What is a bryophyte?
Bryophytes are a group of plants that include mosses,
liverworts and hornworts.
Mosses may be small, but they may also be as complex
as flowering plants. They have stems with leaves, and there is just
about as much variation in the form and size of these plants as there
is in the flowering plants. The 20,000 species range from being microscopic
to over a metre; they may be upright, or creeping and much branched.
They may grow in streams or deserts, on mountain tops or in sea spray,
from the antarctic through tropical rain forests to the arctic, and
in fact just about anywhere except in the sea itself.
Liverworts may be leafy and very similar to mosses
(although the fruit looks quite different). Or they may form flat plates
of apparently leafless tissue, in which case they are called thallose.
Hornworts look like thallose liverworts, but have fruit that is unlike
that of either mosses or liverworts. It is probable that mosses, liverworts
and hornworts are not at all closely related, being united only by sharing
their peculiar life-cycle.
Mosses and Liverworts have been called up-side-down
or rôle-reversal plants. The green and often leafy part underneath
that we would think of as the moss or liverwort itself, is equivalent
to tiny parts within a flower, or to a small, rarely seen part of the
fern. The part that is equivalent to all of the flowering plant or fern
that we normally see, is the fruit of the moss or liverwort.
As in all plants, and indeed animals, these two parts,
or generations, alternate with each other in the life-cycle. The spores
produced by the moss fruit will germinate into green leafy plants. These
plants produce gametes, or eggs and sperm, and the resulting embryos
grow up into new fruits. But what makes bryophytes different from all
other plants, is that the fruit or spore-bearing generation remains
semi-parasitically attached to the green gamete-bearing generation,
and never becomes independent. The truth about this strange life cycle
was not finally established until 1851!
The gamete-bearing generation is the green plant that
most people think of as the main plant. This is the part that traps
light energy and converts it into food for both generations.
One of the most attractive and flower-like structures
of mosses is found on the fruit. The fruit consists of a fruit stalk
whose foot remains embedded in the moss plant, and a spore-producing
capsule. When the spores are ripe, the lid falls off, but the release
of spores is usually controlled by a fringe of flexible teeth round
the capsule mouth.
These are the peristome teeth (meaning ‘around
the mouth’) and they are as varied and beautiful as any flowers,
but very much smaller.
Moreover, they move as they are watched, since small
currents of air cause changes in humidity, making the teeth flex in
and out. These movements assist in the most effective distribution of
spores, flicking out small quantities when air conditions are right,
and stopping the process when they are not. Look at a ripe moss capsule
with a hand lens, and your breathing will make the teeth flex.