Jungermanniales: Structure, Reproduction, Classification

Jungermanniales is the largest order of class Hepaticopsida in the division Bryophyta.

This order contains 244 genera and more than 9000 species.
Members of Jungermanniales are known as leafy liverworts. Because the gametophytes of this order are mostly foliose type and they are differentiated into stem and leaves.
In some genera, gametophytes are thallose types like those of Marchantiales.
Intermediate forms between thallose and leafy types are also noted among some members of this order.
But the presence of any air chamber or any internal tissue differentiation like the gametophytes of Marchantiales is absent.
The gametophytes bear only smooth-walled rhizoids.
Scales are absent.

Table of Contents

Structure of Gametophyte in Jungermanniales

The gametophyte of Jungermanniales is the plant body. The plants are thalloid, except Fossombronia.

The plants are thalloid in which midrib is present (Pellia) but maybe absent (Riccordia).
In Fossobronia, the plant body is erect and foliose, which is divisible in stem and leaves.
In Pellia, the gametophytic plant body is thin, dorsiventrally flattened, prostrate, and dark bright green in color.
Leaves are arranged in lateral margins, and a clear differentiation is observed in Fossombronia.
Scales and rhizoids are absent in most plants. But in Pellia, numerous simple, smooth-walled, unicellular rhizoids are present irregularly on the mid-ventral surface of the thallus.

The internal structure of the thallus is very simple. No tissue differentiation is observed in the thallus.
Simple, thin-walled chlorenchymatous tissues are present between the upper and lower epidermis. From the lower epidermis rhizoids arise in Pellia.
A single large apical cell situated in the apical notch brings out the growth of the thallus.

In some species, sex organs are developed at specific branches called antheridial and archegonial branches.
Fossombronia is heterothallic, hence the male and female organs develop at separate thallus.
In Pellia, the thallus may be monoecious (Pellia epiphylla) or maybe dioecious (P. endivaefolia). Sex organs (antheridium and archegonium) are borne on the thallus.

Antheridium is the male sex organ.
In antheridium, any cell behaves like a superficial cell and divides by transverse division to form an upper cell and a basal cell.
Generally, the basal cell is inactivated and the upper cell divides to form a stalk cell and a primary antheridial cell.
Stalk cell divides to form 2-celled stalk, while antheridial cell divided by transverse division and oblique division through which peripheral cells and central cells are formed.
Peripheral cells are divided and a one-celled thick jacket is developed, while central cells divide to form antherozoid mother cells.
Antherozoid mother cells produce antherozoids which are spiral, biflagellate, and uninucleate.
In such a way, mature antheridium is made up of a 2 celled stalk, and a club-shaped structure.

Archegonium is the female sex organ developed at the female thallus.
In Pellia, The group of archegonia is protected by an involucre (an outgrowth of the thallus tissue developing from the base of the archegonium).
In archegonium, any cell behaves like a superficial cell which is divided by transverse division to form an upper cell and a basal cell.
Basal cell forms stalk. While the upper cell divides by oblique division to form peripheral cells and central cells.
Peripheral cells continue to divide to form a jacket and the central cell divide to form the primary cover cell and central cell.
The primary cover cell produces 4 cover cells and the central cell forms a primary neck initial and a primary venter initial.
The Neck initial divides to form a neck canal cell and the primary venter initial divides to form one ventral canal cell and an egg.
In such a way, mature archegonium consists of 4 cover cells, 6-8 neck canal cells, one ventral canal cell, and an egg.

When archegonium matures and the antherozoid is released towards the archegonium, neck canal cells and ventral canal cells dissolved and water enters in neck portion.
From the neck of the archegonium, some salts or proteins are excreted which stimulates the antherozoids to move in that direction.
Then the antherozoid enters the archegonium male nucleus fused with an egg to form a zygote.

A zygote is the first cell of the sporophyte, immediately after fertilization, the zygote increases in size.
The zygote is divided by transverse division to form epibasal cells and hypobasal cells.
Hypobasal cell is inactivated, while epibasal cell divides through 3 transverse, one longitudinal, and one vertical division.
In this process, 4 tiers of 4-4 cells are formed.
Upper 2-tiers form capsule.
In upper tiers, periclinal division takes place through which outer amphethecium and inner endothecium are formed.
Amphethecium develops in the outer jacket, while outer endothecium develops in the inner jacket.
Inner endothecium changed into archesporial tissue, in which some are fertile and some are sterile.
Fertile cells undergo meiosis to form spore-tetrad but sterile cells are present either at the lower portion (Pellia) or at the upper portion (Riccardia).
These cells are elongated and form elaterophores. The elaterophores produce elaters.
The elaters are spindle-shaped elongated structures provided with 2-3 spiral thickening bands.

The sporophyte consists of the foot, seta, and capsule.
The foot is a small bulbous type and the seta is very short.
The capsule is made up of 2 layers of thick jacket and inner side elaterophores.
In elaterophores, elaters and spore-tetrads are present.

When the capsule is matured then the upper jacket is dried away while the lower jacket is moist, so due to contraction and expansion capsule is opened into 4 slits.
Its opening is similar to a flower opening.

Some common genera of Order Jungermanniales is Pellia, Porella, Fossombronia, Riccardia, etc.

Smith (1938) has divided Order Jungermanniales into two sub-orders: