SUB-KINGDOM IV.
BRYOPHYTA.
The Bryophytes, or mosses, are for the most part land plants, though a
few are aquatic, and with very few exceptions are richly supplied with
chlorophyll. They are for the most part small plants, few of them
being over a few centimetres in height; but, nevertheless, compared
with the plants that we have heretofore studied, quite complex in
their structure. The lowest members of the group are flattened,
creeping plants, or a few of them floating aquatics, without distinct
stem and leaves; but the higher ones have a pretty well-developed
central axis or stem, with simple leaves attached.
There are two classes--I. Liverworts (_Hepaticæ_), and II. Mosses
(_Musci_).
CLASS I.--THE LIVERWORTS.
One of the commonest of this class, and to be had at any time, is
named _Madotheca_. It is one of the highest of the class, having
distinct stem and leaves. It grows most commonly on the shady side of
tree trunks, being most luxuriant near the ground, where the supply of
moisture is most constant. It also occurs on stones and rocks in moist
places. It closely resembles a true moss in general appearance, and
from the scale-like arrangement of its leaves is sometimes called
"scale moss."
The leaves (Fig. 52, _A_, _B_) are rounded in outline unequally,
two-lobed, and arranged in two rows on the upper side of the stem, so
closely overlapping as to conceal it entirely. On the under side are
similar but smaller leaves, less regularly disposed. The stems branch
at intervals, the branches spreading out laterally so that the whole
plant is decidedly flattened. On the under side are fine, whitish
hairs, that fasten it to the substratum. If we examine a number of
specimens, especially early in the spring, a difference will be
observed in the plants. Some of them will be found to bear peculiar
structures (Fig. 52, _C_, _D_), in which the spores are produced.
These are called "sporogonia." They are at first globular, but when
ripe open by means of four valves, and discharge a greenish brown mass
of spores. An examination of the younger parts of the same plants will
probably show small buds (Fig. 54, _H_), which contain the female
reproductive organs, from which the sporogonia arise.
[Illustration: FIG. 52.--_A_, part of a plant of a leafy liverwort
(_Madotheca_), × 2. _B_, part of the same, seen from below, × 4. _C_,
a branch with two open sporogonia (_sp._), × 4. _D_, a single
sporogonium, × 8.]
On other plants may be found numerous short side branches (Fig. 53,
_B_), with very closely set leaves. If these are carefully separated,
the antheridia can just be seen as minute whitish globules, barely
visible to the naked eye. Plants that, like this one, have the male
and female reproductive organs on distinct plants, are said to be
"diœcious."
A microscopical examination of the stem and leaves shows their
structure to be very simple. The former is cylindrical, and composed
of nearly uniform elongated cells, with straight cross-walls. The
leaves consist of a single layer of small, roundish cells, which,
like those of the stem, contain numerous rounded chloroplasts, to
which is due their dark green color.
The tissues are developed from a single apical cell, but it is
difficult to obtain good sections through it.
The antheridia are borne singly at the bases of the leaves on the
special branches already described (Fig. 53, _A_, _an._). By
carefully dissecting with needles such a branch in a drop of water,
some of the antheridia will usually be detached uninjured, and may
be readily studied, the full-grown ones being just large enough to
be seen with the naked eye. They are globular bodies, attached by a
stalk composed of two rows of cells. The globular portion consists
of a wall of chlorophyll-bearing cells, composed of two layers
below, but single above (Fig. 53, _C_). Within is a mass of
excessively small cells, each of which contains a spermatozoid. In
the young antheridium (_A_, _an._) the wall is single throughout,
and the central cells few in number. To study them in their natural
position, thin longitudinal sections of the antheridial branch
should be made.
[Illustration: FIG. 53.--_A_, end of a branch from a male plant of
_Madotheca_. The small side branchlets bear the antheridia, × 2. _B_,
two young antheridia (_an._), the upper one seen in optical section,
the lower one from without, × 150. _C_, a ripe antheridium, optical
section, × 50. _D_, sperm cells with young spermatozoids. _E_, ripe
spermatozoids, × 600.]
