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.
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.
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 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.
Oil bodies are structures in a liverwort's leaves. Although they were first discovered centuries ago, nobody knows what they do. They come in a range of sizes, shapes and colours. Most are brown or colourless, but a few are blue. They probably deter hungry slugs and insects.
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.
Without protection from ultraviolet radiation, the liverworts living at high altitude would soon die. The native Ptilidium ciliare protects itself with reddish pigments known as flavonoids, which act like a sunscreen.
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.