Mosses
Our next plant group
moves a little further along the evolutionary scale. These are the mosses
(Musci or Bryopsida) and they are considered to be more ‘evolved’.
Firstly we’ll look
at the classification of mosses. These follow the usual ranking,
Division - Bryophyta
Class – Musci
Sub-class –
Sphagnales, Andreaeales, Bryales
Family –
Sphaganceae, Andreaeaceae, Polytrichaceae,
Genus – Sphagnum,
Andreaea, Poyltrichum
Then the species
name.
The key features of
the Sphagnales (Family Sphagnaceae) are; the presence of large ‘empty’ cells,
called hyaline cells, and they are what give these mosses their ability to hold
prodigious quantities of water. Hyaline cells
are surrounded by narrow margins of smaller cells that contain chloroplasts.
These mosses are generally limited to very wet, swampy habitats.
Andreaeales (Family
Andreaceae) grow in rocks in high altitude areas. They form small, dense clumps
and are often dark coloured. The capsules are
not readily found as they are small and are also deciduous (they fall off).
Where other mosses have capsules with ‘hood’ (calyptra) and peristome teeth,
these have no calyptra and simply split down the sides of the capsule to shed
their spores.
Bryales (Family
Polytrichaceae) form the largest family of mosses and are the most commonly
found. They occupy quite a wide habitat range.
Next we’ll look at
some of the key characteristics to identify genera and species.
Mosses are generally
still small (our friend the diffusion distance) and there are no thalloid-type
specimens among them. Although they are called leafy and we describe these
structures as leaves they are not in reality complex enough to be true leaves.
The rhizoids are
multicellular and have oblique divisions between the cells. Rhizoids are still
not equivalent to roots. The structure remains simple and the function is
mainly anchorage rather than nutrient uptake.
Mosses (and
liverworts) produce spores and the dominant generation is the gametophyte as
the sporophyte is nutritionally dependent on the gametophyte.
Figure
1. Typical moss life cycle showing haploid and diploid stages. Copyright S.
McCurdy,
The sporophyte is
much more complex and is long lasting on a wiry stem. The upper capsule section
often has a covering (like a hood) and this is called the calyptra. These often
have characteristics that help in identifying a species. For example, they may
be hairy, or smooth or pointed or rounded.
Inside the calyptra
is another smaller ‘cap’. This is called the operculum and also has
characteristics that help in identification. It may have a point, be hooked or
elongate.
Figure 2. Moss
capsule identifying main features. Adapted from Beever et al. (1992).
Usually the opening
of the capsule beneath the operculum also has a specialized structure. These
are teeth-like projections called the peristome teeth (peri = around the
perimeter and stome = mouth). Made from cells that are sensitive to humidity
they expand and contract and this makes the teeth move. Spores that are
contained inside the capsule are flicked out the opening as the teeth move. You
can breathe on a capsule in the lab and watch the result under a dissecting
microscope in the lab. Creepy but fascinating!!
Sometimes the male
reproductive structures are on separate stems. These often appear as splash
cups.
Plate 1. Splash cups
in a moss. Photo from www.bioimages.org.uk.
The sperm are formed
on antheridia and held until rain falls, the sperm cells are released into the
rain drop and when another raindrop hits the cup-like shape, the sperm are
splashed out and hopefully closer to the female reproductive structures
(archegonia). They must then swim (they are flagellated)
to the archegonia, up the neck and finally fertilize the egg cell within. This
all seems a bit hit and miss doesn’t it? But it must work because there’s lots
of mosses around!
Mosses are far
easier to find, as they are less restricted in their habitats. Why? because
they have some water conservation adaptations. These include rudimentary water
transport vessels (hydroids), and stomata.
The diversity of
mosses exceed that of liverworts, meaning that not only are there more species
but also that the complexity of anatomical structure is often greater.
The first step in
identification is, as in all specimens; look at the overall morphology and
growth habit. Are the stems upright or flat to the ground? Are the leaves in
rows or are they in a spiral formation?
You will need to
remove leaves carefully in order to maintain cell integrity (science-talk for
keeping the leaf whole and not ripping it to bits and mashing all the cells
up).
