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.
