_Anthocerotales._--This small and very natural group includes the three
genera _Anthoceros_, _Dendroceros_ and _Notothylas_, and stands in [v.04
p.0650] many respects in an isolated position among the Bryophyta. Three
species of _Anthoceros_ occur in Britain, growing on the damp soil of
fields, ditches, &c. The dark green thallus has an ill-defined midrib, and
is composed of parenchymatous cells. In each assimilating cell there is
usually a single large chloroplast. The apical region, which has a single
initial cell, is protected by mucilage secreted by the mucilage slits,
which are small pit-like depressions between superficial cells of the lower
surface. Mucilage is also often formed in intercellular spaces within the
thallus. Colonies of _Nostoc_ are constantly found living in some of the
mucilage slits which then become enlarged. The sexual organs are scattered
over the upper surface. The stalked globular antheridia are exceptional in
being formed endogenously, and are situated in groups in special
intercellular spaces. The superficial layer of cells bounding the cavity
does not break down until the antheridia are nearly mature. Occasionally
antheridia develop on the surface of shaded portions of the thallus. The
necks of the archegonia hardly project above the general surface of the
thallus. In structure and development they agree with other Hepaticae,
though differences of detail exist. The young sporogonium is protected by a
thick calyptra derived from the tissue of the thallus around the
archegonium. The sporogonium consists of a large bulbous foot, the
superficial cells of which grow out into processes, and a long capsule,
which continues to grow for months by the activity of a zone of cells
between it and the foot, and may attain the length of an inch and a half.
The wall of the capsule is several layers of cells thick, and since the
epidermis contains functional stomata and the underlying cells possess
chlorophyll it is capable of assimilation. In the centre of the capsule is
a strand of narrow elongated cells forming the columella, and between this
and the wall spores mixed with elaters are formed from the dome-shaped
archesporium, the origin of which has already been described (fig. 4, D).
The capsule opens by splitting into two valves from the apex downwards, and
the mature spores escape while others are developing in succession below.
In _Dendroceros_, which grows as an epiphyte in the tropics, the thallus
has a well-defined midrib and broad wings composed of a single layer of
cells. The capsule is similar to that of _Anthoceros_, but has no stomata,
and the elaters have spirally thickened walls. Some species of _Anthoceros_
agree with it in these respects. _Notothylas_ resembles _Anthoceros_ in its
thallus, but the sporogonium is much smaller. In some species, although the
columella and archesporium arise in the usual way, both give rise to
mingled spores and elaters, and no sterile columella is developed.
_Musci_ (Mosses).
Though the number of species of mosses is far greater than of liverworts,
the group offers much less diversity of form. The sexual generation is
always a leafy plant, which is not developed directly from the spore but is
borne on a well-marked and usually filamentous protonema. The general
course of the life-history and the main features of form and structure will
be best understood by a brief account of a particular example.
[Illustration: FIG. 11.--_Funaria hygrometrica._
A, Leafy shoot (g) bearing a young sporogonium enclosed in the calyptra
(c).
B, Similar plant with an almost mature sporogonium; s, seta; f, capsule; c,
calyptra.
C, Median longitudinal section of a capsule, with the seta gradually
widening into the apophysis at its base; d, operculum; p, peristome; a,
annulus; c, columella; s, archesporium; h, air-space between the spore-sac
and the wall of the capsule.
(From Goebel's _Pflanzenmorphologie_, by permission of W Engelmann)]
_Funaria hygrometrica_ is a moss of very common occurrence even in towns on
the soil of paths, at the foot of walls and in similar places. The small
plants grow closely crowded in tufts, and consist of short leafy shoots
attached to the soil by numerous fine rhizoids. The latter, in contrast to
the rhizoids of liverworts, are composed of rows of elongated cells and are
branched. The leaves are simple, and except for the midrib are only one
layer of cells thick. The structure of the stem though simple is more
complicated than in any liverwort. The superficial cells are thick-walled,
and there is a central strand of narrow cells forming a water-conducting
tissue. The small strand of elongated cells in the midrib of the leaf runs
down into the stem, but is not usually connected with the central strand.