Sometimes mosses may
have a ‘nerve’ along the central part of their ‘leaf’. In reality (and when you
look at them under the microscope) the ‘nerve’ is a long narrow collection of
cells that can be several cells in depth, not complex enough to be a vein as we
understand them to be in higher plant leaves.
Key features to look
for are the leaf shape; this
includes the edges and the tips. The following diagrams show you the most
common shapes and their descriptions.
Figure 3. Most
commonly found moss leaf shapes and edge shapes. Adapted from Beever et al. (1992).
You need to look at
the edges of the leaves, are they
incurved, flat, etc?
Using your
microscope, look and see if there is a nerve,
is it straight or forked, and does it extend beyond the tip of the leaf?
Figure 4. Common
nerve arrangement in moss leaves. Adapted from Beever et al. (1992).
Another important
feature may the presence of special cells at the very base of the leaf. These
are very much enlarged compared to the other cells, usually thin walled, and
they are called alar cells.
Figure 5. Alar cells
that are typically found at the leaf base in some mosses. Copyright S. McCurdy,
In a few species of
moss there is sometimes a mass of hairs in the lower regions of the stems,
especially where the stems are clumped together. This can look like lint or
felt, and can be different colours. The scientific
term for this is tomentum. You will come across
this term in many other plants and the colour, length and overall
appearance of the tomentum is a key identifying feature.
Do the cells form a
simple tissue layer or are there large ‘empty’ cells surrounded by small
chloroplast filled cells? This is a typical appearance for sphagnum species.
The large ‘empty’ cells are called hyaline cells
and they are what give these mosses their ability to hold prodigious quantities
of water.
Under very high
power magnification you may see small outgrowths from the leaf surface, rather
like tiny fingers. These are called lamellae
(pl.) (lamella (s.)) – you will come across these in one of the specimens to be
keyed out.
The key is in the
book The Mosses of New Zealand (Beever et
al. 1992). There are several ways to approach moss identifications in this
publication. Page12 gives groups of commonly found genera based on habitat and
continues with genera based on key features, for example, red stems, leaves in
five rows etc. Now this won’t enable you to actually identify a moss but once
you are good at mosses this section is an invaluable tool for jogging the
memory.
The first place we’ll look to find a specimen to key out
is on a clay bank where the road has been cut through a hill. These are fairly
hostile environments in that they are exposed to all weather conditions,
subject to large variations in temperature and exposed to very high light. One
of the common mosses that survive these conditions is called Breutelia pendula.
The key identifying
features to look for in this moss are;
when you separate
the stems is there any tomentum? What colour is it?
You need to collect
specimens with their reproductive structures in tact (this means the capsules
and male splash cups)
What colour is the
seta (stalk that holds up the capsule - remember?)
Now let’s try to key this out from the book.
Section E Leaves
with Lamellae or Filaments Page 166
The next moss we’ll
key out is also easily found. It is also found in similar habitats to
Polytrichadelphus but does prefer less light and more water. Begin at page 155
again to establish a group, then key out from the group characteristics.
Section G Nerve
Short or Absent Page 171
Glossary
Alar cells – Distinctive cells at the base of a
leaf. They are usually larger than surrounding cells, thin-walled and may be
variously coloured.
Antheridiophore – Structure containing the antheridia
that produce sperm cells.
Archegoniophore – Structure containing
the flask-shaped archegonia with the egg cell inside.
Capsule – A structure that holds spores in both liverworts and
mosses. Part of the sporophyte generation.
Crescentic – shaped like a crescent or half-moon
Flagellum – a whip like extension that propels a cell or organism
when moved.
Hyaline cells – usually large transparent colourless
cells that are not living, i.e. there is no protoplast.
Lamella – linear collections of cells that grow from the leaf
surface to form ridges
Nerve – an elongate narrow region in a mosses ‘leaf’. In a
similar position to a mid-vein in a leaf.
Papillose – with minute pimple-like lumps.
Rhizoid – similar to a root but structurally and functionally
less complex than in a higher plant.
Seta – a thin, wiry, long lasting stem the supports the
capsule on a moss plant. Part of the ‘sporophyte’ generation.
Tomentum – a dense collection of rhizoids that resembles felt or
lint. May be various colours.
References
Beever, J., Allison, K.W., and Child, J. 1992. The Mosses of