Anthoceros

 Class : Anthocerotopsida

The class Anthocerotopsida is a small but very
distinct group, which differ in many ways,
especially, its sporophyte which is of indefinite
growth because of presence of a characteristic
intercalary meristem at the base of the capsule.
The sporogenous cells are amphithecial in origin.
The cells of gametophytes, usually, have large
single chloroplasts with a conspicuous
 pyrenoid – a characteristic feature of algae.

Commonly, the members of this class are
called the hornworts. The class
Anthecerotopsida contain only one order
Anthocerotales, and only one family
Anthocerotaceae. The important genera of
the family are Anthoceros, Nothothylos,
Dendroceros and Megaceros.

Anthoceros

Distribution and Habitat:

Anthoceros is the largest genus of the class
Anthocerotopsida, comprising about 250
species. They are worldwide in distribution
found in temperate as well as tropical regions
of world.

In India 25 species have been reported which are
widely distributed in the Himalaya,
Chhota Nagpur, Nilgiris. They usually occur at an
altitude ranging from 5,000 to 8,000 ft. Common
Indian species are A. himalayensis, A. chambenis,
A. erectus.

All the species grow on moist, clayey soil or wet rocks.
They prefer shady places and usually grown in dense
patches. They grow along the banks of streams, ditches,
hill slopes, crevices of rocks. They are suited to dry
conditions.

Gametopphyte

The plant body is gametophyte. It is very simple,
small, prostrate, dorsiventral thallus of yellowish
green or dark green colour. Thallus is lobed with
dichotomous branching. The thallus is several
layer thick but it lacks a definite midrib.

The dorsal surface of the thallus may be smooth or
rough with ridges. The lower ventral surface bears
numerous, unicellular, smooth walled rhizoids.
These anchors the thallus to the substratum.
Scales, tuberculate rhizoids and mucilage hairs
are absent on the ventral surface.

Also, in the ventral surface small, rounded, bluish
green spots are seen. These spots are the Nostoc
colonies which are inhabited by blue-green algae
Nostoc. The Anthoceros-Nostoc association is
symbiotic, the thallus supplies carbohydrate (food)
to the cynobacterium and the latter supplies nitrate
by fixing the atmospheric nitrogen.

The mature gametophyte bears horn-like sporangia on
their dorsal surface. Hence, Anthoceros is called
Horn-wort.

Internal Structure of the Thallus:

Internally, the structure of thallus is very simple.
There is no tissue differentiation and little cell
specialization.

The entire thallus is composed of uniform,
thin-walled, parenchymatous cells. The
surface cells of the thallus show more regular
arrangement. These cells are smaller. There is
thus no organized epidermis.

Each cell of a thallus contains a single large
chloroplast. Each chloroplast is oval or flattened
with large pyrenoid in the centre. Each pyrenoid
is actually an aggregate of 25-50 disc or spindle
shaped bodies. The presence of chloroplasts with
pyrenoid is a feature which Anthoceros shares with
the green Algae. In both starch grains are synthesized
at the periphery of the pyrenoid. This shows the algal
ancestry of the Anthocerotales.

Each cell has a single nucleus very near to the chloroplast.

There are no air chambers or pores in the thallus. But ventral
surface of the thallus shows small, rounded pores or slits.
These are called slime pores. Each slime pore is guarded by
two bean shaped guard cells. These guard cells do not
function to control the size of the pores which remains
completely open.

The slime pores lead inwards into large schizogenously formed
intercellular cavities. These cavities are formed due to
breakdown of cells. These cavities are filled with mucilage
hence called mucilage cavities.

These mucilage cavities are inhabited by Nostoc – a blue-green
algae. Hormogones of Nostoc gain entry into the cavities
through the slime pores. There, they multiply and form
colonies.

The cavities containing Nostoc colonies are visible to the naked
eyes as small, deep blue-green rounded spots on the ventral
surface of the thallus.

Growth:

Growth in Anthoceros takes place by means of a single apical cell.
The apical cell is pyramidal in shape and has four cutting
faces – dorsal, ventral and two lateral.

Reproduction:

Gametophyte of Anthoceros reproduces by (1) Vegetative
reproduction and (ii) Sexual Reproduction.

Vegetative Reproduction:

Vegetative reproduction in Anthoceros takes place by
following methods:-

1. Fragmentation:

The cells in the basal older portions of the thallus die and
disorganize. When the progressive decay and death of cells
reaches the branching region, the thallus lobes becomes
separated. Each separated lobe grow into a new thallus by
continuous apical growth.

This type of vegetative reproduction is no so common in
Anthoceros as in other Bryophytes.

2. Gemmae:

They are formed on short stalks on the upper surface and along
the margin of the thallus in A.       glandulous, A. formosae and
A. propaguliferus. Sometimes, the gemmae are stalked and develop
mucilage pores even before they are detached from the parent plant.
The gemmae detaches from the parent plant and germinate into new
gametophytes.