When ripe, if brought into water, the antheridium bursts at the top
into a number of irregular lobes that curl back and allow the mass
of sperm cells to escape. The spermatozoids, which are derived
principally from the nucleus of the sperm cells (53, _D_) are so
small as to make a satisfactory examination possible only with very
powerful lenses. The ripe spermatozoid is coiled in a flat spiral
(53, _E_), and has two excessively delicate cilia, visible only
under the most favorable circumstances.
The female organ in the bryophytes is called an "archegonium," and
differs considerably from anything we have yet studied, but recalls
somewhat the structure of the oögonium of _Chara_. They are found in
groups, contained in little bud-like branches (54, _H_). In order to
study them, a plant should be chosen that has numbers of such buds,
and the smallest that can be found should be used. Those containing
the young archegonia are very small; but after one has been
fertilized, the leaves enclosing it grow much larger, and the bud
becomes quite conspicuous, being surrounded by two or three
comparatively large leaves. By dissecting the young buds, archegonia
in all stages of growth may be found.
[Illustration: FIG. 54.--_A-D_, development of the archegonium of
_Madotheca_. _B_, surface view, the others in optical section. _o_,
egg cell, × 150. _E_, base of a fertilized archegonium, containing a
young embryo (_em._), × 150. _F_, margin of one of the leaves
surrounding the archegonia. _G_, young sporogonium still surrounded by
the much enlarged base of the archegonium. _h_, neck of the
archegonium. _ar._ abortive archegonia, × 12. _H_, short branch
containing the young sporogonium, × 4.]
When very young the archegonium is composed of an axial row of three
cells, surrounded by a single outer layer of cells, the upper ones
forming five or six regular rows, which are somewhat twisted
(Fig. 54, _A_, _B_). As it becomes older, the lower part enlarges
slightly, the whole looking something like a long-necked flask (_C_,
_D_). The centre of the neck is occupied by a single row of cells
(canal cells), with more granular contents than the outer cells, the
lowest cell of the row being somewhat larger than the others
(Fig. 54, _C_, _o_). When nearly ripe, the division walls of the
canal cells are absorbed, and the protoplasm of the lowest cell
contracts and forms a globular naked cell, the egg cell (_D_, _o_).
If a ripe archegonium is placed in water, it soon opens at the top,
and the contents of the canal cells are forced out, leaving a clear
channel down to the egg cell. If the latter is not fertilized, the
inner walls of the neck cells turn brown, and the egg cell dies; but
if a spermatozoid penetrates to the egg cell, the latter develops a
wall and begins to grow, forming the embryo or young sporogonium.
[Illustration: FIG. 55.--Longitudinal section of a nearly full-grown
sporogonium of _Madotheca_, which has not, however, broken through the
overlying cells, × 25. _sp._ cavity in which the spores are formed.
_ar._ abortive archegonium.]
The first division wall to be formed in the embryo is transverse,
and is followed by vertical ones (Fig. 54, _E_, _em._). As the
embryo enlarges, the walls of the basal part of the archegonium grow
rapidly, so that the embryo remains enclosed in the archegonium
until it is nearly full-grown (Fig. 55). As it increases in size, it
becomes differentiated into three parts: a wedge-shaped base or
"foot" penetrating downward into the upper part of the plant, and
serving to supply the embryo with nourishment; second, a stalk
supporting the third part, the capsule or spore-bearing portion of
the fruit. The capsule is further differentiated into a wall, which
later becomes dark colored, and a central cavity, in which are
developed special cells, some of which by further division into four
parts produce the spores, while the others, elongating enormously,
give rise to special cells, called elaters (Fig. 56, _B_).
[Illustration: FIG. 56.--Spore (_A_) and two elaters (_B_) of
_Madotheca_, × 300.]
The ripe spores are nearly globular, contain chlorophyll and drops
of oil, and the outer wall is brown and covered with fine points
(Fig. 56, _A_). The elaters are long-pointed cells, having on the
inner surface of the wall a single or double dark brown spiral band.
These bands are susceptible to changes in moisture, and by their
movements probably assist in scattering the spores after the
sporogonium opens.
