|Bryophytes - mosses and liverworts
Bryophytes - mosses and liverworts (including hornworts) are tiny plants scarcely noticed by many, but closer
inspection will reveal their fantastic beauty and diversity of form. They are also informative organisms in illustrating
plant evolution and in illustrating how physics can be beautifully exploited by some of the simpler organisms.
Liverworts, like Marchantia above, are small plants with prostrate (lying flat against the substrate) leafy thalli. (A
thallus, plural thalli, is the name given to the body of plants that lack definite roots, such as multicellular algae and
bryophytes). There are no roots, strictly speaking, but there are rhizoids that serve for anchorage (but not water
and mineral absorption). The upper part of the thallus corresponds in some ways to the green shoot-system of a
tracheophyte (tracheophytes are larger more complex plants, such as ferns, conifers and flowering plants) in that
the cells contain chloroplasts. In Marchantia these cells are arranged in air chambers which contain branching
columns of photosynthesising cells which receive atmospheric carbon dioxide through stoma-like pores (which are
apparently incapable of closing like the stomata of tracheophytes). A good place to find liverworts is clinging to the
soil of riverbanks.
Alternation of Generations
The leafy thallus of the bryophyte is haploid and produces gametes by mitosis and is called the gametophyte. The
spermatozoids are biflagellate and swim through the surface film of moisture to reach the eggs which are retained
inside the thallus but open to the outside via pores. The spermatozoids are attracted to chemicals secreted by the
egg and one can fertilise each egg. The result of fertilisation is a diploid zygote which grows into a diploid plant
which produces spores by meiosis and is called the sporophyte. The sporophyte remains attached to the parent
gametophyte and is said to be 'parasitic' on it since it absorbs much of its nutrition from the gametophyte.
However, it is not true parasitism since it is a mutual symbiosis.
Water Transport in Mosses
Mosses differ from liverworts in having definite stems with (spiralling arranged) leaves in addition to anchoring
Ectohydric mosses (a) rely mainly on water transport along the external surface of the plant by capillarity. The
spaces between the leaves and stem and between papillae on the surface of leaves seem to be optimised to act
as capillary channels to draw water up and down the plant. The rhizoids may form a dense felt-like tomentum and
small leaf-like structures called paraphyllia and both these structures can assist capillarity. These mosses
generally have very thin non-waxy cuticles and are not waterproofed, allowing water to be absorbed along any
part of the body. these mosses dry out easily, but can tolerate dryness, rehydrating when water returns and,
contrary to popular belief, they are not confined to damp habitats. Some mosses can absorb fog and morning dew
through their surfaces and so thrive on arid mountains and in deserts. Most mosses are primarily ectohydric and
so have no specialised vascular tissue. Rather water can move slowly from cell-to-cell across the cortex via
plasmodesmata and more readily via the apoplast, though internal movement in these mosses is slow and
external capillarity is rapid. Generally, the upper tissues have the most rapid conduction and are supplied by
water first and some water may then move downwards inside the plant. Ectohydric mosses will usually rehydrate
within minutes when water is added to dry plants.
Endohydric mosses (b) rely mostly on internal water transport and have vascular tissues. They possess thin
wax-like cuticles that confer some degree of water repellency (though not to the extent seen in tracheophytes).