3. Tubers:

Under favourable conditons, many species of Anthoceros produce tubers by
the thickening of marginal patches of the thallus tissue. The tubers are
protected by 2 or 3 layers of cork cells. The cork cells protect the inner
cells that store starch, oil globules and aleurone grains.

The tubers can withstand unfavourable conditions, especially, the drought
to which thillus dies. These tubers regenerate on return of favourable
conditions. Thus, tubers function as organs of perennation and serve as
means of vegetative propogation. Ex., A. tuberosus, A. pearsonii,
A. himalayensis.

4. Persistant Growing Apices:

In few species of Anthoceros like A. pearsonii and A. fusiformis, the plants
dry up during the summer. Only the growing apices of the thallus lobes with
a little of adjacent tissues survive. The apices starts growing on return of
favourable conditions.

5. Apospory:

In few species of Anthoceros, the unspecialized cells of varioius parts of
sporogonium like the intercalary meristematic zone, sub-epidermal and
sporogenous tissue gives rise to the thallus. The formation of thallus
(gametophyte) directly from the vegetative cells of sporogonium is called
apospory. These gametophytes are diploid and sterile.

Sexual Reproduction:

The sexual reproduction is oogamous types. Most of the species of Anthoceros
like A. fusiformis, A. pronctatus, A. himalayensis are monoecious i.e.,
male and female sex organs are produce in the same thallus. Such monoeciosus
thalli are protandrous i.e., antheridia matures before the archegonia.
Archegonia formation begins when antherozoids are mature. This is to
prevent self-fertilisation.  

Some species of Anthoceros like A. erectus, A. chambensis, A. pearsonii are
dioecious and hetertothallic.

The sex organs in Anthoceros are immersed in the thallus tissue on the dorsal
surface. As water is essential for the dehiscence of sex organs and fertilization,
the sex organs develop in fall, winter and spring.

Antheridia:

The antheridia of Anthoceros are unique in being normally endogenous. They
develop singly or in groups on the upper surface of the thallus within closed
cavities called the antheridial chambers. The roof of the antheridial chamber
is single layered.

A mature antheridium is an orange coloured, club-shaped structure with a
multicellular stalk and a cylindrical body. The stalk is slender and multicellular.
The body of the antheridium consists of a jacket layer of an antheridial wall
enclosing a mass of primary androgonial cells. The cells are arranged in four
tiers. The cells of lower three tier are rectangular and elongated. The cells of
uppermost tier are triangular with narrow end towards the apex.

The primary androgonial cells undergo repeated divisions to produce innumerable
androgonial cells. The last cell generation of androgonial cells gives rise to
androcyte mother cells. Each androcyte mother cell divides to form two
androcytes. Each androcyte metamorphose into a biflagellated antherzoid.

The antherozoid is a spindle shaped cell with a curved posterior end. It has a
haploid nucleus, dense cytoplasm and a blepheroplast granule. There are
two equal flagella at the anterior end of the antherozoid.

Dehisence:

At maturity, the roof of the antheridial chamber burst open and expose the
antheridia. The antheridia, thus exposed dehisce following absorption
of water. This leads to rupture at the the distal end of the antheridium.
An aperture is thus formed at the distal end. Through this aperture
oozoes out mass of antherozoids. The liberated antherozoids swim in
water to reach the mature archegonia.

Archegonia:

The archegonia are produced on the dorsal surface and remain embedded
in the thallus in direct contact with the surrounding vegetative cells.
They lie close to the growing point and are developed in acropetal
succession.

Each archegonium consists of an axial row of 4-6 neck canal cell, a
ventral cell and an egg. There is no sterile jacket layer. On at the
tip 4-5 cover cells are present, which project above the upper
surface of the thallus, where it is surrounded by funnel-shaped
mass of mucilage called the mucilage mound. The place of an
archegonium on a thallus can be identified by the presence
of mucilage mound.

Fertilisation:

At the time of fertilization, on access of water the venter canal cells and cells of neck disintegrate
releasing a mucilaginous mass which becomes continuous with the mucilage mound on the
surface of the thallus. The cover cells are thrown off. An open passage down to the egg
is formed and egg now becomes directly exposed.

The antherozoids caught in mucilage reach the egg and fuses with it to form a diploid zygote.
The zygote swells to fill the venter cavity completely. Then it secretes a cellulose wall
around itself. A number of zygotes are formed on a single thallus.

Sprophytic Phase:

The diploid zygote is the first cell of the sporophytic
phase. The first division of zygote is longitudinal,
in this way hornworts differ from rest of
Bryophytes. The subsequent division which is
right angle to the first produces 4 cell stage –
the quadrant.

Another division, results in formation of 8 celled
‘octant stage’. Eight cells are arranged in two tiers
of 4 cells each. The lower tier of four cells form the
sterile foot. The lower most cells of the foot grow
into short rhizoids, which increase absorptive
surface for sucking food from the gametophyte.

The upper tier of cells and its lower cells usually
form an intermediate part, which later develops
an intercalary meristematic tissue.  The upper
cells undergoes periclinal divisions and separate
out the inner endothecium and the outer
amphithecium.

