Marchantia

Hepaticopsida

The class Hepaticopsida comprises of 330
genera and 8000 species.
They are commonly known as ‘liverworts’
(latin Hepatic-liver).

Characteristics:

1. They are comparatively small in size never
more than a few inches in length.

2. The plant body is gametophyte, independent
and autotrophic.
It is either thallose in form or  leafy in nature.
The leaf has no midrib.

3. The gametophyte is attached to the ground by
simple, one-celled,
absorptive processes- the rhizoids.

4. The thallus is externally and internally
differentiated into distinct dorsal and ventral side.
The dorsal side contains assimilatory tissue.
The cell constituting it have numberous, small  
chloroplasts. The chloroplast lacks pyrenoids

5. Ventral side has storage tissue.

6. Sex-organs develop either on the dorsal side of
the thallus or on special erect branches called
‘gametophores’.

7. Sporophyte is small and generally without
Chlorophyll, hence it is dependent on gametophyte
for its nourishment.

8. Sporophyte is either differentiated into foot,
seta and capsule or foot and capsule. In Riccia it is
very simple with both foot and seat lacking.

9. the sporogenous tissue originates from endothecium.
A major portion of the sporogenous tissue forms the spores
as compared to the other two calss of Bryophytes. The capsule
lacks sterile region. The wall of the capsule develops from
ampithecium. It has no stomata.

10. the sterile cells elongates to form elaters with spirally
thickened walls. The elaters are sterile and unicellular.

11. The columella is absent.

12. The spores, on germination, generally directly and in
some indirectly, develop into the gametophyte.

Order – Marchantiales (Chambered –hepatics)

This order includes 27 genera, 450 species.
About half of these species belong to the genus Riccia.
This group is represented world-wide.

Salient Features:

1. Typical thallose liverworts.

2. Thallus is dorsiventrally differentiated and
dichotomously branched.

3. Thallus is differentiated into two regions:   
 a. upper phtotosynthetic region- of
chlorophyllous cells, air-chambers and air pores.

 b. lower storage region – of non-chlorophyllous
cells withous air spaces.

4. Rhizoids and scales arises from the ventral
surface of the thallus.

5. Rhizoids are unicellular and are of two types
1. Smooth walled and 2. Tuberculate. Scales are
multicellular.

6. The archegonial branch-archigoniophore-
normally bears several sporophyte.

7. The sporogonium jacket is unistratose-composed
of single layer of sterile cells.

MARCHANTIA

           Class: Hepaticopsida

Order: Marchantiales

Family: Marchantiaceae

Genus: Marchantia

Distribution and Habit:

The name Marchantia has been given by French botanist
N. Marchant in memory of his father. The genus Marchantia
with about 65 species is found in all parts of the world.
In India there are about 11 species,distributed all over the
Himalayan region. M. palmata, M. nepalansis,
M. polymorpha and M. indica are common Himalayan species
found in Kashmir, Himachal Pradesh, Garhwal and Kumaon hill.
M.polymorpha, M. indica occurs in hilly regions of South India.

Marchantia prefers moist shady places either on forest floors
or banks of streams or shaded cliffs. M.polymorpha grows
as a pioneer in the burnt soil of the forest after fire.

Vegetative plants with characteristics gemma cups can be
collected all over the year but sexual plants bearing –
antheridiophore and archegoniophores – can be collected
from March to May in Himalayan region and during October-
November from hills of South India.

The Gametophyte

The gametophyte is the dominant phase in the life cycle of
Marchantia. It is haploid.

External Morphology of the Thallus

The thallus is dark green, prostrate, flat, dorsiventrally
differentiated and dichotomously branched.

The thalli are fairly large extending upto 10cm or more in
length with distinct, broad, central  midrib, represented by
a shallow groove on dorsal side and a ridge on ventral side.

Each thallus branch posses a notch at its apex. At the bottom
of notch is located growing point.