The sexual organs are developed in groups at the apices, the antheridial
group usually terminating the main axis while the archegonia are borne on a
lateral branch. The brown tint of the hair-like paraphyses mixed with
antheridia (fig. 15) makes the male branch conspicuous, while the
archegonia have to be carefully looked for enclosed by the surrounding
leaves (fig. 16, B). The sporogonium developed from the fertilized ovum
grows by means of a two-sided apical cell (fig. 16 A), and is at first of
uniform thickness. After a time the upper region increases in diameter and
forms the capsule, while the lower portion forms the long seta and the foot
which is embedded in the end of the stem. With the growth of the
sporogonium the archegonial wall, which for a time kept pace with it, is
broken through, the larger upper part terminated by the neck being carried
up on the capsule as the calyptra, while the basal portion remains as a
tubular sheath round the lower end of the seta (cf. figs. 16, C, and fig.
11, A, B). The seta widens out at the base of the capsule into a region
known as the apophysis. The peripheral cells of the seta are thick-walled,
and it has a central strand of elongated conducting cells. In the epidermis
of the apophysis functional stomata, similar to those of the higher plants,
are present and, since cells containing chlorophyll are present below the
superficial layers of the apophysis and capsule, the sporogonium is capable
of independent assimilation. The construction of the capsule will be best
understood from the median longitudinal section (fig. 11, C). The central
region extending between the apophysis and the operculum is composed of
sterile tissue and forms the columella (c). Immediately around this is the
layer of cells from which the spores will be developed (s), and the layers
of cells on either side of this form the walls of the spore-sac, which will
contain the spores. Between the wall of the capsule, which is composed of
several layers of cells, and the spore-sac is a wide intercellular space
(h) bridged across by trabeculae consisting of rows of
chlorophyll-containing cells. At the junction of the operculum (d) with the
rest of the capsule is a circle of cells forming the annulus (a), by help
of which the operculum is detached at maturity as a small lid. Its removal
does not, however, leave the mouth of the capsule wide open, for around the
margin are two circles of pointed teeth forming the peristome. These are
the thickened cell-walls of a definite layer of cells (p), and appear [v.04
p.0651] as separate teeth owing to the breaking down of the unthickened
cell-walls. The numerous spores which have been developed in the spore sac
can thus only escape from the pendulous capsule through narrow slits
between the teeth, and these are closed in damp air. The unicellular spores
when supplied with moisture germinate (fig. 12) and give rise to the sexual
generation. A filamentous protonema is first developed, some of the
branches of which are exposed to the light and contain abundant
chlorophyll, while others penetrate the substratum as brown or colourless
rhizoids. The moss-plants arise from single projecting cells, and numerous
plants may spring from the protonema developed from a single spore.
[Illustration: FIG. 12.--_Funaria hygrometrica._ (After Goebel.)
A, Germinating spores. s, Wall of spore; v, vacuole; w, rhizoid.
B, Part of a developed protonema. h, Creeping filament with brown walls
from which the filaments of chlorophyll-containing cells (b) arise; k,
young moss-plant; w, its first rhizoid.]
The majority of the mosses belong to the same great group as _Funaria_, the
Bryales. The other two subdivisions of the Musci are each represented by a
single genus. In the Andreaeales the columella does not extend to the upper
end of the capsule, and the latter opens by a number of lateral slits. The
Sphagnales also have a dome-shaped spore-sac continued over the columella,
and, though their capsule opens by an operculum, they differ widely from
other mosses in the development of the sporogonium as well as in the
characters of the sexual generation. The three groups are described
separately below, but some more general features of the mosses may be
considered here.