They have a primitive form of vascular tissue – the hydrome (consisting of hydroids and sclereids) conducts
water generally down the stem and although non-lignified resembles xylem. The hydroids are narrow elongated
cells that join together to form longitudinal tubes and are joined by slanted end-walls and resemble tracheids. Like
tracheids they lose their protoplasts when mature. Hydroids also appear to contain lignin-like polyphenolic
compounds in their cell walls. However, hydroids differ from the tracheids and vessel members of tracheophytes in
that their end-walls are not perforated by large pores to form pore/perforation plates (with few exceptions), but
instead contain many plasmodesmata; thus water transport through these vessels will encounter more resistance
and be much slower than in tracheophyte xylem. The sclereids are cylindrical cells that connect together to form
long thin fibres with thick-walls and narrow lumens and probably have both a mechanical supporting function and
a water-conducting function. Thickened cells in the cortex, especially the outer cortex also contribute to support
and may conduct water very slowly, probably mostly in a radial direction, via the apoplast and through
plasmodesmata (symplast). The leptoids constitute the leptome and resemble the phloem of tracheophytes and
do indeed conduct photoassimilates from sources to sinks. The leptome encloses the hydrome. Like phloem tube
elements these cells have degenerate nuclei at maturity, but unlike phloem they lack highly porous sieve plates,
instead their end-walls contain numerous plasmodesmata, so again conduction in the leptome is much slower than
in phloem. True leaf traces may be present – branches of vascular tissue that connect the midribs of the leaves to
the hydrome of the stem. However, some species have pseudo leaf-traces – vascular strands from the leaf
midribs that end blindly in the cortex and do not connect directly to the central strand of the stem.
Myxohydric mosses employ both the ectohydric and endohydric water-conducting pathways in varying ratios. It
can be argued that most, if not all, mosses are really mixohhydric in that conduction of water occurs by both
external and internal routes to varying degrees, even when a central strand is absent. Internal conduction may
account for as little as 1% of water transport (ectohydric) to 70% or more (endohydric).
The diagrams below illustrate the structures of cross-sections through the stems of an ectohydric moss (a) and an
endohydric moss (b).
Left: epiphytes growing on the bark of an oak tree; the
grey-green encrusting epiphytes at the top of the
picture are lichens (a fungus host and an algal or
cyanobacterial endosymbiont) and lower down is a mat
of epiphytic moss. The tip of one of these mosses is
shown magnified above. This moss is probably a
species of either Brachythecium or Isothecium,
similar-looking mosses that both grow as epiphytes.
Characteristic of Brachythecium this moss has oval
concave leaves with distinctly pointed tips and small
marginal teeth (denticles) and midribs reaching to just
above half-way along the leaf.
The photomicrographs below show the structure of the moss pictured at the top of this page.
The tip of a single moss stem. These
stems may be several cm long and run
mostly horizontally along the
substratum (tree bark in this instance)
but the tip regions may stand upright.
The leaves are sheets that are only one-cell thick. The cells are elongate and
packed with chloroplasts for photosynthesis.
Click photos to enlarge. Above and
below: details of the marginal spines
Below: close-up view of the midrib which consists of cylinders of more
elongated cells. Notice how the slanting end-walls of these cells are often
tightly and closely pressed together - suggestive of water-conducting tissue.
The midrib also provides the leaf with additional support.
Left: bundles of red-brown root-like
rhizoids branch from the under-surface
of the prostrate (horizontal or lying
down) part of the stem. This moss is a
pleurocarpous moss (a moss with
prostrate habit). These anchor the
moss plants in the tree bark, or in
matter that has accumulated on the
bark. They may have some role in
water-transport by acting as wicks to
draw water up along the outside of the
rhizoids by capillarity, but they are
probably not major organs of water
Below: a section through the moss stem. The image is out unfocused in places due to
the thickness of the section which was simply cut with a scalpel and not sectioned on
a microtome. Nevertheless, the stem can be seen to comprise two regions - an outer
cortex of smaller green thick-walled cells that give support to the stem (and probably
carry out some photosynthesis) and an inner region of larger cells containing
granules (starch grains and/or chloroplasts? This was a smallish stem, and the larger
stems, though still tiny, can be surprisingly tough to cut.
It is hard to stay whether or not this moss has specialised water-conducting tissue; a
longitudinal section would be needed to show this (there may be elongated
parenchyma for food transport). Certainly when slicing the stems, the inner cells
often frayed from inside their tough cortex, suggesting a filamentous nature. I have
not tried to cut lengthwise with my scalpel, this would be tricky, but I might have a go.
The greenish structures covering the surface of the stem are epiphytes - probably
cyanobacteria, so epiphytes have epiphytes!
Reproduction in Bryophytes
Bryophytes have extraordinary life-cycles in which the generations alternate between tow genetically distinct
forms - the haploid gametophyte, which have already looked at and the diploid sporophyte. This is the so-called
alternation of generations.
