Reproduction In Bryophytes

Reproduction in bryophytes occur by both sexual and vegetative means. Sexual reproduction occurs the combination of genes from two parent plants, potentially resulting in offspring that are genetically distinct from either parent. Whereas, vegetative reproduction does not involve genetic mixing; each new plant originates from a single parent.

Types of Reproduction in Bryophytes

A. Vegetative Reproduction in Bryophytes
B. Sexual Reproduction in Bryophytes

A. Vegetative Reproduction in Bryophytes

Vegetative reproduction in bryophytes is very notable and extensive, which occurs during seasons favorable for vegetative growth.

Most bryophytes propagate through vegetative reproduction, employing various methods to do so. Some of them as follows;

By Fragmentation

In bryophytes, the growing point is located at the tip of the thallus.
The older, basal portion of the thallus begins to rot or disintegrate due to aging or drought.
When this decay extends to the point of dichotomy, the lobes of the thallus become separated.
These detached lobes or fragments then grow into independent plants through apical growth.
Fragmentation is one of the most common vegetative method of reproduction in bryophytes, occurs extensively in species like Riccia, Marchantia, Anthoceros, and some mosses such as Sphagnum.

By Persistent Apices

In some bryophytes, such as Riccia, Anthoceros, and Cyathodium, prolonged dry conditions, summer, or the end of the growing season cause the entire thallus to dry out and deteriorate, except for the growing point.
This growing point then penetrates the soil and becomes thicker.
When conditions become favorable again, it develops into a new thallus.

By Tubers

Tubers form in species exposed to desiccation. As the growing season ends, the subterranean branches swell at their tips to create underground tubers.
These tubers develop two to three layers of water-resistant, corky, hyaline cells on their periphery, surrounding inner cells that contain starch, oil globules, and albuminous layers.
During unfavorable conditions, the thallus dies, but the dormant tubers remain unaffected.
Tubers also serve as organs of perennation is also one of the most occurring mode of vegetative reproduction in bryophytes during favorable condition as in Anthoceros, Conocephalum, Conicum, and Fossombronia.

By Gemmae

Gemmae are green, multicellular reproductive structures of various shapes.
They are formed in gemma cups, on leaf surfaces, at the stem apex, or even inside cells.
Once detached from the parent plant and landing on a suitable substrate, gemmae can directly develop into a new individual, as seen in Marchantia, or indirectly, as in mosses.

By Adventitious Branches

Adventitious branches grow from the ventral surface of the thallus in species like Riccia fluitans and Anthoceros.
Once detached from the parent plant, these branches develop into new thalli.
In Marchantia and Dumortiera, the branches emerge from the archegoniophore, whereas in Pellia, they arise from the dorsal surface or margins of the thallus.

By Regeneration

The liverworts possess an amazing power of regeneration.
Part of the plant or any living cell of the thallus (e.g., rhizoid, scales) are capable of regenerating the entire plant, e.g., Riccia, Marchantia etc.

By Innovation

In Sphagnum, one branch in the apical cluster grows more vigorously than the others, continuing to grow upward instead of forming drooping or divergent branches.
This tall, upright branch exhibits all the characteristics of the main axis and is called an innovation.
As the parent plant progressively dies and decays, these innovations detach and establish themselves as new parent plants.

By Primary Protonema

The primary protonema is a filamentous stage produced by developing moss spores, which gives rise to leafy gametophores.
It breaks into short cell filaments due to the death of cells at intervals.
Each detached fragment grows into a new protonema that bears a crown of leafy gametophores, as seen in Funaria serves as the mode for vegetative reproduction in bryophytes.

By Secondary Protonema

The secondary protonema forms by methods other than spore germination.
It can develop from any living cells of the leafy gametophore, such as those from the leaf, stem, rhizome, injured portions of the leafy gametophore, antheridium, paraphysis, or archegonium.
Leafy gametophores or lateral buds emerge from this secondary protonema in the same way as from the primary protonema, as observed in Funaria and Sphagnum.

By Bulbils

These are small resting buds develop on rhizoids.
Bulbils are devoid of chlorophyll but full of starch.
On germination bulbils produce a protonema which bears leafy gametophores

By Apospory

Apospory is the production of a diploid gametophyte from an unspecialized sporophyte without meiosis, as seen in Anthoceros.
In Funaria, green protonemal filaments can arise from unspecialized cells in various parts of the sporogonium.
These filaments bear lateral buds that develop into leafy gametophores.

