Table of Contents
- Sphagnum Classification
- Structure of Gametophyte
- Reproduction
- Antheridium
- Archegonium
- Fertilisation
- Structure of Sporophyte
- Dehiscence of Capsule
- Germination of Spore
Sphagnum is a genus of moss that is also commonly known as peat moss, sphagnum moss and bog moss. They are of ecological importance as they form peat that serve as carbon sinks in the ecosystem. It grows in wet and boggy areas and requires acidic soil.
Sphagnum Classification
|
Kingdom |
Plantae |
|---|---|
|
Division |
Bryophyta |
|
Class |
Sphagnopsida |
|
Order |
Sphagnales |
|
Family |
Sphagnaceae |
|
Genus |
Sphagnum |
Structure of Gametophyte
- The gametophyte of Sphagnum species is represented in two stages: juvenile protonema and leafy gametophore.
- Young gametophytes bear multicellular rhizoids with oblique septa. However, mature gametophytes do not have rhizoids.
- The gametophyte is differentiated into a main axis with branches and leaves.
- The main axis is soft and delicate when young but becomes erect and stout at maturity.
- The axis is profusely branched. At the apex of the main stem, tufts or branches of limited growth, known as coma, are seen.
- The leaves are borne on both the main axis and branches. They are small, sessile and without a distinct midrib.
Reproduction
Like other bryophytes, Sphagnum shows alternation of generation with the haploid gametophytic stage being the dominant one. The species of Sphagnum can either be unisexual or bisexual. The gametophytes reproduce by fragmentation to produce most of the living materials.
The sexual reproduction in Sphagnum is of advanced oogamous type. The sporophyte remains attached to the gametophyte and is short-lived. It is shiny green in colour with spherical spore capsules. The antheridia and archegonia grow on specialised male and female branches that occur at the coma or the lower nodes of the main axis.
Antheridium
The antheridial branches are short and the antheridia are borne on them in an acropetal fashion. They are club-shaped with a long multicellular stalk and a globular body. The body has a single layer of jacket that encloses numerous androgonial cells.
The androgonial cells develop into spirally coiled sperms that have a pair of flagella. Upon maturation, sperms are released from the antheridia by separation of the jacket cells.
Archegonium
The archegonial branches are short, thick and purple in colour. 3-5 archegonia grow in groups at the tip of the branch. A mature archegonium is flask-shaped with a swollen neck and long twisted venter. The neck has 6 vertical layers of neck cells and cover cells. The venter has a ventral canal cell and a basal egg cell.
Fertilisation
Fertilisation in Sphagnum takes place only in the presence of water. The sperm cells swim up to the archegonia. The neck canal cells and ventral canal cells get disorganised to form a passage for the entry of sperm cells. It reaches the egg cell and forms a diploid zygote.
Refer: Life Cycle of a Moss Plant – An Overview
Structure of Sporophyte
The sporophyte is a dark-coloured spherical structure that grows attached to the female branch. A mature sporophyte is divided into a foot, seta and capsule. The foot is embedded in the tissues of the gametophyte. The seta is a narrow neck-like region that connects foot and capsule.
The capsule has a central core of sterile cells called columella. It is surrounded by a thin spore sac. The columella and the spore sac are surrounded by 4-6 layered capsule walls that have non-functional stomata. A disc-shaped lip or operculum is found at the top of the capsule that is differentiated from the rest of the capsule by a ring-like structure called annulus.
The remains of archegonia cover the tip of the capsule by forming calyptra. At maturation, the operculum is blown off after rupturing of annulus cells and the spores are released in the air.
Dehiscence of Capsule
The capsule blows off with an explosive mechanism. The columella and the capsule wall become dry and shrink. This creates an air space in the capsule. The capsule then becomes cylindrical in shape and a pressure is created inside the structure. Due to this pressure the operculum bursts open through the annulus making a distinguishing sound. The spores are catapulted and released in the air. This whole mechanism is known as the air-gun mechanism.
Germination of Spore
The spore released from the sporophyte is the first cell of the gametophytic generation. It is arranged in tetrads with a triradiate ridge. The spore has two layers: an outer exine and an inner intine.
The spores germinate in favourable conditions to form a flat thallus of the primary protonema. The protonema then gives rise to buds which further develop into erect leafy gametophytes.
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