On the whole mosses grow in drier situations than the liverworts, and the
arrangements they present for the conduction of water in the plant are also
more complete and suggest in some cases comparisons with the higher plants.
In spite of this, however, they are in great part dependent on the
absorption of water through the general surface of the shoot, and the power
of rapid imbibition possessed by their cell-walls, the crowded position of
the small leaves on the stem, and special adaptations for the retention of
water on the surface, have the same significance as in the foliose
liverworts. The different appearance of exposed mosses in dry weather and
after a shower illustrates this relation to the water supply. The protonema
is always a well-marked stage in the life-history. Not only does a
moss-plant never arise directly from the spore, but in all cases of
vegetative reproduction, apart from the separation of branches by decay of
older regions of the plant, a protonema is found. Usually the protonema is
filamentous and ceases to be evident after the plants have developed. But
in some small mosses (e.g. _Ephemerum_) it plays the chief part in
assimilation and lives on from year to year. In _Sphagnum_, _Andreaea_ and
some genera of the Bryales the protonema or some of its branches have the
form of flat plates or masses of cells. The formation of the moss-plant on
the protonema is always from a single cell and is similar in all mosses.
The first three walls in this cell intersect one another, and define the
three-sided pyramidal apical cell by means of which the shoot continues to
grow. In _Fissidens_ and a few other mosses the apical cell is two-sided.
The leaves formed by the successive segments gradually attain their normal
size and structure. Each segment of the initial cell gives rise to a leaf
and a portion of the stem; the branches arise from the lower portion of a
segment and stand immediately below a leaf. The leaves may form three
vertical rows, but usually their arrangement, owing to the direction of the
segment walls at the apex, becomes more complicated. Their growth proceeds
by means of a two-sided apical cell, and the midrib does not become more
than one cell thick until later. In addition to the leaves the stem often
bears hair-like structures of different kinds, some of which correspond to
modified branches of protonema. The branched filamentous rhizoids which
spring from the lower region of the stem also correspond to protonemal
branches. The structure of both stem and leaf reaches a high grade of
organization in some mosses. Not only are thick-walled sclerenchymatous
cells developed to give rigidity to the periphery of the stem and the
midrib of the leaf, but in many cases a special water-conducting tissue,
consisting of elongated cells, the end walls of which are thin and oblique,
forms a definite central strand in the stem. In the forms in which it is
most highly developed (Polytrichaceae) this tissue, which is comparable
with the xylem of higher plants, is surrounded by a zone of tissue
physiologically comparable to phloem, and in the rhizome may be limited by
an endodermis. The conducting strands in the leaves show the same tissues
as in the central strand of the stem, and in the Polytrichaceae and some
other mosses are in continuity with it. The independent origin of this
conducting system is of great interest for comparison with the vascular
system of the sporophyte of the higher plants.
The sexual organs, with the exception of the antheridia of _Sphagnum_, are
borne at the apices of the main shoot or of branches. Their general
similarity to the mature antheridia and archegonia of liverworts and the
main difference in their development have been referred to. The antheridia
open by means of a cap cell or groups of cells with mucilaginous contents.
The details of construction of the sporogonium are referred to below. In
all cases (except _Archidium_) a columella is present, and all the cells
derived from the archesporium produce spores, no elaters being formed. In a
few cases the germination of the spore commences within the capsule. The
development of the sporogonium proceeds in all cases (except in _Sphagnum_)
by means of an apical cell cutting off two rows of segments. The first
periclinal division in the region forming the capsule separates an inner
group of cells (the endothecium) form the peripheral layer (amphithecium).
In _Sphagnum_, as in _Anthoceros_, the archesporium is derived from the
amphithecium; in all other mosses it is the outermost layer of the
endothecium.
Vegetative propagation is widely spread in the mosses, and, as mentioned
above, a protonema is always formed in the development of the new plant.
