Mosses and Liverworts

by John Walsby

Moist pond and stream banks are often carpeted with green mosses and liverworts. These primitive plants are closely related, though many have quite contrasting growth forms.

Mosses form springy cushions of tightly packed slender stems. Each stem has a close spiral of small pointed leaves with mid-ribs.

All liverworts have their leaves flattened against the ground. Some are quite moss-like but their small leaves are placed alternately on each side of a stem and have no midribs. Easiest to distinguish are the thaloid liverworts with large multilobed leaves which often have vase shaped reproductive structures, called gemmicups, on the upper surfaces.

Like ferns, mosses and liverworts have two distinct generations in their life cycles. One releases spores and the other produces gametes - eggs and sperm. With ferns, it is the spore producing plant (or sporophyte) which we recognise as the fern. Its gamete-producing generation (or gametophyte) is short lived and very small - smaller than a fingernail. A sporophyte sprouts from the gametophyte if fertilisation is successful and grows up into a large plant.

In mosses and liverworts the gametophyte is the dominant generation. The sporophytes are just single stems, each topped with a spore capsule and are obvious on many mosses but very small on liverworts. The male gametes require a film of water in which to swim to the stationary eggs before fertilisation can occur. Damp conditions are also necessary for the adult plants to flourish. Their leaves do not have thickened outer skins, or hairs to reduce evaporation, nor storage organs for holding water. They do not even have a proper root system to absorb water from below the soil surface, just fine rootlet hairs, called rhizoids, growing out of the stem bases.

Many mosses and liverworts are attractive in miniature and are homes for small animals that like to live close to water but not in it. Check the banks of your local pond and stream to find the different mosses and liverworts and look amoungst their leafy growths for sheltering inhabitants.

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