Mosses, liverworts & moss reproduction
Bryophyte is a general term used to cover mosses, liverworts and hornworts.
They appear to have developed from green algae.
Figures given for the number of genera vary from 12,000 up to 20,000.
They are small plants that do not have flowers or seeds but reproduce by spores.
They are non-vascular plants as, on the whole, they do not have specialised structures
to transport water and nutrients throughout the plant.
Almost all contain chlorophyll so can photosynthesise their food from water and carbon
dioxide absorbed through their surfaces.
They prefer moist conditions but can survive in a large range of habitats.
Some only live in specific areas or conditions, others are more widespread.
They can grow on almost any substrate including wood, leaves, soil and rocks.
They can become dormant in dry conditions then resume growth when wet.
They lack true roots.
Is it a moss or liverwort?
- Mosses are always leafy but only some liverworts are.
- Moss leaves are unlobed; liverworts are variously lobed.
- Moss leaves are spirally arranged; liverworts are in 2 or 3 rows.
- Spore capsule stalks are rigid and often pigmented in mosses;
in liverworts they are delicate and transparent.
A generalised description of mosses.
Most are green but they can be other colours.
They usually grow in thick mats with most being less than 10 cm high.
Upright mosses are usually unbranched and the sporophyte grows from the tip of the stem.
These usually grow on the ground.
Prostrate mosses are branched and the sporophyte grows from the side of the stem.
These usually grow on trees, other plants or litter.
Mosses aways have leaves, often only one cell thick, sometimes with a midrib.
Some with a central vein have a rib or flap (lamella) on either side of it.
The leaves are unlobed and most are spirally arranged on a thin stem.
There may be wrinkles running across or down a leaf, the margins may be toothed, curve
up or down and some have a bristle at the tip.
They lack true roots but many are anchored by short root-like rhizoids.
There may be a long, stiff, opaque, unbranched stalk or seta supporting the spore capsule.
In mosses the stalk forms before the spore capsule matures.
There is only 1 capsule per stalk and they come in a large variety of sizes, shapes and textures.
Most species have capsules with a protective cap (operculum or calyptra) which may be smooth
or hairy and which falls off as the stalk grows.
Most capsules have a rim of peristome teeth which can vary from a few large ones up to so many
they look like hairs and some have 2 rows of teeth.
In some genera the tips of the peristome teeth are attached to a white ‘disc’.
1. The Gametophyte.
The most prominent and long lasting part of the life cycle of most Bryophytes is the vegetative
phase (the gametophyte) which produces the eggs and sperm (the gametes).
Mosses and some liverworts have gametophytes consisting of stems and leaves.
Hornworts and some liverworts have gametophytes that are sheet-like thalli undifferentiated
into leaves or stems.
When mature the gametophyte produces special (microscopic) structures in which the eggs
and sperm are produced.
Archegonia are flask-shaped structures on a stem.
The egg is at the base of the flask and there is a long narrow neck down which the sperm enter.
Antheridia are small sacs about 0.5 mm across that produce the sperm.
Both structures are at the tips of the stems and may be surrounded by modified leaves.
They can be on the same or separate plants.
A sperm swims through the surface water to fertilise an egg to produce a zygote which
develops into a sporophyte.
2. The Sporophyte.
A sporophyte consists of a capsule (the sporangium) in which the spores are formed, and it can
be sessile or on a stalk supported by a foot.
They can take months to mature and are dependent on the gametophyte for nutrition.
Spores dispersed, mostly by wind, and landing on a suitable substrate develop into a new gametophyte.
Bryophytes can also reproduce asexually by fragmentation.
Gemmae are small collections of cells, either just on the surface or in specialised structures.
These break off and when dispersed grow into new gametophytes.