The upper or dorsal surface of thallus has distinct rhomboidal
or polygonal areas called the areolae.
The areolae indicate the boundaries of underlying air-chambers.
Each area is with a small and distinct air-pore in the centre,
which leads to an air chamber underneath. The pore is visible on
the surface as light dot.

Also present on the dorsal surface along the midrib are
characteristics cup-shaped structures called the gemma-cups.
The margin of a cup can be smooth, dented or frilled; a feature
helpful in recognizing different species.

From the bottom of the cup arise numerous, green, flat,
multicellular bodies called the gemmae. The detached
gemmae give rise to two new gametophyte plants.

The lower or ventral surface of the thallus is violet in colour.
On the ventral surface arise rhizoids and scales on both side
of the midrib.

Rhizoids

The rhizoids are numerous, elongated, single-celled, hair like
outgrowths.
They arise beneath the scale. They anchor the plant to the
substratum. In addition they absorb water and minerals.
The rhizoids are to two kinds smooth walled and tuberculate.

The smooth walled rhizoids penetrate the substratum to absorb
water and to fix the thallus to it. They are borad, thin walled
with colourless contents. They are first to be formed on
germination.

The tuberculate rhizoids are thick walled, narrow and appressed to the surface
of the thallus. the tuberculate or peg-like rhizoids have pronounced peg-like
invaginations of the wall called the tubercles. They extend across the
cavity but do not form  complete partitions. They function as a capillary
conducting system which serves to carry water to all the absorptive parts of the
thallus and useful in retaining water.

Scales

Scales are multicellular, one celled in thickness. They are usually violet
in colour due to the presence of anthocyanin pigments. They are arranged
in 2-4 rows on either side of the midrib. Generally, each scale is divided
into two parts, the body and the appendage. Based on presence or absence
of appendage, the scales are of two types – (i) Appendiculate – has appendage
and (ii) Ligulate – has no appendage.
The appendiculate scales from the inner row near the midrib and ligulate form
the outer row.

On the periphery of scales develops a mucilage papillae. These arise as idioblast
cells, elongate and appear club-shaped.
The mucilage papillae secrete mucilage which surrounds and protects the growing
region of the thallus from dessication.

The mature thalli produce sex organs on special stalked erect structures called
gametophores or gametangiophores. They are of two kinds, antheridiophores and
archegoniophores. These branches arise from the growing apex situated in the
apical notch.
The antheridiophores bears the male sex organs called the antheridia and
archegoniophores bears the female sex organs called the archegonia.

Marchantia is a dioecious plant – male plant bears antheridiophores and female
plant bears archegoniophores.

INTERNAL STRUCTURE

A vertical section of the thallus shows two zones; upper assimilatory or
photosynthetic zone and lower storage zone.

Assimilatory or Photosynthetic zone:-

The uppermost layer of the photosynthetic zone is upper epidermis. It is single
layered and consists of thin-walled cells. These cells are green with few .
chloroplasts The outer walls of the epidermal cells are water proof. Thus
epidermis is protective and checks transpiration.

The upper epidermis is interrupted by special, chimney like or barrel shaped
air pores which lead to air chambers below. Each pore is surrounded by 4-8
super imposed tier of concentric rings, each ring again composed of 4-5 cells.
The cells are arranged in the form of a small chimney which encloses a wide
passage. The passage is broad in the middle and narrow above and below.
The lower most ring has cell projecting inwards giving a star-shape or cruciate
appearance of the pore when viewed from the above.

The air pores facilitates gaseous exchange necessary for photosynthesis and respiration.
The air pores are analogous to the stomata of the higher plants, but they do not exhibit
any regulatory mechanism.

The air pores opens into the air-chambers which are present beneath the upper epidermis.
They are arranged in single horizontal layer. The chambers are partitioned by one cell
layer thick partitions which connect the upper epidermis with the inner cells. The partitions
are 3-4 cells in height. Each chamber opens to the exterior by means of air pore.
From the floor of the air chambers arise numerous branched assimilatory or photosynthetic
filaments. They nearly fill the cavity of the chamber. All the cells of the floor, side walls
and the photosynthetic filaments contains numerous, ovoid chloroplasts. The chambered
region constitutes the ventilated photosynthetic tissue. It is the principal centre of
photosynthesis in the thallus.