By Cladia

Cladia are small or broad detachable branches that aid in vegetative reproduction in bryophytes.
Cladia which arises from individual leaf cells are called leaf cladia found as in Plagiochila, Bazzania, and Frullania spp.
Cladia which arises from the stem are called stem cladia as seen in Bryopteris.

By Separation of Whole Shoots

Deciduous, catkin-like branches develop over the surface of the gametophytic plant.
When these branches separate, they develop into new plants, such as seen in Pohlia nutans.

By Separation of Shoot Tips

In Campylopus piriformis, the separated shoot tips grow into new plants.

By Rhizoidal Tips

The apical part of young rhizoids divides and re-divides to form a gemma-like mass of cells, as in Riccia glauca.
These cells contain chloroplasts and can develop into a new thallus.

B. Sexual Reproduction in Bryophytes

Gametophytic Phase

Sexual reproduction in bryophytes is highly oogamous.
The male and female reproductive organs are called antheridia (singular: antheridium) and archegonia (singular: archegonium), respectively usually formed in receptacles.
These reproductive organs are surrounded by multiple layers of protective tissue.
The antheridium is stalked, pear-shaped or oblong, with a one-cell-thick outer layer that encloses fertile cells known as androcytes. Each androcyte transforms into a biflagellate antherozoid.
The archegonium is a stalked, flask-shaped structure with a swollen base called the venter and an elongated neck. The neck contains many neck canal cells, while the venter holds a large egg cell and a small venter canal cell.
Antherozoids are chemically attracted to the neck of the archegonium by substances such as sugars, malic acid, proteins, and potassium salts present in the mucilaginous material formed by the degeneration of neck canal cells and the venter canal cell.
Water is essential for fertilization during reproduction in bryophytes.
The fertilized egg, or zygote, marks the beginning of the sporophytic phase and is retained within the venter of the archegonium.

Reproduction in bryophytes (typical Liverwort) — Fig: Reproduction in bryophytes (Typical Liverwort) [Image Source: by **LadyofHats** , wikimedia / Public Domain]

Sporophytic Phase

Without a resting period, the zygote undergoes continuous divisions to form a multicellular structure known as the embryo.
The zygote’s first division is always transverse, and the outer cell develops into the embryo. This type of embryogenesis is called exoscopic.
The embryo develops into a sporophyte or sporogonium.
The sporophyte typically differentiates into a foot, seta, and capsule.
The sporophyte remains attached to the parent gametophytic plant body throughout its life and relies on it partially or completely for nutrition.
The foot is a basal, bulbous structure embedded in the tissue of the parent gametophyte, primarily functioning to absorb nutrients from the parent gametophyte.
The seta is located between the foot and capsule, elongating to push the capsule through protective layers and conducting food from the foot to the capsule.
The capsule is the terminal part of the sporogonium, responsible for producing spores.
All bryophytes are homosporous, meaning all spores are similar in shape, size, and structure.
The capsule produces sporogenous tissue, which entirely develops into spore mother cells only or in some species it differentiates into spore mother cells and elater mother cells.
Spore mother cells undergo diagonal division to produce four haploid spores arranged in tetrahedral tetrads.
Elater, which are of hygroscopic in nature are formed in liverworts but not in mosses.
The venter wall enlarges with the developing sporogonium, forming a protective multicellular layer called the calyptra, which is the gametophytic tissue enclosing the sporophyte.
The meiospore, which is formed after meiosis, is the initial cell of the gametophytic phase.

Each spore is unicellular and haploid. It germinates directly into a young gametophytic plant, as seen in Riccia or Marchantia, or it first forms a filamentous protonema that produces buds, which then develop into a young gametophytic plant, as observed in Funaria.

Alternation of Generation

The life cycle of a bryophyte involves a regular alternation between gametophytic and sporophytic generations.
The haploid phase (n) is the gametophyte or sexual generation, which bears the sexual reproductive organs that produce gametes, such as antherozoids and eggs.
The union of these gametes forms a zygote, which develops into the sporophyte, the diploid phase (2n).
The sporophyte produces spores that always germinate into gametophytes.
During spore formation, spore mother cells undergo meiotic division to produce haploid spores, marking the beginning of the gametophytic or haploid phase.
These spores germinate and develop into the gametophytic phase, which bears sex organs.
Eventually, gametic union occurs, resulting in a diploid zygote (2n), marking the start of the sporophytic or diploid phase.

Thus, the sporophyte generation spans from fertilization (syngamy) to meiosis (reduction division), while the gametophyte generation spans from meiosis to fertilization, showing alternation of generation.