Bryology (mosses,
liverworts and hornworts)
What are bryophytes?
Bryophytes are the oldest land plants on earth,
and have been around for 400 million years or more.
Although small, they can be very conspicuous
growing as extensive mats in woodland, as cushions
on walls, rocks and tree trunks, and as pioneer
colonists of disturbed habitats.
They comprise three main taxonomic groups:
mosses (Bryophyta), liverworts (Marchantiophyta) and
hornworts (Anthocerotophyta) which have evolved
quite separately.Worldwide there are possibly 10,000
species of mosses, 7000 liverworts and 200 hornworts.
Most bryophytes have erect or creeping stems and tiny
leaves, but hornworts and some liverworts have only a
flat thallus and no leaves.
How do bryophytes live their lives?
Bryophytes have a two-stage life cycle (alternation
of generations). The ‘gametophyte' generation is
the green photosynthetic part (the familiar moss or
liverwort plant) attached to the substrate by threads
(rhizoids), and the ‘sporophyte' generation which
consists of a stalk and capsule which are
dependent on the gametophyte for support and
nutrients.
Marchantiophyta
A division of bryophyte plants commonly
referred to as hepatics or liverworts.
Most liverworts are small, usually from 2–20
millimetres (0.08–0.8 in) wide with individual plants
less than 10 centimetres (4 in) long, so they are
often overlooked. The most familiar liverworts
consist of a prostrate, flattened, ribbon-like or
branching structure called a thallus (plant body);
these liverworts are termed thallose liverworts.
LIVERWORTS ARE PRIMITIVE
One reason liverworts are so curious is that in
terms of the evolution of life on Earth, these plants are
old. The first liverworts arose as green alga was making
its transition onto land during the Devonian Era some
400,000,000 years ago -- and that's a long, long time
before more advanced plants such as flowering plants,
ferns and mosses appeared. In fact, liverworts are often
referred to as "the simplest true plants." Here are some
easy-to-see aspects of their primitive nature:
• Instead of bearing regular roots, liverworts anchor
themselves with rather primitive and simple, onecelled appendages known as rhizoids
•Unlike tree leaves which have veins that conduct
water, nutrients and other materials, in liverworts
there is little or no conducting tissue
•Tree leaves have window-like stomata which close
when the leaf is threatened with drying out; liverworts
have nothing like stomata, so the whole plant shrivels
when dry
How they Reproduce
Liverworts, like all bryophytes, have two forms of
reproduction. Asexual or vegetative reproduction,
and sexual reproduction.
Sexual Reproduction
The sexual reproduction for leafy liverworts is very
similar to the mosses. The sexual parts are contained in
small and inconspicuous structures known as antheridia
(male) and archegonia (female), which develop on
separate plant bodies.
In the thallose liverworts, things are a little different.
Marchantia species the antheridia and archegonia are
produced on an umbrella like structure. While in
others species they are hidden in small pockets on
the leafs
In leafy liverworts the antheridia produce mobile
antherozoids (sperm), which require a film of water
in which to move to the archegonia, where
fertilisation takes place. After fertilisation, a new
plant develops, which remains attached to the
parent plant.This is the sporophyte.
Asexual Reproduction
The gametophyte can propagate itself vegetatively, and also
produce the gametes, which give rise to the saprophyte.
Vegetative reproduction can occur as a result of older parts
of a plant dying off so that the newer branches become
separated; by specialised whip-like branches; or by leaves that
drop off the plant.
In the thallose liverworts a more complicated system is used,
with propagative structures called gamma cups forming on
the leaves. Each gamma cup gives rise to numerous gametes
that are released when water droplets splash into the cup
thus transported the gametes to favourable sites to grow into
new plants.
In most species, a haploid liverwort spore
germinates and gives rise to a single-celled protonema,
a small filamentous cell. In general, the haploid
gametophyte develops from the protonema. In most
liverworts, the gametophyte is procumbent, although in
some species it is erect. Typically, the gametophyte has
a subterranean rhizoid, a specialized single-celled
structure which anchors the liverwort to its substrate
and
takes
up
nutrients
from
the
soil.
Male and female reproductive organs, the
antheridia and archegonia, grow from the gametophyte.
These arise directly from the thallus or are borne on
stalks. About 80% of the liverwort species are dioecious
(male and female on separate plants) and the other 20%
are monoecious (male and female on the same plant).
Each archegonium produces a single egg; each
antheridium produces many motile sperm cells, each
with two flagella. The sperm cells must swim through
water to reach the archegonium. Then, the sperm
fertilizes the egg to form a diploid cell. This eventually
develops into a multicellular diploid sporophyte.
The sporophyte of liverworts, like that of mosses, has a
terminal capsule borne on a stalk, known as a seta. As the
sporophyte develops, haploid spores form inside the
capsule. In general, the sporophytes of liverworts are
smaller and simpler in morphology than those of mosses.
Another difference is that the liverwort seta elongates
after capsule maturation, whereas the moss seta
elongates
before
capsule
maturation.
The hornwort gametophyte thallus is not much
different from a thallose liverwort. The thallus is a onecell-thick sheet at the margins and forms a pad of cells
closer to the middle of the thallus. The thallus is
irregular in outline and sometimes "frilled" at the
margins. Beneath the thallus the lower epidermis
anchors itself with rhizoids. Hollow areas of the
central pad region may house symbiotic cyanobacteria.
These are likely providing a particular mineral nutrient
for the hornwort.