Storage Zone

The storage zone is present on the ventral side of the thallus, below the air-chambers.
It consists of a uniform tissue made up of large, colourless, thin-walled polygonal,
parenchymatous cells. This cells lacks chloroplast. The storage zone is thick towards
the midrib and thin towards the margins. Some cells may contain a single large oil
body or may be filled with mucilage. The forms cells are called oil body cells and latter
mucilage cells.

The main function of the storage region is the storage of starch, proteins, oils and
mucilage.
The lower most layer of the storage zone is the lower epidermis. It is composed of a
single layer of compactly arranged cells.

Rhizoids and scales arise from the lower epidermis. The scales help to retain the moisture
below the thallus. This enables Marchantia to grow in drier places.

Growth

The growing point lies at the bottom of a notch at the apex of each lobe. It consists of
horizontal row of meristematic cells. By the activity of these the thallus grows in length.

Reproduction

Marchantia reproduces by vegetative and sexual methods.

Vegetative Reproduction

The thallus reproduces vegetatively in the growing season by:

Fragmentation

This is most widely occurring method of vegetative reproduction
in Marchantia. It occurs by progressive death and decay of the
older portions of the thallus and younger branches separate.
The young branches by apical growth develops into an independent
gametophyte. In this way there is rapid increase in the number of
plants in a particular area and constant invasion of the new territories.

Formation of Adventitious branches

Special types of branches develops from the ventral surface of the thallus.
In M. assamica and M. palmate, the adventitious branches arise from the
stalks of archegoniophores. These branches when detach from the thallus
develop into new individuals.

Gemmae Formation

The most common method of vegetative reproduction in Marchantia is by the
formation of multicellular bodies called the gemmae. They are produced inside
shallow, cup-like structures called the gemma cups. The gemma cups develop
on the dorsal surface of the thallus in the midrib region.

A gemma cup is approximately 0.2 cm in diameter and o.3 cm in height.
Its margins are hyaline, lobed or toothed. Intermingled with the gemmae
in the cup are the club-shaped mucilage hairs which secrete mucilage.
They absorb water, swells and causes the gemmae to get detach and help in
dissemination of gemmae from the gemma cup.

Structure of Gemmae:

Gemmae are small, green, discoid, lens-shaped, multicellular structures. They
are several cell thick in the middle region and gradually thins towards the
margins. Each gemma has a short, delicate, single cell stalk, the stalk attaches
it to the bottom of the gemma cups. Over the stalk a multicellular, discoid body
is present. The discoid body has two notches on lateral side, opposite to each
other in each notches lies growing apex. Most of the cells constituting the body
of Gemma are green and contains abundant chloroplast and are green or
chlorenchymatous  cells.
Some cells situated in the margin contain oil bodies instead of chloroplast
they are called as oil cells.  A few cells in the central region of ventral side
are colorless and are known as “rhizodial cells”. These cells give rise to rhizoids
on germination.

Dispersal:

The mucilage hairs associated with gemmae help in dispersal. With the
absorption of water, the mucilage cells swells and cause the gemmae to
get detached from their stalks. The gemmae are them washed on to the
soil and carried by a current of water far from the plant.

Germination of Gemmae:

Gemma, after falling on the suitable substratum starts germination. The
side of gemma in contact with the soil becomes ventral surface. The
rhizoidal cells present on the ventral surface develops rhizoids.

The meristems or growing points present in the two lateral notches grow
simultaneously in opposite direction. On decay of central part of gemma,
the two thalli become independent and develops into two independent
gametophyte plants.
Thus, a single gemma gives rise to two independent plants.
Usually, the gemmae produced from male thallus gives rise to male
gametophyte and those produced from female thalli produce female
gametophyte.

