In this Division a well-marked alternation of generations is
to be found. The development of the first or sexual generation
(_gametophyte_),[16] which bears the sexual organs, antheridia
and archegonia, commences with the germination of the spore, and
consists, in the Liverworts, of a thallus, but in the true Mosses of a
filamentous protonema, from which the Moss-plant arises as a lateral
bud. The second or asexual generation (_sporophyte_), developed from
the fertilised oosphere, consists of a sporangium and stalk.
=The sexual generation, the gametophyte.= The protonema in the
Liverworts is very insignificant, and not always very sharply
demarcated from the more highly developed parts of the nutritive
system. In the true Mosses the protonema is well-developed, and
consists of a branched, alga-like filament of cells, the dividing
cell-walls being always placed obliquely. In the parts exposed to the
light it is green, but colourless or brownish in those parts which are
underground (Fig. 186). The protonema is considered to be a lower form
of the stem, and grows in the same manner by means of an apical cell;
at its apex it may directly develope into a leaf-bearing stem, or these
arise from it as lateral branches (Fig. 186 _k_).
The more highly differentiated part of the vegetative system, the
“Moss-plant,” which is thus developed from the protonema, is in the
“thalloid” Liverworts generally a dichotomously-branched thallus
without any trace of leaf-structures (Fig. 194); in _Marchantia_
(Fig. 197) and others, scale-like leaves (_amphigastria_) are found
on the under surface. The higher Liverworts and the Leafy-Mosses are
differentiated into a filamentous, ramified stem with distinct leaves
arranged in a definite manner, resembling the stem and leaves of the
higher plants (Figs. 186, 195, 200).
_True roots are wanting_, but are biologically replaced by _rhizoids_.
These are developed on the stems or thallus: in the Liverworts they
are unicellular, but in the Leafy-Mosses generally multicellular and
branched. In the latter group they are considered identical with
the protonema, and may become true protonema, and new plants may be
developed from them (Fig. 186 _b_).
[Illustration: FIG. 186.--_A_ Lower portion of a Moss-plant with
rhizoids (_r_), one of which bears a reproductive bud (_b_). The
dotted line indicates the surface of the ground; the portions
projecting above this become green protonema (_p_); _k_ is a
young Moss-plant formed on one of these. _B_ Germinating spore of
_Funaria hygrometrica_, with exospore still attached. _C_, _D_
Older stages of the protonema.]
The internal structure of the sexual generation is very simple. The
leaves in nearly all cases are formed of a single-layered plate of
cells; in the Leafy-Mosses, however, a midrib is very often formed, and
sometimes, also, marginal veins; and along these lines the leaves are
several layers of cells in thickness. The stem is constructed of cells
longitudinally elongated, the external ones of which are narrower and
sometimes have thicker walls than the more central ones. _Vessels are
not found_, but in several Mosses there is in the centre of the stem
a conducting strand of narrow, longitudinal cells, which represents
the vascular bundle in its first stage of development. This strand
contains elements for conveying water as well as sieve-tubes. Stomata
are entirely wanting in the sexual generation of the Leafy-Mosses; they
are found in a few Liverworts (_Marchantia_), but their structure is
not the same as in the higher plants.
VEGETATIVE REPRODUCTION takes place by gemmæ or buds which arise on
the protenema, the rhizoids, the thallus, or the shoots, and become
detached from the mother-plant; or else the protonema and the older
parts of the plant simply die off, and their branches thus become
independent plants. This well-developed vegetative reproduction
explains why so many Mosses grow gregariously. In certain Marchantiaceæ
special cupules, in which gemmæ are developed, are found on the surface
of the thallus (Fig. 197 _A_, _s-s_). Again, protonema may also arise
from the leaves, and thus the leaves may act as reproductive bodies.
Certain Mosses nearly always reproduce vegetatively, and in these
species the oospheres are seldom fertilised.
[Illustration: FIG. 187.--_Marchantia polymorpha_: _a_ mature
antheridium.]
[Illustration: FIG. 188.--Spermatozoids.]
The first generation bears the SEXUAL ORGANS; both kinds are found
either on the same plant (monœcious), or on separate plants (diœcious).
In the thalloid Liverworts they are often situated on the apex of small
stems (_gametophores_), springing from the surface of the thallus.