Haploid: Possessing only one set of chromosomes (like spermatozoa and ova in humans).
Diploid: Possessing two sets of chromosomes - one maternal and one paternal (like humans).
Curiously, the haploid form is the dominant growing stage and the adult form in bryophytes. It produces gametes
by mitosis (instead of by meiosis as in diploid organisms) and so is the mature sexual life-stage. The biflagellate
spermatozoids (or antherozoids)are produced inside containers called antheridia (singular antheridium),
from which they are released when ripe, and they use their two flagella to swim toward the archegonium (plural
archegonia), which is typically a vase-like structure with an egg cell at the bottom of it. The spermatozoids are
attracted to the ova which releases chemicals that the sperm sense. They will swim across in the film of surface
moisture covering clumps of bryophytes to reach her. One spermatozoid only will fertilise an ovum, producing a
diploid cell or zygote. This cell then develops into the sporophyte whilst still attached to the parent gametophyte.
The sporophyte comprises an elongated stalk or seta (plural setae) rooted in the gametophyte tissue (at the
base of what was the archegonium) by its 'foot'. The free tip of the seta swells and develops into a spore-packed
Pellia epiphyta is another liverwort which grows on damp soil in streams
and ditches. It is another non-leafy thallous liverwort anchored by
unicellular rhizoids and differentiated into a photosynthetic epidermis
and cortex and a starch-storing medulla. The thallous is
parenchymatous (comprised of largely undifferentiated plant cells that
non-fibrous)). The antheridia produce the male gametes (spermatozoa)
and the involucre is a flap which shields the flask-shaped archegonia
which contain the eggs. The sporophyte grows up from a fertilised
archegonium, protruding from the flap as a stalk several cm long
bearing a dark sphere containing the spores. Unlike Marchantia, Pellia
has no stomata or air-chambers.
Diagrams based on information from Plant Types 2: Mosses, Ferns, Conifers and Flowering Plants - an excellent
book by Ruth Miller (Pub. Hutchinson).
Above: photographs of moss spore capsules (taken with a compound microscope, which is not ideal for such large
structures, but it's all I have). These spore capsules belong to an epiphyte, collected from near the base of an oak
tree, which was a different genus to the one whose gametophyte is shown above. This latter species had leaves
without midrib (but with more elongated cells) which were lanceolate (pointed) as shown below:
The sporophyte derives some of its nutrition from the parent gametophyte, via its foot and is said to be
'parasitic' on the gametophyte. However, the sporophyte is capable of photosynthesis when young and green,
and so produces some of its own food. (Also, this isn't true parasitism, since the gametophyte is really
investing in its offspring). There is a small space between the edge of the foot and the adjacent cells of the
gametophyte and the gametophyte cells are thought to actively pump nutrients into this space and may be
modified into transfer cells (that is they have wall invaginations to increase the surface area of their
cell-surface membrane to accommodate more pumps). The foot absorbs these nutrients (and may also have
transfer cells to pump the nutrients in) and these are then conducted along the seta (which has leptoid and
hydroids in at least some cases) to the developing cells. The seta and then later the spore capsule typically
turn from green to red-brown in mosses as the cells die and dehydrate during spore release. The tip of the
developing sporophyte is typically covered in a sheath or calyptra, which is the remains of the neck and venter
of the flask-shaped archegonium in which the zygote germinates.
Spore Dispersal Mechanisms
Bryophytes have many ingenious mechanisms to assist in dispersal of their spores.