Just before the spores are ripe, the stalk of the sporogonium
elongates rapidly, carrying up the capsule, which breaks through the
archegonium wall, and finally splits into four valves, and discharges
the spores.
There are four orders of the liverworts represented in the United
States, three of which differ from the one we have studied in being
flattened plants, without distinct stems and leaves,--at least, the
leaves when present are reduced to little scales upon the lower
surface.
The first order (_Ricciaceæ_) are small aquatic forms, or grow on damp
ground or rotten logs. They are not common forms, and not likely to be
encountered by the student. One of the floating species is shown in
figure 57, _A_.
The second order, the horned liverworts (_Anthoceroteæ_), are
sometimes to be met with in late summer and autumn, forms growing
mostly on damp ground, and at once recognizable by their long-pointed
sporogonia, which open when ripe by two valves, like a bean pod
(Fig. 57, _B_).
The third order (_Marchantiaceæ_) includes the most conspicuous
members of the whole class. Some of them, like the common liverwort
(_Marchantia_), shown in Figure 57, _F_, _K_, and the giant liverwort
(Fig. 57, _D_), are large and common forms, growing on the ground in
shady places, the former being often found also in greenhouses. They
are fastened to the ground by numerous fine, silky hairs, and the
tissues are well differentiated, the upper surface of the plant having
a well-marked epidermis, with peculiar breathing pores, large enough
to be seen with the naked eye (Fig. 57, _E_, _J_, _K_) Each of these
is situated in the centre of a little area (Fig. 57, _E_), and beneath
it is a large air space, into which the chlorophyll-bearing cells
(_cl._) of the plant project (_J_).
The sexual organs are often produced in these forms upon special
branches (_G_), or the antheridia may be sunk in discs on the upper
side of the stem (_D_, _an._).
[Illustration: FIG. 57.--Forms of liverworts. _A_, _Riccia_, natural
size. _B_, _Anthoceros_ (horned liverwort), natural size. _sp._
sporogonia. _C_, _Lunularia_, natural size, _x_, buds. _D_, giant
liverwort (_Conocephalus_), natural size. _an._ antheridial disc. _E_,
small piece of the epidermis, showing the breathing pores, × 2. _F_,
common liverwort (_Marchantia_), × 2. _x_, cups containing buds. _G_,
archegonial branch of common liverwort, natural size. _H_, two young
buds from the common liverwort, × 150. _I_, a full-grown bud, × 25.
_J_, vertical section through the body of _Marchantia_, cutting
through a breathing pore (_s_), × 50. _K_, surface view of a breathing
pore, × 150. _L_, a leafy liverwort (_Jungermannia_). _sp._
sporogonium, × 2.]
Some forms, like _Marchantia_ and _Lunularia_ (Fig. 57, _C_), produce
little cups (_x_), circular in the first, semicircular in the second,
in which special buds (_H_, _I_) are formed that fall off and produce
new plants.
The highest of the liverworts (_Jungermanniaceæ_) are, for the most
part, leafy forms like _Madotheca_, and represented by a great many
common forms, growing usually on tree trunks, etc. They are much like
_Madotheca_ in general appearance, but usually very small and
inconspicuous, so as to be easily overlooked, especially as their
color is apt to be brownish, and not unlike that of the bark on which
they grow (Fig. 57, _L_).
CLASS II.--THE TRUE MOSSES.
The true mosses (_Musci_) resemble in many respects the higher
liverworts, such as _Madotheca_ or _Jungermannia_, all of them having
well-marked stems and leaves. The spore fruit is more highly
developed than in the liverworts, but never contains elaters.
A good idea of the general structure of the higher mosses may be had
from a study of almost any common species. One of the most convenient,
as well as common, forms (_Funaria_) is to be had almost the year
round, and fruits at almost all seasons, except midwinter. It grows in
close patches on the ground in fields, at the bases of walls,
sometimes in the crevices between the bricks of sidewalks, etc. If
fruiting, it may be recognized by the nodding capsule on a long stalk,
that is often more or less twisted, being sensitive to changes in the
moisture of the atmosphere. The plant (Fig. 58, _A_, _B_) has a short
stem, thickly set with relatively large leaves. These are oblong and
pointed, and the centre is traversed by a delicate midrib. The base of
the stem is attached to the ground by numerous fine brown hairs.
