Sphagnum leaf section
Sphagnum Bogs
Diagram of Sphagnum leaf in surface view
Sphagnum mosses (peat or bog mosses) maybe small, but they are remarkable plants
with considerable ecological importance. The main body of the plant is haploid (has only
one set of chromosomes) and is called the
gametophyte. There are over 300 species
(about 380) of
Sphagnum. They vary in colour from green, to yellow, to pink, orange and
red according to species and light levels. The red pigment is an
anthocyanin deposited
in the cell walls and is produced  more in bright light (protecting the chlorophyll from
damage due to over-illumination and UV light). Shaded
Sphagnum mosses tend to be
greener.
Sphagnum belongs in the class of mosses called Sphagnopsida, which
includes three other genera of one species each.

Most mosses are either
acrocarpous: forming clustered upright stems with reproductive
parts borne at the stem tip, or
pleurocarpous: prostrate (more-ore-less horizontal) with
reproductive organs at the ends of side branches. Sphagnum is in a class of its own: it
forms compact upright masses but the reproductive organs are borne at the ends of
terminal or lateral whorls of short reproductive branches. Typically, a whorl of short male
branches develop first and produce
antheridia (which produce sperm or antherozoids
which swim to receptive female organs on a different plant through the film of surface
moisture by means of two
flagella) followed by a whorl of short female branches (with
larger leaves) bearing the female
archegonia at their tips, each containing an egg. The
gametes (egg cells and antherozoids) are also necessarily haploid (as in animals) and so
are produced from the parent gametophyte by
mitosis (rather than meisosis as in
animals).

Once the egg within one archegonium on the branch is fertilised, it produces a stalk,
called the
pseudopodium which bears the sporophyte. The sporophyte is derived from
the zygote, the cell produced by fertilisation of an egg cell by an antherozoid, which is
diploid (it has two sets of chromosomes, one from each parent). The stalk of most mosses
is part of the sprophyte, whereas in
Sphagnum it is part of the gametophyte and bears
the sporophyte as a spore-producing capsule at its end.
The last terminal whorl of long lateral branches forms the rays of the capitulum, above
which the shorter reproductive branches are borne. Other whorls of lateral branches
occur along the stem, along with some pendulous branches which hang downwards and
spiral around the stem. Whorls near the apex are crowded together initially (sometimes
called the comal tuft but these will get displaced downwards with growth of the stem and
may elongate). The pendulous branches enclose
capillary spaces between themselves
and the stem, which draws water up or down the plant by capillary action (Sphagnum is
an
ectohydric moss - relying mainly on the external transport of water). However,
transport of nutrients and photoassimilates (food manufactured by photosynthesis) are
transported within the plant body, passing from cell to cell through
plasmodesmata.

