Mosses (Bryophytes)

What are mosses?

Mosses belong to a group called bryophytes. They were among the first plants to live on land. Most mosses are only 1 to 10 cm tall and grow in thick green patches on damp soil, rocks, walls and tree trunks.

Why so small? A moss has no pipes (no xylem or phloem). Water and food move slowly from cell to cell. Pipes are needed to carry water to a tall plant, so mosses cannot grow tall.

Structure of a moss plant

A moss plant has three simple parts.

• Rhizoids: thin brown threads at the bottom. They hold the plant to the soil. They are not true roots, because they are only a few cells thick and do not carry water up through pipes.
• Stem: a thin, upright stalk.
• Leaves: very small and flat, often only one cell thick. They make food by photosynthesis and soak up water straight from rain and mist.

The whole green plant is the gametophyte. Its cells have one set of chromosomes (n).

Life cycle of a moss

A moss life cycle has two bodies that take turns. This is called alternation of generations.

1. The green plant (n) grows male tips that make sperm and female tips that make an egg.
2. In wet weather the sperm swim through a film of water to the egg. This is fertilisation. The sperm have tails, which is why water is needed.
3. The fertilised egg (zygote, 2n) grows into a sporophyte: a stalk with a capsule on top. It stays attached to the green plant and takes food from it.
4. Inside the capsule, cells divide by meiosis and make many spores (n).
5. The capsule opens when it is dry. Wind carries the spores away.
6. A spore that lands on damp ground grows into a green thread, the protonema, and then into a new moss plant.

In a moss, the green plant you see is the main stage (n). The capsule (2n) is small and depends on it.

Peat bogs and sphagnum

Sphagnum (peat moss) grows in cold, wet places called bogs. Its leaves have big empty cells that hold water, so one handful can hold many times its own weight in water, like a sponge.

The top layer stays alive and green. The lower layers die. In the wet, sour, low-oxygen water, bacteria and fungi cannot rot the dead moss fully. New moss keeps growing on top, and the dead layers are pressed down and slowly turn into peat. A bog gains very roughly 1 mm of peat a year, so a metre of peat takes about a thousand years.

• Dried peat can be burned as fuel in some countries.
• Gardeners use it to hold water in soil.
• Bogs store a lot of carbon and water, so draining them harms the climate and wildlife.

Why mosses matter

Mosses are pioneers. They grow on bare rock and make a thin layer of soil, so other plants can follow. They hold water on slopes and reduce soil wash-away. They are also food and shelter for tiny animals.

Try it: your own moss study

Find a moss patch in the rainy season. Gently lift a small piece and look with a magnifier. Count the leaves on one stem. Find the rhizoids at the bottom. Look for thin stalks with a capsule on top. Next, sprinkle water on a dry brown moss and wait ten minutes. What happens? Write what you saw. (Please put the moss back where you found it.)

Key formulas and definitions

• Key term: bryophyte = plant with no pipes, needs water for fertilisation
• Key term: rhizoid = root-like thread that only holds the plant
• Green moss plant = gametophyte (n); capsule with stalk = sporophyte (2n)
• Order: sperm + egg → zygote (2n) → sporophyte → spores (n) → protonema → moss plant
• Peat bog: about 1 mm of peat per year

Worked examples

Common mistakes

• Calling rhizoids roots. Rhizoids only hold the moss; they do not carry water up pipes.
• Saying mosses have flowers or seeds. They make spores.
• Mixing up n and 2n: the green plant is n, the capsule is 2n.
• Thinking peat is coal. Peat is young and soft; coal is much older and harder (see ferns and coal).