Archegonia and antheridia are partially or
completely embedded in the upper surface of the thallus
in the padded central region. Both have sterile jackets
which open in response to free water. The sperm are
chemotactically attracted to the egg across a film of
water trapped on the surface of the pad.
The zygote and resulting young sporophyte is
initially dependent on the gametophyte for nutrition.
The archegonium neck proliferates at the base of the
seta and around the foot of the sporophyte. The seta
outgrows the neck, however and emerges into the
sunlight. It begins to do its own photosynthesis.
Its epidermis is cutinized and includes stomata. The
cortical layer inside carries out photosynthesis.
Surrounding hydroids and leptoids in the center of the
seta, are cells which become the sterile jacket and the
sporocytes. The latter undergo meiosis to produce
spores. As the seta matures, it splits open longitudinally
to shed the spores from within.
A truly amazing aspect of the sporophyte is that
just above the foot the seta is meristematic and can keep
making new seta for a long period of time...even after
most of the gametophyte has disintegrated! The
sporophyte then is more-or-less indeterminate in
growth! This is the "horn" of the "hornwort.”
The plant body may have conducting tissue and some
of this has a familiar look. The xylem-like water-andmineral-conducting tissue is called hydroid. The
phloem-like sugar-and-amino-acid-conducting tissue
is called leptoid.
Mosses are small, soft plants that are typically 1–10 cm
(0.4-4 in) tall, though some species are much larger. They
commonly grow close together in clumps or mats in
damp or shady locations. They do not have flowers or
seeds, and their simple leaves cover the thin wiry stems.
At certain times mosses produce spore capsules which
may appear as beak-like capsules borne aloft on thin
stalks.
mosses are bryophytes, or non-vascular plants. They can
be distinguished from the apparently similar liverworts
(Marchantiophyta or Hepaticae) by their multi-cellular
rhizoids.
In addition to lacking a vascular system,
mosses have a gametophyte-dominant life cycle,
i.e. the plant's cells are haploid for most of its life
cycle. Sporophytes (i.e. the diploid body) are
short-lived and dependent on the gametophyte.
This is in contrast to the pattern exhibited by
most "higher" plants. In seed plants, for example,
the haploid generation is represented by the
pollen and the ovule, whilst the diploid
generation is the familiar flowering plant.
The life of a moss starts from a haploid spore. The
spore germinates to produce a protonema (pl.
protonemata), which is either a mass of thread-like
filaments or thalloid (flat and thallus-like).
This is a transitory stage in the life of a moss, but
from the protonema grows the gametophore ("gametebearer") that is structurally differentiated into stems and
leaves. A single mat of protonemata may develop several
gametophore shoots, resulting in a clump of moss.
From the tips of the gametophore stems or
branches develop the sex organs of the mosses. The
female organs are known as archegonia (sing.
archegonium) and are protected by a group of modified
leaves known as the perichaetum (plural, perichaeta). The
archegonia are small flask-shaped clumps of cells with an
open neck (venter) down which the male sperm swim.
The male organs are known as antheridia (sing.
antheridium) and are enclosed by modified leaves called
the perigonium (pl. perigonia). The surrounding leaves in
some mosses form a splash cup, allowing the sperm
contained in the cup to be splashed to neighboring stalks
by falling water droplets.
In the presence of water, sperm from the
antheridia swim to the archegonia and fertilization
occurs, leading to the production of a diploid sporophyte.
The sperm of mosses is biflagellate, i.e. they have
two flagellae that aid in propulsion. Since the sperm must
swim to the archegonium, fertilisation cannot occur
without water.
After fertilisation, the immature sporophyte pushes
its way out of the archegonial venter. It takes about a
quarter to half a year for the sporophyte to mature.
The sporophyte body comprises a long stalk, called a
seta, and a capsule capped by a cap called the
operculum.
The capsule and operculum are in turn sheathed
by a haploid calyptra which is the remains of the
archegonial venter. The calyptra usually falls off when
the capsule is mature. Within the capsule, sporeproducing cells undergo meiosis to form haploid
spores, upon which the cycle can start again.
The mouth of the capsule is usually ringed by a
set of teeth called peristome. This may be absent in
some mosses.
Raindrop
1 Spores develop into
threadlike protonemata.
Key
Male
gametophyte
Haploid (n)
Diploid (2n)
Sperm
“Bud”
2 The haploid
protonemata
produce “buds”
that grow into
gametophytes.
Protonemata
4 A sperm swims
through a film of
moisture to an
archegonium and
fertilizes the egg.
Antheridia
3 Most mosses have separate
male and female gametophytes,
with antheridia and archegonia,
respectively.
“Bud”
Egg
Spores
Gametophore
Female
Archegonia
spores develop in the sporangium gametophyte
of the sporophyte. When the
Rhizoid
sporangium lid pops off, the
peristome “teeth” regulate
6 The sporophyte grows a
gradual release of the spores. long stalk, or seta, that emerges
Seta
from the archegonium.
8 Meiosis occurs and haploid
Peristome
Sporangium
MEIOSIS
Mature
Mature
sporophytes
sporophytes
Capsule
(sporangium)
FERTILIZATION
(within archegonium)
Calyptra
Zygote
Embryo
Archegonium
Foot
Capsule with
peristome (LM)
Female
gametophytes
Young
sporophyte
7 Attached by its foot, the
sporophyte remains nutritionally
dependent on the gametophyte.
5 The diploid zygote
develops into a
sporophyte embryo within
the archegonium.