Mosses. When mature, the spore capsule dries (sometimes aided by stomata-like pores which become
uncovered when the calyptra is shed) typically a lid detaches from the spore capsule. The opening or mouth of
and inner peristome when it is double) of which there are often 16 teeth. These teeth have a two-play structure
and uneven thickenings in their cell walls, which causes them to open or close with changes in humidity. In
humid conditions the teeth close, preventing spore discharge, whilst in dry conditions the peristome opens,
allowing spores to be discharged and carried on the wind. (It is also possible that repeated movements of the
peristome can flick spores from the capsule). The seta, typically one to a few cm in length, helps raise the
capsule above the boundary layer of stagnant air, to facilitate dispersal. The seta may also twist rapidly one
way and then the other as it becomes moist or dries, which presumably aids dispersal by shaking the spore
Liverworts. Liverwort spore capsules lack peristomes. That of Pellia is spherical and when dry splits into four
valves which open to expose a mass of spores intermingled with hair-like structures called elaters, some of
which are attached inside the capsule base and others are loose. The elaters undergo writhing movements as
they dry, dislodging the spores for gradual dispersal.
Once the spore germinate they develop into new gametophytes.
Asexual reproduction. Both mosses and liverworts have a variety of means of reproducing asexually. Mosses
may regenerate from broken or fragmented parts, including single leaves. specialised structures, called
gemmae (singular gemma) are multicellular balls, ellipsoids or discs which are easily detached and which
produce new gametophyte plants on germination. Gemmules may be born on any part of the plant. In some
liverworts they are borne in pretty cup-like structures. In the moss, whose sporophyte is shown in the
photographs above, the adxaial (upper) surfaces of many of the leaves carried spherical balls of 2 or 4 cells
each, with upto 3 found on any single leaf. These structures are most probably gemmae and are shown below:
Above: the tips of developing sporophytes, with the calyptra (accidentally) removed. These sporophyte
tips were originally covered in the calyptra sheath, as shown below left, and detaching (below right).
The leaves of this moss lack midribs
and are extremely tapered and
pointed. The margins are smooth (or
sometimes with very tiny denticle-like
projections) and they frequently had
some browned cells (I don't know why).
Bryophytes, or bryophyte-like plants were possibly among the first plants to colonise the land on Earth some
400 million years ago. (It is possible that today's bryophytes evolved from larger and more complex plants by
miniaturisation, but they are certainly ancient, the oldest known moss fossil is some 360 million years old).
Giant myriapods - millipedes the size of human beings, bulldozed through these primordial forests. In later
times, the bryophytes may have become overshadowed by trees of one kind or another, but they continue to
thrive, covering soil, rocks and trees. Indeed, those that grow on trees are helped off the ground, toward the
light and the fresh air which brings in carbon dioxide for photosynthesis and dispersal spores. It is easy when
walking in a wood not to pay too much attention to these ancient and miniaturised plants, but closer inspection
reveals amazing diversity and beauty of form in the tiny, and often overlooked, forests that these plants often
form. It is remarkable how such small plants have made such diverse use of chemistry and physics in so many
ingenious ways in order to solve life's problems.
Above: the life-cycle of Pellia epiphylla. The spermatozoid-producing antheridia are spherical flasks, each
inside a pit which opens to the outside via a pore. Each antheridium will give rise to many spermatozoids. The
archegonia are flask-shaped structures situated beneath the involucre shield at the tips of the ripe thallus (in
mosses the archegonia are usually on the shoot tips, protected by a sheath of leaves) and each contains one
developing ovum (oosphere). After fertilisation the sporophyte grows out from beneath the involucre, but is
diploid, but produces haploid spores by meiosis, which give rise to haploid gametophytes.
Funaria hygrometrica is a common moss found on bare soil in woodland, moorland and in gardens and
occurs frequently on burnt soil, such as at the sites of recent bonfires.
Above: cross-section through a leaf of Funaria. Some mosses have more complicated midribs with various cells
which transport materials, strengthen the midrib or have unknown functions. Mosses are bryophytes, along with
liverworts and hornworts. An example of a liverwort is shown below:
Sporophyte Spore Dispersal
Above: left, top - sporophyte capsule still with eleytra in place and left, bottom - with the elytra and
operculum shed, revealing the peristome. Above, right - close-up view of the epristome with spores.
Left: spores from a capsule.
