Most mosses are small, green leafy plants attached to soil, bark or rock by fine filaments known as rhizoids. Some have single erect stems, whereas others have branching stems and develop into a sprawling carpet or drape from trees. New Zealand’s tallest moss, Dawsonia superba, grows to a height of half a metre, but few others are anywhere near that tall – most are about 1–2 centimetres tall.
Life cycle: two plants in one
The moss life cycle has two different plants. One of those two plants produces spores, and in mosses it’s always attached to and partially dependent on the other plant, which produces sex cells.
Where to find mosses
Mosses occupy a wide range of habitats in New Zealand. Many favour damp, shady sites, and thrive on the forest floor and around the base of trees. Others flourish best in light, and grow as epiphytes on the branches of trees, or in open areas such as clay banks and the edges of tracks. A few are aquatic, submerged in streams and lakes. All sphagnum or peat mosses grow in swamps and bogs where there is standing water year-round. Granite mosses (Andreaea species) are a hardy group and form dense, dark cushions on alpine rocks.
In the city, few plants can take root in concrete and bitumen, and even fewer can cope with the petrochemicals and dust in the air. But half a dozen moss species can grow far better in the cracks and crevices in footpaths, walls, gutters and drains than they ever could in the bush. They are called urban mosses. One in particular, the silvery bryum (Bryum argenteum), has established itself in cities everywhere, and is now the most widespread plant in the world.
Silvery bryum looks silvery because the cells of the upper third of its leaves die and lose their chlorophyll. Their skeletal remains are white and shiny, especially when dry. The dead tips act like a layer of sunscreen, protecting the still-living lower two-thirds of the leaves from damage by ultraviolet radiation. When the dead leaf tips are dampened by rain, they turn glass-like, allowing light to penetrate the moss cushion.
Survival in tough conditions
Many mosses have some things in common with the stalwart silvery bryum. They are small and can fit into cracks and knotholes, where bigger plants cannot survive. They do not die if they dry out, and do not suffer if their habitat becomes dry, or if wet and dry conditions alternate. During dry spells their metabolism shuts down until the rains return, when their processes are re-launched and repairs are quickly made. Large plants cannot do that, because drying fatally airlocks the complex internal pipes that carry water and dissolved minerals and food.