In the Leafy-Liverworts and true Mosses the leaves which enclose the
sexual organs often assume a peculiar shape, and are arranged more
closely than the other leaves to form the so-called “Moss-flower.” The
male sexual organs are called _antheridia_. They are stalked, spheroid,
club- or egg-shaped bodies whose walls are formed of one layer of cells
(Fig. 187), enclosing a mass of minute cubical cells, each one of which
is a mother-cell of a spermatozoid. The spermatozoids are self-motile;
they are slightly twisted, with two cilia placed anteriorly (Fig. 188),
while posteriorly they are generally a trifle club-shaped, and often
bear at that part the remains of the cytoplasm, the spermatozoid itself
being _formed from the nucleus_. In the presence of water the ripe
antheridium bursts, and its contents are ejected; the spermatozoids,
being liberated from their mother-cells, swarm about in the water in
order to effect fertilisation.
[Illustration: FIG. 189.--_Marchantia polymorpha. A_ A young,
and _B_ a ripe archegonium with open neck. _C_ An unripe
sporangium enclosed by the archegonium _a_: _st_ the stalk; _f_
the wall of the sporangium. Elaters are seen between the rows of
spores.]
The female sexual organs are termed _archegonia_. They are flask-shaped
bodies (Fig. 189), the lower, swollen portion (_venter_) having a wall,
in most cases from 1–2 cells thick, enclosing the oosphere (Fig. 189
_B_, _k_): the long neck is formed of tiers of 4–6 cells, enclosing
a central row of cells--_the neck-canal-cells_ (Fig. 189 _A_). When
the archegonium is fully developed, the walls of the neck-canal-cells
become mucilaginous and force open the neck of the archegonium. The
mucilage thus escapes, and, remaining at the mouth of the archegonium,
acts in a somewhat similar manner to the stigma and conducting tissue
of a carpel, by catching and conducting the spermatozoids to the
oosphere (Fig. 189 _B_, _m_), with whose cell-nucleus they coalesce.
With regard to the formation of the oosphere, it may further be
remarked that the lower part of the archegonium originally encloses the
so-called “central cell”; but shortly before the archegonium is ripe,
this cuts off a small portion, _the ventral-canal-cell_, which lies
immediately beneath the neck, and the larger, lower portion becomes the
oosphere.
The organs mentioned here, antheridia and archegonia, are
present in the Cryptogams (Pteridophyta) and the Gymnosperms.
They have always the same fundamental structure, but with slight
modifications of detail. These plants are therefore known as the
ARCHEGONIATA.
The fertilisation of the Mosses cannot be effected without water. Rain
and dew therefore play a very important part in this process, and for
this end various modifications of structure are found.
[Illustration: FIG. 190.--_Andreæa rupestris._ Longitudinal
section through a sporangium at the time when the mother-cells
of the spores are dividing: _p_ pseudopodium; _f_ foot; _v_
vaginula; _h_ neck; _c_ columella; _w_ wall of the sporangium;
_e_ external row of cells; _s_ the spore-sac; _t_ the
spore-mother-cells; _r_ the calyptra with the neck of archegonium
(_z_).]
[Illustration: FIG. 191.--_Andreæa rupestris._ Transverse
section through a ripe sporangium. In the middle is seen the
four-sided columella, surrounded by the numerous spores,
drawn diagrammatically. Surrounding them is seen the wall of
the sporangium, whose outer layer of cells is thickened and
coloured. The layer of cells is unthickened in four places (_x_),
indicating the position of the clefts (see Fig. 193).]
Among the sexual organs, paraphyses--filamentous or club-shaped
bodies--are to be found.
=The asexual generation, the sporophyte= (Moss-fruit or sporogonium).
As the result of fertilisation the oosphere surrounds itself with a
cell-wall, and then commences to divide in accordance with definite
laws.[17] The embryo (Fig. 189 _C_) produced by these divisions
remains inside the wall _a-a_ of the archegonium (Figs. 190, 199 _D_,
_E_), and developes into the _sporogonium_, which remains attached
to the mother-plant, often nourished by it, as if the two were one
organism. The lower extremity of the sporogonium, _the foot_ (Figs.
190 _f_; 199 _D_), very often forces its way deep down into the
tissue of the mother-plant, but without an actual union taking place.