Right: a cross-section through the seta
or stalk of the sporophyte. There are
three distinct types of cells here - the
small and very thick walled outer
protective and supporting cells
(orange) and the intermediate yellow
cells (leptoids?) and the central strand
of small cells (hydroids) which formed
the conductive tissue which carried
nutrients and water to the sporophyte
tip as it was developing. Some mosses
exceptionally have setae 5 to 10 cm in
Below: the life-cycle of liverworts is similar. The spores are shed as the capsule splits open into a series of
valves. Elongated elaters then writhe about as they dry, gradually shaking the spores loose.
Section through the spore capsule of the liverwort
Close up of the elaters. Note the spiral thickenings
which cause the elaters to twist as they dry.
Elaters extending from the lower wall of the spore
Close up of the elaters. As the elaters dry and twist,
they help scatter the spores.
Close up of the spores. Note the elaters entwined
Close up of the spore capsule wall.
Liverworts growing on a post on a stream bank.
Above: Moss capsules which have shed their opercula caps removed to show the
Above and left: A transverse section through a
moss identified as Polytrichum (an endohydric
moss). The central strand with its hydroids is
clearly visible. Typically in Polytrichum,
however, there is a central strand of hydroids
(hydrome) surrounded by a ring of leptoids
(leptome). Some mosses have just the central
strand of hydroids whilst others have only
elongated parenchyma cells which transport
food along the stem.
Rhizoids of Marchantia in longitudinal
section (above) and transverse section
(left). Each rhizoid is an extension of a
single epidermal cell. Some
cyanobacteria associated with the
rhizoids are visible in the
photomicrograph at top left.
Above: an air chamber filled with
photosynthetic cells in the thallus of
Above: a multicellular scale in
Left: a sector of cortex from the moss Polytrichum sp. showing
the outer cortex of 4 or 5 cell layers of small and thick-walled
cells which are clearly adapted to provide additional
mechanical support. The middle cortex consists of larger cells,
which like those of the outermost cortex, contain starch grains
as a food reserve. The innermost cortex consists of large and
angular parenchyma cells (assuming that these are not
elongated cells in longitudinal section) with non-thickened
walls. Each cell acts as a pressurised unit, inflating the stem
when turgid and giving the plant mechanical support. Inside this
is a central strand of conducting tissue (as illustrated above).
Right: a section through the thallus of
Marchantia sp. Note the curious
shapes of the innermost parenchyma
cells: is this due to mechanical
stresses or an adaptation for transport
of materials from cell to cell?
Right: a section through the cortex of polytrichum sp.
showing a leaf trace - a cluster of about 20 or so small cells
which extends into a leaf higher up the axis, continuing
along the midrib of the leaf. In gymnosperms and
angiosperms such a trace would contain conducting tissue,
such as xylem, in mosses, however, the leaf trace cells are
less specialised and are hydroids in this case. In this
cross-section there were 3 leaf traces in total. (Many
bryophytes have three ranks of leaves).
Above: left, cells in the leaf of an unidentified species of moss, with large chloroplasts. The apoplast
system (cell walls and the middle lamella which glues neighbouring cells together) provides both
support and acts as a conduit for water movement through the leaf. Right, elongated cells of the leaf
midrib provide mechanical support and transport photoassimilates from cell to cell, via
plasmodesmata, in the symplast transport pathway.
Below: left, spore capsules of a moss; right, the leaf margin of a moss, up close.
Above and below: one of the anatomically most sophisticated moss leaves is that of Polytrichum. The
photosynthetic cells are arranged in vertical longitudinal sheets, each one cell wide (seen in transverse or
cross-section in these diagrams) on the upper surface of each leaf. Most of the chloroplasts occur in these cells.
The outermost end or terminal cell of each lamella has a characteristic notch. These notches are thought to form
capillary channels for the conductance of an external water film, whilst the spaces between the lamellae
presumably remain dry for gas exchange during photosynthesis. Internally, the supporting rib or costa of each
leaf has a high degree of differentiation into different cell and tissue types: thick-walled stereids provide
mechanical support in mature leaves, an internal layer of large hydroids conduct water internally, and on either
side of the layer of hydroids are groups of leptoids which transport the sugars made in the leaf to other parts of
the plant. A thick-walled layer of epidermal cells covers the external surface of the back of the leaf.