The social growth of the plants characteristic of many mosses is a result
of the formation of numerous plants on the original protonema and on
developments from the rhizoids. Besides this, gemmae may be formed on the
protonema, on the leaves or at the apex, and some mosses have specialized
shoots for their better protection or distribution. Thus in _Georgia_ the
stalked, multicellular gemmae are borne at the ends of shoots surrounded by
a rosette of larger leaves, and in _Aulacomnium androgynum_ they are raised
on an elongated leafless region of the shoot. In other cases detached
leaves or shoots may give rise to new plants, and when a moss is
artificially divided almost any fragment may serve for reproduction.
Even in those rare cases in which the sexual generation can be developed
without the intervention of spore production from the tissues of the
sporogonium, a protonema is formed from cut pieces of the seta or in some
cases from intact sporogonia still attached to the plant. This phenomenon
of _apospory_ was first discovered in mosses, but is now also known in a
number of ferns (see PTERIDOPHYTA).
[Illustration: FIG. 13.--_Sphagnum acutifolium._ (After Schimper.)
A. Longitudinal section of apex of a bud bearing archegonia (ar), enclosed
by the large leaves (y); ch, small perichaetial leaves.
B. Longitudinal section of the sporogonium borne on the pseudopodium (ps);
c, calyptra; ar, neck of archegonium; sg', foot; sg, capsule.
C. _S. squarrosum._ Ripe sporogonium raised on the pseudopodium (qs) above
the enclosing leaves (ch); c, the ruptured calyptra; sg, capsule; d,
operculum.]
_Sphagnales._--The single genus _Sphagnum_ occupies a very distinct and
isolated position among mosses. The numerous species, which are familiar as
the bog-mosses, are so similar that minute structural characters have to be
relied on in their identification. The plants occur in large patches of a
pale green or reddish colour on moors, and, when filling up small lakes or
pools, may attain a length of some feet. Their growth has played a large
part in the formation of peat. The species are distributed in temperate and
arctic climates, but in the tropics only occur at high levels. The
protonema forms a flat, lobed, thalloid structure attached to the soil by
rhizoids, and the plants arise from marginal cells. The main shoot bears
numerous branches which appear to stand in whorls; some of them bend down
and become applied to the surface of the main axis. The structure of the
stem and leaves is peculiar. The former shows on cross-section a
thin-walled central tissue surrounded by a zone of thick-walled cells.
Outside this come one to five layers of large clear cells, which when
mature are dead and empty; their walls are strengthened with a spiral
thickening and perforated with round pores. They serve to absorb and
conduct water by capillarity. The leaves have no midrib and similar empty
cells occur regularly among the narrow chlorophyll-containing cells, which
thus appear as a green network. The antheridia are globular and have long
stalks. They stand by the side of leaves of special club-shaped branches.
The archegonial groups occupy the apices of short branches (fig. 13, A.).
The mature sporogonium consists of a wide foot separated by a constriction
from the globular capsule (B). There is no distinct seta, but the capsule
is raised on a leafless outgrowth of the end of the branch called a
pseudopodium (C, qs). The capsule, the wall of which bears rudimentary
stomata, has a small operculum but no peristome. There is a short, wide
columella, over which the dome-shaped spore-sac extends, and no air-space
is present between the spore-sac and the wall. In the embryo a number of
tiers of cells are first formed. The lower tiers [v.04 p.0652] form the
foot, while in the upper part the first divisions mark off the columella,
around which the archesporium, derived from the amphithecium, extends. The
sporogonium when nearly mature bursts the calyptra irregularly. The capsule
opens explosively in dry weather, the operculum and spores being thrown to
a distance. The spore on germination forms a short filament which soon
broadens out into the thalloid protonema. Some twelve species of _Sphagnum_
are found in Britain.
[Illustration: FIG. 14.--_Andreaea petrophila_. Plant bearing opened
capsule.
(k) ps, Pseudopodium.
c, Calyptra.
spf, Foot of sporogonium.