Liverworts and hornworts

Features and life cycle

Liverworts and hornworts are significant in New Zealand, but often overlooked or mistaken for mosses. In New Zealand these small plants grow from the coasts to alpine zones, but most abundantly in rainforests. In the past, they were grouped with mosses under the name bryophytes. We now known that, although they share common features, liverworts, hornworts and mosses evolved independently.

Key features

Four key features are common to liverworts, hornworts and mosses, and distinguish them from more complex plants such as ferns, conifers and flowering plants:

• They lack specialised fluid-conducting tissues (xylem and phloem). That limits their size, because they cannot draw water and mineral nutrients more than about half a metre up their stem.
• Their water-producing cells are not stiffened with lignin.
• The plant in their life cycle that produces spores is smaller and attached to the plant that produces sex cells.
• The spore-producing plant is unbranched and bears a single spore capsule.

Life cycle

In common with all plants, liverworts and hornworts have a life cycle with two generations. The green plant that we call a liverwort or a hornwort produces sex cells (eggs and sperm). Inside the green plant an egg and sperm unite into a single cell, which then begins to grow into a spore-producing plant. This new plant remains attached to its parent, which it depends on it for water and nutrients.

Liverworts

New Zealand has over 500 species of liverwort – 7% of the world’s estimated 7,000 species. Worldwide there are about half as many liverwort species as there are moss species, but in New Zealand the two groups are nearly equal in number.

Features

Liverworts have three unique traits:

• Most have special structures known as oil bodies in their leaf cells.
• Their spore-shedding generation is not green and lives only a short time.
• Their spore capsules swell and the spores mature before the capsule’s stalk starts to lengthen.

Types

There are two main types of liverwort:

• Leafy liverworts have leaves.
• Thalloid liverworts do not have leaves – they are sheets of cells. If they are thick and have internal air spaces with pores to the outside, they are called complex thalloid liverworts. If they are thin and have no air spaces or pores, they are called simple thalloid liverworts.

Leafy liverworts

Leafy liverworts come in a huge variety. New Zealand’s largest species, Schistochila appendiculata, grows up to a metre long in damp environments. The leaves of some species are covered in so many fine hairs that they look woolly. The hairs are important because they hold a volume of water weighing several times the plant’s dry weight, as insurance against drying out. A few leafy liverworts have water sacs, which harbour tiny swimming animals and bacteria, which provide nitrogen to the liverwort.

Thalloid liverworts

There are fewer thalloid species, but because they are bigger than leafy liverworts they are more noticeable. Some can reproduce asexually as well as sexually (with eggs and sperm).

Species of Marchantia are easily recognised by the many splash cups on their surface. The cups are filled with tiny green discs called gemmae. When a raindrop hits a cup, the energy of its fall is exploited by the shape of the cup to hurl the gemmae out. They can land up to a metre away. If they fall onto a moist surface, the gemmae grow into new plants. Not all liverwort splash cups are circular – Lunularia cups are shaped like a crescent moon.

New Zealand’s largest thalloid species, Monoclea forsteri, grows up to 20 centimetres by 5 centimetres. In favourable conditions, individual plants growing together can cover square metres of damp bank or stream-side rocks.

Differences between leafy liverworts and mosses

It is easy to confuse leafy liverworts with mosses. There are a number of ways to tell the difference between the two:

• Liverwort leaves are lined up in straight rows, never have midribs, and are often lobed. Moss leaves are spiralled around the stem, usually have a midrib, and are never lobed.
• Most leafy liverworts have a row of small leaves on one side of their stems, but few mosses do.
• The stalk of a leafy liverwort’s spore capsule is transparent and fragile. On a moss, the stalk of a moss capsule is coloured and strong.
• The spore capsule of a leafy liverwort looks like a brown sausage, and splits into four segments. The capsule of a moss is brightly coloured and usually opens from a central mouth ringed with teeth.

Hornworts

Hornworts get their name from their horn-shaped spore capsules. These slender, upright capsules are where the plants make and shed their spores.

Hornworts grow as a thick sheet of green tissue (a thallus). They are a small group of plants with about 100 species worldwide. Thirteen species of hornwort are known in New Zealand.

Differences between hornworts and liverworts

Although they look similar to thalloid liverworts, hornworts differ in four important ways:

• Most hornworts trap sunlight with only one or two massive chloroplasts in each of their cells (liverworts have dozens).
• Their chloroplasts can store carbon dioxide, a vital raw material for photosynthesis (liverworts cannot).
• They never have oil bodies in their cells (at least 90% of liverworts have oil bodies).
• Hornwort capsules continue to make and shed new spores for weeks (a liverwort spore capsule usually sheds all its spores within a few hours).

Habitats

Hornworts are found in a variety of habitats, but are most abundant in damp places such as clay banks. Most settle on soil or rock, although some species prefer bark, and others overrun mosses and liverworts. Anthoceros species are often found on damp banks, while Dendroceros giganteus lives on swampy ground.