Adiantum_), with one young leaf, and
the base of an older one, x 1. _D_, three cross-sections of a leaf
stalk: i, nearest the base; iii, nearest the blade of the leaf,
showing the division of the fibro-vascular bundle, x 5. _E_, part of
the blade of the leaf, x 1/2. _F_, a single spore-bearing leaflet,
showing the edge folded over to cover the sporangia, x 1. _G_, part of
the fibro-vascular bundle of the leaf stalk (cross-section), x 50.
_x_, woody part of the bundle. _y_, bast. _sh._ bundle sheath. _H_, a
small portion of the same bundle, x 150. _I_, stony tissue from the
underground stem, x 150. _J_, sieve tube from the underground stem,
x 300.]

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