The mature capsule is broadly oval in form (Fig. 58, _C_), and
provided with a lid that falls off when the spores are ripe. While the
capsule is young it is covered by a pointed membranous cap (_B_,
_cal._) that finally falls off. When the lid is removed, a fine fringe
is seen surrounding the opening of the capsule, and serving the same
purpose as the elaters of the liverworts (Fig. 58, _E_).
[Illustration: FIG. 58.--_A_, fruiting plant of a moss (_Funaria_),
with young sporogonium (_sp._), × 4. B, plant with ripe sporogonium.
_cal_. calyptra, × 2. _C_, sporogonium with calyptra removed. _op._
lid, × 4. _D_, spores: i, ungerminated; ii-iv, germinating, × 300.
_E_, two teeth from the margin of the capsule, × 50. _F_, epidermal
cells and breathing pore from the surface of the sporogonium, × 150.
_G_, longitudinal section of a young sporogonium, × 12. _sp._ spore
mother cells. _H_, a small portion of _G_, magnified about 300 times.
_sp._ spore mother cells.]
If the lower part of the stem is carefully examined with a lens, we
may detect a number of fine green filaments growing from it, looking
like the root hairs, except for their color. Sometimes the ground
about young patches of the moss is quite covered by a fine film of
such threads, and looking carefully over it probably very small moss
plants may be seen growing up here and there from it.
[Illustration: FIG. 59.--Longitudinal section through the summit of a
small male plant of _Funaria_. _a_, _aʹ_, antheridia. _p_, paraphysis.
_L_, section of a leaf, × 150.]
This moss is diœcious. The male plants are smaller than the female,
and may be recognized by the bright red antheridia which are formed at
the end of the stem in considerable numbers, and surrounded by a
circle of leaves so that the whole looks something like a flower.
(This is still more evident in some other mosses. See Figure 65, _E_,
_F_.)
The leaves when magnified are seen to be composed of a single layer
of cells, except the midrib, which is made up of several thicknesses
of elongated cells. Where the leaf is one cell thick, the cells are
oblong in form, becoming narrower as they approach the midrib and
the margin. They contain numerous chloroplasts imbedded in the layer
of protoplasm that lines the wall. The nucleus (Fig. 63, _C_, _n_)
may usually be seen without difficulty, especially if the leaf is
treated with iodine. This plant is one of the best for studying the
division of the chloroplasts, which may usually be found in all
stages of division (Fig. 63, _D_). In the chloroplasts, especially
if the plant has been exposed to light for several hours, will be
found numerous small granules, that assume a bluish tint on the
application of iodine, showing them to be starch grains. If the
plant is kept in the dark for a day or two, these will be absent,
having been used up; but if exposed to the light again, new ones
will be formed, showing that they are formed only under the action
of light.
[Illustration: FIG. 60.--_A_, _B_, young antheridia of _Funaria_,
optical section, × 150. _C_, two sperm cells of _Atrichum_. _D_,
spermatozoids of _Sphagnum_, × 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, × 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, × 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, × 300. _B_, an older
embryo freed from the archegonium, × 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_), × 50. _B_, outline of a leaf, showing also the thickened
midrib, × 12. _C_, cells of the leaf, × 300. _n_, nucleus. _D_,
chlorophyll granules undergoing division, × 300. _E_, cross-section of
the stem, × 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
(_Sphagnaceæ_); II. _Andreæaceæ_; III. _Phascaceæ_; IV. The common
mosses (_Bryaceæ_).
[Illustration: FIG. 64.--_A_, a peat moss (_Sphagnum_), × ½. _B_, a
sporogonium of the same, × 3. _C_, a portion of a leaf, × 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_,
× 3. _B_, fruiting plant of _Atrichum_, × 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, × 2.
_F_, a male plant of _Mnium_, × 4. _G_, pine-tree moss (_Clemacium_),
× 1. _H_, _Hypnum_, × 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 _Andreæaceæ_ are very small, and not at all common. The capsule
splits into four valves, something like a liverwort.
The _Phascaceæ_ 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.