Sexual Reproduction:

The sexual reproduction in Marchantia is oogamous type. The sex organs
are formed only under special environmental condition like long day length,
high humidity and low nitrogen content.

The sex organs are borne on special erect and stalked branches called
gametophores or receptacles.
The receptacles bearing male sex organs i.e., antheridia are called
antheridiophores. The receptacle bearing female sex organs i.e., archegonia
are called the archegoniophores or carpocephalum.

The gametophores or receptacles are actually direct prolongations of the thallus.
The thallus like nature of the gametophore is shown by the presence of dorsiventral
nature with scales and rhizoids within the grooves on the outer (ventral) side and
photosynthetic chambers on the inner (dorsal) side. Marchantia is sexual dimorphic
and heterothallic (dioecious i.e., male and female thalli are separate).

Abnormal receptacles bearing both archegonia and antheridia are seen in M. palmate
and M. polymorpha. Such receptacles are called androgynophorous. Archegonia
develop on the under surface and antheridia in cairtier on the upper side of the
receptacles.

Antheridiophore:

The erect branches that bear antheridia is called antheridiophore. They are 1 to
3 cm in length. Each antheridiophore consists of a stalk and a disc shaped receptacle.

The stalk is cylindrical. The internal structure of the stalk shows air-chamber with
assimilatory filaments and pores on its posterior side. On anterior side it has two
grooves or furrows, they run longitudinally the entire length of the stalk. They
contain rhizoids and scales.

The receptacle is flat and typically has eight lobes. M. germinate has only four
lobes. The internal structure of the peltate disc is similar to that of thallus. There
is upper epidermis with air pores.
The pores opens into underlying air-chambers containing the photosynthetic
filaments. Beneath, the chambers is compact region of colourless parenchyma
cells. The lower epidermis bears scales and rhizoids.

Alternating with the air-chamber on the upper side of the receptacle are the
flask shaped antheridial chamber. Each antheridial  chamber contains a single
antheridium. Each chamber opens externally through a pore called ostiole.

The antheridia are formed in acropetal succession, older antheridia at the
centre and younger at the margins.

Structure of Antheridia:

Each antheridium is an ovoid structure having a short, multicellular stalk and
a large oval body. The stalk attaches the antheridium to the floor of the
antheridial chamber.

The body of antheridium has a jacket layer of sterile cells constituting its wall.
The wall encloses a mass of androcyte mother cells. Each androcyte mother cell
divides to form two androcytes or sperm cells. The androcytes metamorphoses
into a antherozoids. The antherozoids are small, rod shaped or coiled,
uninucleate and biflagellate.

Dehiscence of antheridium:

The antheridia dehisce in the presence of water provided by rain or dew drops.
Water enters the antheridial chambers through the ostiole. Few cells of jacket
absorb water and disintegrate. Thus, antheridia ruptures and large mass of
androcytes comes out just like a smoke column. The antherozoids swim in the
film of water with the help of flagella.

Archegoniphore:
It is also called as carpocephallum. It consists of a stalk bearing eight lobed
disc-like receptacle. The growing apices of these lobes bend downwards and
inwards towards the stalk.

The archegonia are formed on the upper surface of the young receptacles with their
neck directed upwards. The archegonia develop in acropetal manner i.e., older
archegonia at the centre and the younger ones at the margins. The archegonia occur
in 8 radial rows corresponding to the 8 lobes of the disc. In each row there are 12-14
archegonia.

After fertilization, the stalk of the archegoniophore starts elongating and there is
tissue proliferation in the central area of receptacle. This results in inversion of
marginal portion of receptacle. Consequently, the archegonia are transferred to the
lower surface with their neck downward. The position of archegonia is reversed,
the oldest archegonia are at the periphery of the receptacle and youngest near the
stalk.

Along with these changes, due to intercalary growth, the central part of disc
produces long, sterile, green, cylindrical lobes of tissue called the rays. These
rays alternate with the archegonia bearing fertile lobes. They are usually 8 in
number. They hang downwards like the ribs of a tiny umbrella. At maturity, they
spread widely and give stellate (star-shaped) appearance to the female receptacle.

