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.
Four key features are common to liverworts, hornworts and mosses, and distinguish them from more complex plants such as ferns, conifers and flowering plants:
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.
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:
There are two main types of liverwort:
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.
It is easy to confuse leafy liverworts with mosses. There are a number of ways to tell the difference between the two:
Hornworts get their name from their horn-shaped spore capsules. These slender, upright capsules are where the plants make and shed their spores.
Hornwort is also the common name for the water weed Ceratophyllum demersum, an introduced flowering plant that has spread to many lakes and rivers in New Zealand.
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.
Although they look similar to thalloid liverworts, hornworts differ in four important ways:
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.
Malcolm, Bill, and Nancy Malcolm. The forest carpet: New Zealand’s little-noticed forest plants. Nelson: Craig Potton, 1989.
Allison, K. W., and John Child. The liverworts of New Zealand. Dunedin: University of Otago Press, 1975.