Each shoot grows as a result of cell division in the apical cell, situated at the shoot tip,
which is shaped like a triangular pyramid. This produces, by means of cell division, three
rows of leaves along each branch, though developmental rearrangements give rise to the
appearance of 5 rows of leave sin some species. The branch leaves are often tapered to
a hooded apex. Stem leaves occur, typically at the bases of the branches, and these are
more rounded and their precise shape is useful in distinguishing the various species of
Sphagnum.
Above: a cross-section through the stem of a Sphagnum moss. There is a core of
parenchyma surrounded by a couple of layers of small thick-walled cells and a layer of
large dead cells called
retort cells. In some species there are several layers of large
dead cells called
hyaline cells. The sheath of small thick-walled cells offers some
mechanical support (and the cell walls may also provide an easy root for water transport
into the stem). The parenchyma cells will also offer support when they are turgid
(pressurised with water) but the thicker-walled cells offer toughness and some support
when the parenchyma dehydrate. Although the stems are substantial (up to 1.2 mm in
diameter) its relative lack of specialised supporting cells makes it somewhat flimsy, but
since
Sphagnum grows in dense mats extra support is not needed as the plants lean on
each other for mutual support, whilst in some species the plants live submerged in water
and so the water gives them buoyancy.
The retort cells are open to the outside through large circular pores (one of which is
visible in the section above, along with an emerging leaf base). These cells, as well as
filling with water, house useful
symbionts: methane-metabolising bacteria which convert
methane, abundant in bogs, into forms the plant can use for carbon, such as acetate;
and nitrogen-fixing bacteria which can convert atmospheric nitrogen into forms the plant
can utilise as a nitrogen source, such as nitrate. This is important since available
nitrogen is often limiting in bogs.
Drosera, a bog plant often found associated with
Sphagnum, is carnivorous, obtaining nitrogen from its insect prey.
Dead hyaline cells also occur in the leaves, as well as around the stem in some species.
The leaves are a single layer of cells in thickness, and consist of alternating rows of
hyaline cells and thin elongated photosynthetic cells.
Above: sections through Sphagnum leaves. The large open cells are the dead
hyaline cells, alternating with densely-staining photosynthetic cells. The leaves of
Sphagnum have no vein-bearing midribs.
Above: the leaves form capillary spaces around the main stem
and branches for the external transport of water by capillary
action.
Sphagnum stem T.S.
Sphagnum stem T.S. showing opening of retort cell
Sphagnum leaf section
Above: a diagrammatic view of a small section of Sphagnum leaf as seen in surface
view. The hyaline cells have large circular pores connecting them to the outside and
internal spiral thickenings to prevent their collapse. Their function is to absorb water
(and potentially to house symbiotic bacteria) as a reserve to reduce the risk of drying
out. When fully filled, more than 95% of the weight of the moss may be water. Mosses
are
poikilohydric, which means that they dry out easily in the absence of water to
enter a dormant state and rapidly rehydrate and spring back to life when water returns.
Having a store of water means that the moss can spend longer hydrated and hence can
photosynthesise and grow for a larger proportion of the time. The moss
Leucobryum
also has water-storing dead hyaline cells, but with a layer of green photosynthetic cells
sandwiched between two layers of hyaline cells. The alternating arrangement seen in
Sphagnum is unique and characteristic. The water-absorbing ability of Sphagnum made
it useful for mopping up blood and dressing wounds in World War I.

Mature
Sphagnum have no root-like rhizoids, as are found in most bryophytes, since
the mosses continue to grow upwards as the lower portion dies. In this way,
Sphagnum
forms mats up to 50 cm deep, in which only the upper portion is green and living. The
rest serves to soak up water, acting as a water reserve.


The Sporophyte

The foot of the sporophyte is enclosed in gametophyte tissue (the vaginula) forming
the obvious disc-like swelling just underneath each spore capsule. The sporophyte
depends upon the gametophyte for its nutrition, The placenta is the region of the foot
where nutrients cross from the gametophyte to the sporophyte. In most bryophytes
(mosses, horn mosses and liverworts) there are transfer cells on one or both sides of
the
placenta. These are cells with wall ingrowths which serve to increase the surface
areas of the cell membranes for more rapid transfer. These transfer cells are lacking in
Sphagnum (and also in the liverwort Pellia). Instead, in at least the species studied, the
cells on the gametophyte side of the placenta break down to form mucilage. This
mucilage preseumably contains nutrients which the sporophyte absorbs.

In most bryophytes the foot of the sporophyte is at the base of a sporophyte stalk or
seta. In
Sphagnum, there is no sporophyte stalk, or at most a very short one, the stalk
instead being produced by the gametophyte (the pseudopodium) with the sporophyte
anchored on the end of it.

Inside the sporophyte spore capsule, there is a dome-shaped floor, or
columella, with
the spore sac above it. As spores are generated inside the spore sac the columella
collapses to provide more space. The innermost layer of the dome-shaped spore sac is
called the
amphithecium and produces the dome-shped archesporium which
consists of cells that give rise to spore mother cells. The diploid spore mother cells
undergo meiosis (a cell division in which each daughter is haploid, i.e. has only one set
of chromosomes) to produce the haploid spores.