After the inversion of archegonia, there is a formation of one layered, plate-like
tissue on either side of every group of archegonia. This two lipped, curtain-like
sheath is known as perichaetium or involucre.

Structure of Archegonium:

The archegonium has a short and distinct, multicellular stalk and flask-shaped
body. The stalk attaches the archegonium to the surface of the receptacle.

The body of archegonium consists of two parts, the basal swollen venter and a
long slender, tubular neck. The venter consist of a wall of single layer of sterile
cells. Inside the wall there is a venter cavity. The venter cavity has two cells –
the larger egg cell at the base and smaller venter canal cell.

The neck is made of six longitudinal rows of neck cells, characteristic of the
Marchantiales order. Inside the neck there are 6-8 neck canal cells. the apical
part of the neck consists of 4 lid or cover cells.
The venter canal cell and neck canal cells degenerate at maturity forming a
mucilage. The mucilage absorb water and swells. This exerts a pressure on the
cover or lid cells and thus they separate and forms an opening.

Fertilisation:

Fertilization is dependent on the presence of water and is possible if male and
female plants grow together. The flattened, slightly concave upper surface of the
male disc acts as a shallow splash cup. Rain drops falling on male disc can splash
the androcytes upto two feet. This is called splash cup mechanism.

All the splash droplets falling on the neighbouring female receptacle will contain
antherozoids. And antherozoids splashed on the ground by the rain drops from
male disc may swim the whole way and reach the female receptacle.

From the female receptacle, they are attracted towards the archegonia by the
chemical protein or Potassium salts present in the drop of mucilage, that oozes
through the open necks of archegonia. Many antherozoids enter the  archegonia
and swim downwards in the neck canal.
One of the antherozoids fuses with the egg and produce a zygote. The zygote is
diploid.

Post Fertilisation Changes:

As a stimulus of fertilization, the stalk of an archegonium elongates becoming
a couple of mm in length. The fertilized egg enlarges and fills up the cavity of
the venter of the archegonium. It develops into the sporophyte initiating
sprorophytic generation.

The cells of venter wall divides to form a2 to 3 layered calyptra. It envelops
the growing sporogonium. A ring of cells at the base of venter divide and
re-divide to form one layered covering around the growing sporogonium and
is called perigynium or pseudoperianth.

On either side of each row of archegonia, a membranous flap known as
perichaetium or involucre is developed. It is one cell in thickness and has
fringed margins. It separates one row of archegonia from the other.

Thus, a developing sporophyte has 3  concentric sheaths of gametophytic
origin; the calyptra, perigynium or pseudoperianth and perichaetium or involucre.
The main function of these sheaths is to provide protection against drought.

Sporophyte Phase:

The diploid zygote is the first cell of the sporophytic phase. It consists of the
zygote, embryo and the sporogonium. Spore mother cells and elaters are the
last structure of this phase.

Development of sporogonium:

The zygote starts dividing mitotically within 48 hours after the fertilization.
The first division of zygote is transverse and results in the formation of upper
cell called the epibasal cell and the lower cell called the hypobasal cell.

The next division is at right angle to the first and forms a four celled embryo
called the quadrant stage. The next division is vertical forming an 8- celled
embryo called the octant stage.

The four epibasal octant by repeated cell division give rise to the capsule.
Since, the capsule which constitutes the apex of the sporophyte, is formed
from the outer or epibasal region, the embryogeny is described as exoscopic.

The four hypobasal octants form the foot and seta.

When the embryo is about a dozen or more cells, periclinal division in the
capsular region result in the differentiation of an outer layer called the
amphithecium and the inner layer called the endothecium.

The amphithecium remains one-cell-thick and form the wall of capsule. The
cells of endothecium divide repeatedly to form the archesporium. The
archesporial cells divide and redivide to form massive sporogenous tissue.