The endothecium gives rise to the middle sterile
columella, which in the beginning has 4 rows of
cells, but at maturity the rows of cells increase
to 16.

The amphithecial cells divide periclinally and its
outer layer form the jacket or wall of the
capsule and the inner cells become the
archesporium.

A protective sheath or involucre encloses
the developing sporophyte. The upper
part of the involucre is derived from
the venter of the archegonium and the
lower part from cells of thallus. With
the elongation of sporophyte, the
involucre is pierced through the
apex. A part of involucre remains
at the apex of sporophyte- it is
reminiscent of calyptra of mosses –
and the lower part forms a collar
around the base of sporophyte.
The collar helps to retain moisture
and protects the intercalary
meristem.

Structure of Sporophyte:

The sprophyte of Anthoceros occupies a
significant position in the evolutionary
sequence of plants because of its
structure and degree of independence.
The sporophyte is an elongate structure –
giving the name ‘hornwort’ to plants,
which projects out 2 to 15 cms in some
cases.

The sporophyte is differentiated into three
regions : (i) the foot, (ii) the intercalary
or intermediate zone and (iii) capsule.
The seta is absent. Its place is taken up
by the intercalary zone which is
meristematic.

Foot:

The lower rounded, bulbous structure of the sporophyte is the foot. It is deeply
embedded in the tissue of the thallus. The foot mainly consists of a mass of
parnchymatous cells. The surface cells of foot grow out into short, tubular,
rhizoid-like outgrowths. They serve to increase the absorptive surface
of the foot. The foot of Anthoceros sporangium is thus specialized to
function as a haustorium.

Intermediate or Intercalary Meristem:

Between foot and capsule, there is no seta but a short intermediate zone which
is meristematic. The meristem constantly adds new cells to the capsule at its base.
They become progressively differentiated into Columella, Archesporium and
Capsule wall. Thus, the capsule is always in different stages of growth; it is in the
embryonic stage at the base an very mature at the tip. It enables the capsule
to grow for a long period and form spores.

Capsule
The capsule is cylindrical and it is distinguished as capsule wall, sporogenous tissue
and columella.
Capsule wall:
The wall of the capsule is 4-6 layers in thickness. It is derived from amphithecium.
the outermost layer of the wall is the epidermis. It consists of narrow, vertically
elongated cells with their outer walls cutinized. The epidermal layer is interrupted
by stomata.
The wall layers beneath the epidermis are chlorenchymatous. They have intercellular
spaces between them. Each cell contains two large chloroplasts.
The capsule wall is thus actively photosynthetic in function, hence the sporophyte
is partly independent.

Sporogenous tissue or Archesporium
The archesporium is derived from the amphithecium. It is in the form of a cylinder
between the columella and the capsule wall. In some species the archesporium remains
single layered throughout (A. erecturs). In other species it becomes two- layered above
the base of the capsule and in still others it becomes 2-4 layers thick.
It extends over the top of the columella like a dome. The cells of the archesporium
can easily be distinguished from the sterile cells of the columella by their denser
protoplasm.
The archesporium gradually differentiates into oval or spherical fertile spore mother
cells and slender sterile pseudoelater mother cells.
The diploid nucleus of spore mother cells undergoes meiosis and form four haploid
nuclei which are arranged tetrahedrally. In the top most mature portion of the capsule
the spore tetrads seperate as individual spores.
The Pseudoelater mother cells increase in length but do not divide. They form chains of
one to four elongaged, sterile cells of irregular shape. This cells are now called the
pseudoelaters. They are smooth-walled and are nutritive in function. They lack
spiral thickenings and are pluricellular. They are hygroscopic in nature. They are useful
in dispersal of spores.

At the base of the capsule the sporogenous tissue is single to double layered, a little
higher it is differentiated into spore mother cells and pseudo elater mother cells.

A little higher tetrads of spores and many pseudo elaters are seen, in the upper most
region of the capsule separated spores and pseudo elaters are seen

Columella:

The columella is the central part of the capsule. It is formed from endothecium.
It extends from the base to the top of the capsule. The columella consists of 16
rows of cells. The cells are vertically elongated and their walls are uniformly
thickened. It helps in the conduction of food and mineral nutrients and also
provides mechanical support to the long capsule.

Dehiscence of Capsule:
The spores ripen basipetally. As they ripen the mature portion of the capsule
turns grey, brown or black. It loses water, shrinks and consequently ruptures
longitudinally. Capsule usually dehisces along two longitudinal slits which
extend downwards from near the apex. The portion of the capsule wall
between the slits are called the valves.
As the upper part of the capsule dries up, the valves seperate and begin
to twist longitudinally. The twisting exposes the spores and pseudoelaters
. the spores are shed from the capsule by hygroscopic movements of the
valves, pseudoelaters and the columella.

Spore:
The spore is the first cell of the gametophytic generation. The mature spores
are tetraheddral in shape and thick walled. The wall is two layered -
the outer thick exospore and the inner thin endospore. It contains a single
nucleus, a colourless plastid, oil and other reserve food materials