The central portion of the sporogonium becomes a shorter or longer
_stalk_ (_seta_), while the sporangium itself is developed at the
summit. At a later stage, during the formation of the spores, the
sporangium very often assumes the form of a _capsule_, and dehisces
in several ways characteristic of the various genera (Figs. 192, 193,
194, 195, 200). The basal portion of the archegonium grows for a
longer or shorter period, forming a sheath, the _calyptra_, in which
the capsule is developed, but eventually it ceases to enlarge, and
is then ruptured in different ways, but quite characteristically, in
each group. Anatomically, the asexual generation is often more highly
differentiated than the sexual; thus, for instance, stomata are present
on the sporangia of the true Mosses, but are absent in the sexual
generation.
As the capsule developes, an external layer of cells--the
_amphithecium_--and an internal mass--the _endothecium_--are
differentiated. As a rule the former becomes the wall of the capsule
while the latter gives rise to the spores. In this Division, as in
the Pteridophyta, the name _archesporium_ (Fig. 190 _t_) is given
to the group of cells inside the sporangium which gives rise to
the mother-cells of the spores. The archesporium is in general a
unicellular layer; in _Sphagnum_ and _Anthoceros_ it is derived from
the most internal layer of the amphithecium, but with these exceptions
it arises from the endothecium, usually from its most external layer.
In the true Mosses and in _Riccia_ only spore-mother-cells are produced
from the archesporium, but in the majority of the Liverworts some
of these cells are sterile and become elaters (cells with spirally
thickened walls, Figs. 196, 189), or serve as “nurse-cells” for the
spore-mother-cells, which gradually absorb the nutriment which has been
accumulated in them. In _Anthoceros_, and almost all the Leafy-Mosses,
a certain mass of cells in the centre of the sporangium (derived
from the endothecium) does not take part in the formation of the
archesporium, but forms the so called “column” or “columella” (Figs.
190, 191).
The _spores_ arise in _tetrads_, _i.e._ four in each mother-cell, and
are arranged at the corners of a tetrahedron, each tetrahedron assuming
the form of a sphere or a triangular pyramid. The mature spore is a
nucleated mass of protoplasm, with starch or oil as reserve material.
The wall is divided into two layers: the external coat (exospore) which
is cuticularized and in most cases coloured (brown, yellowish), and the
internal coat (endospore), which is colourless and not cuticularized.
On germination the exospore is thrown off, the endospore protrudes, and
cell-division commences and continues with the growth of the protonema
(Fig. 186, _B-D_).
[Illustration: FIG. 192.--_Andreæa petrophila._ A ripe
sporogonium: _a_ an archegonium which has been raised with the
pseudopodium; _p_ the foot; _b_ the neck; _d-e_ the dark-coloured
portion of the sporangium, whose outer cell-walls are
considerably thickened; _c_-_c_ the thin-walled portions where
the dehiscence occurs; _o_ the lower extremity of the spore-sac;
_f_ calyptra; _g_ the apex of the sporangium. (Mag. 25 times.)]
[Illustration: FIG. 193.--_Andreæa petrophila._ An empty capsule;
the calyptra has fallen off. (Mag. 25 times.)]
The morphological explanation which Celakovsky has given of
the sporogonium, and which is not at all improbable, is, that
it is homologous with an embryo consisting of a very small
stem-portion and a terminal spore-producing leaf. This will be
further explained in the introduction to the Flowering-plants
(p. 236).
In the Liverworts the young sporogonium lives like a parasite, being
nourished by the sexual generation (only in _Anthoceros_ has it a
slight power of assimilation). In the Leafy-Mosses, on the other
hand, with regard to the power of assimilation, all transitions are
found from abundant assimilation (_Funaria_, _Physcomitrium_) to
almost complete “parasitism” (_Sphagnum_, _Andreæa_). In the majority
of the operculate Mosses the sporogonium has a more or less perfect
system of assimilation, and is able itself to form a large portion of
the material necessary for the development of the spores, so that it
chiefly receives from the sexual generation the inorganic substances
which must be obtained from the soil. The more highly developed the
assimilative system of the sporogonium, the more stomata are present.
APOSPORY. In some operculate Mosses it has been possible to
obtain a protonema with small Moss-plants from the seta, when
severed from its Moss-plant, and grown on damp sand.
The Mosses are the lowest plants which are provided with stem and
leaf. They are assigned a lower place when compared with the higher
Cryptogams, partly because there are still found within the Division
so many forms with a mere thallus, partly because typical roots are
wanting and the anatomical structure is so extremely simple, and partly
also because of the relation between the two generations. The highest
Mosses terminate the Division, the Muscineæ and Pteridophyta having had
a common origin in the Algæ-like Thallophyta.