In Marchantia sporophyte, the cells of foot, set and capsule are
chlorophyllous, indicating that the sporophyte is not fully dependent on
the gametophyte.

Structure of sporogonium:

The mature sporogonium is differentiated into three regions, (i) foot,
(ii) seta, (iii) capsule.

Foot:

The basal bulbous portion of the sporogonium is called the foot. It is made
up of parenchymatous cells and is embedded in the tissue of female
gametophyte. It helps in the anchorage of sporophyte and absorption of food
from the gametophyte for the developing sporogonium.

Seta:

It is a stalk that connects the foot with the capsule. It is made up of
parenchymatous cells. First it is very short. But with the formation
of spore tetrads in the capsule, the seta enlogates rapidly and ruptures
the calyptra. It pushes the mature capsule through the perigynium and
perichaetium.
Thus, seta not only acts as stalk but also helpsin the spore dispersal.

Capsule:

The upper fertile region of the sporogonium is called capsule. It is oval in
shape and yellow in colour. The capsule wall is single layered made up of
sterile cells.

Enclosed within the capsule wall, there is sporogenous tissue. Initially, the
cells of sporogenous tissue are all alike and vertically elongated. Later, they
get differentiated into two types of cells; spore mother cells and elater
mother cells.

Elater mother cells elongates further and form long, slender, diploid elaters.
The elaters have tapering ends, lacks protoplasm and have double spirally
thickened bands on their walls.
They are dead and hygroscopic i.e., they absorb moisture. They help in the
dehiscence of mature capsule and in the gradual dispersal of spores.

The spore mother cells undergoes meiosis to form spore tetrads. The
individual spores separate at maturity of the capsule. The meiospores are
small in size, spherical and has a thick wall.
The spore wall is differentiated into outer smooth, thick exine or exosporium
and inner thin endospore or intine. Inside the spore wall there is a tiny mass of
granular cytoplasm.
It contains a single nucleus and some reserve food.

Morphologically, the spores are all alike and genetically each tetrad produces
two different kinds of spores.  Two of these produce male thalli and other two
female thalli. This is an example of functional or physiological heterospory.

Dehiscence of Capsule:

With ripening of spores, the seta elongates rapidly, so that the calyptra
enclosing the capsule is ruptured. It projects beyond the perigynium and
the pericheatium.
As the exposed capsule dries the capsule wall ruptures and split open
along 4-6 lines.
The wall later splits in an irregular manner by 4-6 lobes or valves.
Hygroscopic changes in the elaters assist in the dispersal of spores by
wind.

Germination of spore:

On falling on a suitable moist substratum, the spores germinate
immediately.
The spores imbibe water and increases considerably in size. It is
accompanied by the reappearance of chlorophyll.
Spore undergoes asymmetric division, the smaller cell which is
achlorophyllous extends to form a germ-rhizoid. The bigger
chlorophyllous cell form the protonema.
The protonema divides repeatedly to form the young gametophyte.

Life Cycle:

The life cycle of Marchantia is diplohaplontic and thus is characterized
by sporogenic meiosis and heteromorphic alternation of generation. The
haploid gametophyte alternates with a diploid sporopohyte.

The haploid gametophyte is most conspicuous of the two and is
independent.
It vegetatively reproduces by fragmentation, adventitious branches
and gemmae.
Sexually, it reproduces by gametes. The gametophyte is dioecious.
The male plant bears antheridiophores and female plant bears
archegoniophores. The antheridiophores bear antheridia which produce
numerous biflagellate antherozoids.

The archegoniophores bears archegonia in which non-motile female gamete
ova or egg cell is produced. The antherozoid fuses with the female gamete egg
to form diploid zygote (2n). The zygote divides and give rise to the sporophyte
or sporogonium. It is parasite on the female gametophyte.

The sporophyte consists of a bulbous foot, elongated seta and a spherical
capsule.
In the capsule, meiosis or reduction division takes places and haploid spores
are formed. The spores upon falling on a suitable substratum germinate and
produce a haploid gametophyte.