Explosive Spore Discharge

Sphagnum mosses have a curious mechanism to achieve efficient spore dispersal.
Each spore is 22 to 45 micrometres in diameter. The spore capsule is initially green,
turning reddish or brown upon drying and maturing. As it continues to dry it shrinks,
placing the air trapped inside it under considerable pressure. The
operculum or lid is
joined to the capsule by a narrow layer of thin-walled cells and when the pressure inside
becomes critical the lid suddenly blasts off and the spores are propelled 10 to 20 cm
into the air in an 'air-gun' mechanism. The spores are launched with an initial speed
above 15 m/s and sink back down with a terminal velocity of about 1 cm/s. Calculations
of ballistic trajectories underestimate the maximum height reached and this has been
attributed to a vortex generated beneath the spores on discharge producing additional
lift.


Ecology - Peat Bogs

Sphagnum dominates moorland peat bogs along with ling heather (Calluna vulgaris),
bell heather (
Erica teralix), cotton grasses (Eriophorum), purple-moor grass (Molinia
caerulea
), deer grass (Tricophorum caespitosum), insectivorous sundews (Drosera),
bog asphodel (
Narthecium ossifragum) and sometimes shrubby bog myrtle (Myrica
gale
).

These bogs are characterised by raised hummocks, up to 2 feet high, which may dry
out in summer interspersed with pools. There is zonation: aquatic
Sphagnum species
may grow submerged in the pools, along with the carnivorous bladderwort (
Utricularia
minor
) and the bog bean (Menyanthes trifoliata). Sphagnum cuspidatum occurs in the
pools and on flats. The pool margins are occupied by the purple
Sphagnum
magellanicum
, sundews and bell heather. On drier ground there may be a zone of
Sphagnum papillosum and the drier tops of the hummocks are occupied by Sphagnum
rubellum
and ling.

The conditions, and the
Sphagnum itself, make the bogs very acidic. This reduces the
rate of decomposition and the dark semi-decomposed remains of plants accumulate
over the centuries as peat, which has been mined as fuel.

This landscape is in constant flux: as pools fill in, the tallest hummocks erode. This
erosion follows colonisation of drier hummocks by ling, the roots of which break up the
top layer of peat. The proposed model of erosion is then as follows. The ling dies (of old
age and maybe lichen attack) leaving the looser aerated layers of peat exposed to wind
erosion. This exposes the more acidic layers of peat beneath which are too acidic for
many plants to colonise and stabilise, this erosion continues. Due to the compact layers
of peat deeper down, the eroded tops of old hummocks may become flooded to form a
new pool, which can become over-topped by the continued growth of nearby
hummocks. Thus overall the height of the landscape changes, but the nature of its
topography does not: as old pools fill in and dry to form hummocks, so old hummocks
erode to form new pools. This also depends on the water table: if it rises then new pools
will form, if it lowers then erosion may perhaps exceed formation for a time. Excessive
rainwater also adds to erosion, resulting in splits in the peat at the surface and the flow
of peat downslope, which can be very extensive on occasion, resulting in the formation
of ridges and tears.


References, Bibliography and Suggested Reading

Walker, D. 1960. Bogs. The New Scientist (5 May 1960).

Watson, E.V. 1964. The structure and life of bryophytes. Hutchinson University Library.

Whitaker, D.L. and J. Edwards, 2010.
Sphagnum moss disperses spores with vortex
rings.
Science, 329: 406.

Bragina, A., C. Berg, M. Cardinale, A. Shcherbakov, V. Chebotar and G. Berg, 2012.
Sphagnum mosses harbour highly specific bacterial diversity during their whole
lifecycle.
The ISME Journal 6: 802–813.

Ligrone, R. and K. S. Renzaglia, 1989. The ultrastructure of the placenta in
Sphagnum.
New Phytol. 111: 197-201.

Sphagnum leaf - whole mount
Article created:
12/11/2016


Article updated:
18/2/2017
Above: a photomicrograph of a leaf of Sphagnum (whole mount). Note the spiral
thickenings in the elongated hyaline cells.