Why Mosses Are Called the Amphibians of the Plant Kingdom

The green scum on a village pond, the seaweed washed up on a beach and the soft green carpet on a damp wall in the monsoon are all simple plants. Algae live mostly in water, while bryophytes such as mosses live on land — yet still need water to reproduce.

This lesson covers the three classes of algae and their uses, the features of bryophytes, liverworts and mosses, the life cycle of Funaria, and the importance of bryophytes.

Algae are simple, chlorophyll-bearing, mostly aquatic plants; green algae (Chlorophyceae), brown algae (Phaeophyceae) and red algae (Rhodophyceae) differ in their pigments, stored food, cell walls and flagella.

Chlorophyceae (green algae):

- Pigments: chlorophyll a and b
- Stored food: starch; cell wall: cellulose
- Flagella: 2 to 8, equal, at the tip of the cell
- Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara

Phaeophyceae (brown algae):

- Pigments: chlorophyll a and c, with the brown pigment fucoxanthin
- Stored food: laminarin or mannitol; cell wall: cellulose with a gelatinous coating of algin
- Flagella: 2, unequal, attached at the side
- Body often divided into a holdfast, a stalk called the stipe, and a leaf-like frond
- Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus

Rhodophyceae (red algae):

- Pigments: chlorophyll a and d, with the red pigment r-phycoerythrin
- Stored food: floridean starch; cell wall: cellulose, pectin and polysulphate esters
- Flagella: absent
- Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium

Economic importance:

- Algae carry out a large share of the photosynthesis on Earth
- Agar from Gelidium and Gracilaria is used to grow microbes and to set jellies and ice creams
- Algin from brown algae and carrageen from red algae are used as thickeners
- Chlorella is used as a protein-rich food supplement

An everyday example. Coastal communities in Tamil Nadu that farm and harvest seaweed supply red algae from which gels such as agar and carrageen are extracted.

The substance. Red algae can grow deeper than most other algae — r-phycoerythrin absorbs the blue-green light that still reaches deep water.

Bryophytes are small, non-vascular land plants with a dominant haploid gametophyte, rhizoids instead of true roots, and a sporophyte that stays attached to the gametophyte; they need water for fertilisation, which is why they are called the amphibians of the plant kingdom.

General features:

- Grow in damp, shady places
- Plant body is thallus-like or has simple stem-like and leaf-like parts, but no true roots, stems or leaves
- No xylem or phloem
- Male sex organs are antheridia; female sex organs are flask-shaped archegonia
- Male gametes, called antherozoids, swim through water to reach the egg

Liverworts versus mosses:

- Plant body — liverworts are usually flat and thalloid, as in Marchantia; mosses have an upright leafy stage, as in Funaria
- Rhizoids — unicellular in liverworts; multicellular and branched in mosses
- Asexual reproduction — liverworts form gemmae in gemma cups; mosses reproduce by fragmentation and by budding of the protonema
- Protonema — absent in liverworts; a green, thread-like early stage in mosses

An everyday example. The velvety moss that covers compound walls and roof tiles in the monsoon dries and turns brown in the hot months, then turns green again when the rains return.

The substance. Bryophytes stay small because they lack vascular tissue — without xylem, water cannot be carried far up a stem.

The life cycle of the moss Funaria alternates between a dominant, haploid gametophyte that produces gametes and a diploid sporophyte that grows on the gametophyte and produces spores by meiosis.

Stages of the life cycle:

- A haploid spore germinates into the protonema, a green, branched, thread-like stage
- Buds on the protonema grow into the leafy gametophyte, with multicellular rhizoids
- The gametophyte bears antheridia, which release antherozoids, and archegonia, each containing an egg
- Antherozoids swim through a film of water to the egg; fertilisation forms a diploid zygote
- The zygote grows into the sporophyte, made of a foot, a stalk called the seta and a capsule
- Inside the capsule, meiosis produces haploid spores
- The capsule opens, and a ring of teeth called the peristome helps release the spores gradually in dry weather

Alternation of generations. The haploid gametophyte (n) and the diploid sporophyte (2n) follow each other in every cycle.

An everyday example. The thin brown stalks with tiny capsules rising above a patch of moss are sporophytes, each still growing out of the green gametophyte below.

The substance. The moss sporophyte is not free-living — it depends on the gametophyte for water and part of its food, the reverse of the situation in ferns and flowering plants.

Bryophytes provide peat fuel and packing material, serve as food for some animals, and help build and hold soil as pioneer plants on bare rock.

Uses:

- Some mosses are food for grazing mammals and birds
- Peat forms from the moss Sphagnum and is dried and used as fuel
- Sphagnum holds a great deal of water, so it is used as packing material to keep living plants moist during transport

Ecological roles:

- Mosses, along with lichens, are pioneers that colonise bare rock
- They slowly break down rock, forming soil in which larger plants can grow
- Dense moss mats soften the impact of falling rain and prevent soil erosion

An everyday example. Nurseries in hill towns use sphagnum moss to line hanging baskets and keep orchid roots moist.

The substance. Pioneer mosses change the habitat that later replaces them — the soil they build lets grasses and shrubs move in.