Course 3.2Lesson 1 of 3

Liverworts and mosses

Bryophytes live on land but need water for sexual reproduction, so they are called the amphibians of the plant kingdom. Their main plant body is the haploid gametophyte, held by rhizoids, with no true roots, stems or leaves and no vascular tissue. The sporophyte grows on the gametophyte and depends on it. Liverworts such as Marchantia form flat thalli and gemmae; mosses grow from a protonema into leafy shoots.

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Plants that still need water

After the monsoon, an old brick wall often turns green and soft, like velvet. Look closely and you will see tiny plants, packed close together. These are bryophytes: the liverworts and the mosses.

Bryophytes live on land, but they need water for sexual reproduction. So they are called the amphibians of the plant kingdom. They grow in damp, humid and shaded places: on wet soil, rocks, tree bark and walls.

The plant body

The body is more developed than that of the algae. It is thallus-like, lying flat (prostrate) or standing up (erect).
It is attached to the ground by rhizoids, which are unicellular or multicellular threads.
There are no true roots, stems or leaves. Some parts only look like them.
There is no vascular tissue, no xylem and no phloem. So bryophytes stay small, and water moves slowly from cell to cell.
The main plant body is haploid. It makes the gametes, so it is called the gametophyte.

Sexual reproduction

The sex organs are multicellular.

The male sex organ is the antheridium. It makes biflagellate antherozoids, male gametes with two flagella.
The female sex organ is the archegonium. It is shaped like a flask, and it makes a single egg.
The antherozoids are released into water. They swim to the archegonium and one of them fuses with the egg. The result is a zygote.

The zygote does not divide by meiosis at once. It grows into a multicellular body called the sporophyte. The sporophyte is not free-living. It stays attached to the green gametophyte and takes its food from it. Some cells of the sporophyte then divide by meiosis and make haploid spores. Each spore grows into a new gametophyte.

Liverworts

Liverworts grow in moist, shady places: on the banks of streams, on marshy ground, on damp soil, on tree bark and deep in woods.

In Marchantia, the body is a flat thallus, pressed close to the ground. It is dorsiventral: its upper and lower sides differ.
Leafy liverworts have tiny leaf-like parts in two rows on a stem-like axis.
Asexual reproduction is by fragmentation of the thallus, or by gemmae. Gemmae are green, multicellular buds that form in small gemma cups on the thallus. They break off and grow into new plants.
The male and female sex organs grow on the same thallus or on different ones.
The sporophyte has three parts: a foot, a seta (stalk) and a capsule. Spores form inside the capsule by meiosis.

Two drawings of the liverwort Marchantia. On the left, a flat green thallus seen from above: a ribbon that forks into two lobes, with a midrib and a small notch at each tip. Two round gemma cups sit on its upper surface, each holding a few small green gemmae, and thin rhizoids hang from its lower side. On the right, two stalks seen from the side, each rising from a thallus. The male stalk, the antheridiophore, ends in a flat disc that bears the antheridia on top. The female stalk, the archegoniophore, ends in a small head with finger-like rays, and the archegonia hang below it.

Mosses

In a moss, the gametophyte has two stages.

The protonema grows directly from a spore. It is creeping, green, branched and often thread-like.
The leafy stage grows from the protonema as a side bud. It has an upright, slender axis with leaves arranged in a spiral. Multicellular, branched rhizoids fix it to the soil. It bears the sex organs at its tip.

Mosses also reproduce vegetatively, by fragmentation and by budding of the protonema. After fertilisation, the zygote grows into a sporophyte with a foot, a seta and a capsule. It is more elaborate than in liverworts, and mosses have a special way of scattering their spores. Examples are Funaria, Polytrichum and Sphagnum.

A moss and its two stages. On the left, lying on the soil, the protonema: branched green threads that grow from a spore, with a dark bud that will become a leafy shoot. On the right, an upright leafy shoot with small leaves arranged around a slender axis, held in the soil by branched rhizoids. This leafy shoot is the gametophyte. On its tip grows the sporophyte, drawn in orange: a foot sunk in the gametophyte, a long stalk called the seta, and a capsule with a lid at the top, where spores form by meiosis. A note says that the sporophyte grows on the gametophyte and depends on it.

Why bryophytes matter

Some mosses are food for grazing mammals, birds and other animals.
Sphagnum forms peat, which has long been used as a fuel. Dry Sphagnum holds a great deal of water, so gardeners use it to pack living plants for long journeys.
Mosses and lichens are among the first plants to grow on bare rock. They break the rock down slowly, and make it ready for larger plants.
Mats of moss soften the blow of falling rain and hold the soil, so they reduce soil erosion.

You should find that $40$ g of dry moss that holds $20$ times its mass carries $800$ g of water. More than nine tenths of the wet moss is water.

Try it in Python

The program works out the water in a box of wet Sphagnum.

You should see $800$ g of water, a wet mass of $840$ g, and a share of $95$ %. With a multiple of $10$ the share is still about $91$ %, because the dry moss is always a small part.

Practise with new numbers

Common mistakes

Calling a moss a small fern. A moss has no vascular tissue, and its main plant body is the haploid gametophyte.
Thinking rhizoids are roots. Rhizoids hold the plant in place. They are simple threads, not true roots.
Thinking the moss sporophyte is a separate plant. It grows on the gametophyte and takes food from it.
Mixing up gemmae and spores. Gemmae are many-celled buds made without meiosis. Spores are made by meiosis in the capsule.

Remember these

Bryophytes need water for fertilisation: the amphibians of the plant kingdom.
The main plant body is the haploid gametophyte, with rhizoids and no vascular tissue.
Antheridia make biflagellate antherozoids; archegonia make one egg each.
The sporophyte has a foot, a seta and a capsule, and depends on the gametophyte.
Liverworts: thalli, gemma cups. Mosses: protonema, then a leafy stage.