Common
Name: Moss, Broom moss,
Wind-blown moss, Fork moss – Moss may well have been one of the earliest words
in the Germanic languages. The word mēos in Old
English stems from the Old High German
mos which means swamp. In Latin it is muscus,
which is related to dampness or wetness. The boggy habitat of most mosses is
the basis for the etymology.
Scientific
Name: Dicranum scoparium – The generic name is Latin for ‘two
heads’ which may refer to the structure of the sporophyte. Scopa is Latin for ‘broom’, the specific name intimates either that
the moss looks like a broom or that the leaves appear to have been swept to one
side by a broom.
There are many types of moss - about 15,000 species
in some 600 genera; most of these are in the order Bryales, which are called
the ‘true mosses.’ Mosses are found on every continent including Antarctica;
they form an integral part of the arctic tundra. The only exclusion areas for
mosses are marine habitats due to the salinity of the water. Many mosses have
evolved to occupy a unique niche. For example the mosses of the Splachnaceae
family require large amounts of nitrogen and accordingly live on dung and
decaying carcasses; one habitat is Point Barrow, Alaska where the native
Inupiat once dismembered whales for blubber.
However, the most prolific mosses occupy the shady recesses of mesic
(well watered) forest regions. The key to understanding the nature of mosses
relative to other plants is the manner in which they acquire nutrients in the
environment. Mosses are assigned to the Division Bryophyta, which is comprised
of the land plants (embryophytes) that are non-vascular; the liverworts and the
hornworts in addition to the mosses. The mosses are distinguished from the
liverworts and hornworts (sometimes called horned liverworts as they are
similar) by their physiology; moss leaves are arrayed with radial symmetry
about a central axis whereas liverwort leaves are lobed or branched (not
radial) and are dorsiventral (the upper or dorsal side is different from the
ventral or lower side). The vascular plants, or tracheophytes, are the more
advanced land plants that comprise most of the familiar flower bearing
angiosperms and coniferous gymnosperms. Vascular means that they have ducts that convey fluids and salts up
from the roots (through the xylem) and sugars from the leaves back to the roots
or to growth regions (through the phloem). They can get quite large and complex
but they require and extensive resource base of soil for water and minerals. Mosses
do not have the complex plumbing of the vascular plants but one of two
alternative non-vascular methods for water transport. One group has root-like
structures called rhizoids that move water from the soil to a central strand in
the leaf. The other group has no rhizoids and must absorb all nutrients from
the leaf surface. In either case, the
moss habitat is determined by the need for water which is more prevalent in
shaded areas to which they have adapted as an evolutionary consequence of
epigeal growth. The non-vascular
distinction is more subtle than the presence or absence of vessels, since some
mosses do have root-like rhizoids. The
difference is that vascular plants have vessels made from lignin, the
connective tissue of the cellulose, the essence of the woody plant; bryophytes
have no lignin.
The proliferation of mosses in forests is a matter
of the habitat and preference of ecology. They predominate in areas with a high
water table which are adequately shaded to protect against the desiccation
attendant to the radiant heat of direct sunshine. The hackneyed mnemonic that associates the
presence of moss growing on tree boles as an indication of the northern
cardinal point in an otherwise labyrinthine forest is not universal. Moss grows
where there is enough moisture. While this is more likely on the north side of trees
which are less prone to being dried out by the sun it can equally occur on the
downhill side of an upslope. In the northern hemisphere the Sun trends to the south
so the north side would be shadier; however, in the southern hemisphere, moss
would grow on the south side with syllogistic reason. And it may not be moss at
all; the tightly adherent green growth on a tree is more likely to be the ubiquitous
green alga of the genus Pleurococcus.
Mistaken identity is responsible for the etymology of a number of other
moss-like growths that aren’t mosses. Reindeer moss (Cladonia rangiferina) is a lichen which is a combination of an alga
and a fungus, Spanish moss (Tillandsia
usneoides) is an epiphytic (absorbing nutrients from the air) angiosperm in
the bromeliad family and Club moss (Lycopodium
clavatum) is a primitive evergreen vascular plant. Irish moss is Atlantic
seaweed that is used to clarify beer as an integral part of the brewing
process.
Bryophytes
(and ferns) have a much more profound and probably primordial distinction that
differentiates them from the seed plants: spores are an integral part of the
lifecycle, which is dominated by the haploid, or ‘n – chromosome’ state. Most
land plants and all animals have cells which have a double set of chromosomes
(2n) called a diploid; only the sex cells use the haploid or n-chromosome so
that the combination of the sperm (n) and the egg (n) from different
individuals results in a diploid (2n) offspring. The life cycle of the moss
starts with a spore (n) that germinates into a root-like anastomosis called a
protonema if it lands in an auspicious locale. The protonema grows into what is
recognizably the green, leafy, photosynthetic gametophyte (n) phase of the moss
cycle. At maturity, the gametophyte develops the female sex organ called an
archegonium that produces an egg (n) and a male sex organ called an antheridium
that produces a number of biflagellate (two-tail) sperm (n), which must swim to
the egg; the obligatory moist habitat of the mosses is thus manifest – no
water, no sex, no moss. Fertilization
can occur with both sex organs on the same plant, which is called monoicous
(the same as monoecious for flowering plants) or with the sex organs on
different plants which is called dioicous (dioecious). In either case, the
fertilized egg, which is now a 2n sporophyte, will grow to maturity, a process
that can take up to six months. The sporophyte rises above the gametophyte on a
stalk with the spore-bearing sporangium at its apex as shown in the picture. At
maturity, the 2n mother cells in the sporangium undergo the meiosis of reduction
division to produce the n spores which are shed through a hole called an
operculum to disseminate and continue the cycle.
Mosses are thought to be one of the first plants to
emerge from the “primordial swamp” to establish a terrestrial presence.
However, due to their herbaceous nature, the fossil record is relatively devoid
of any evidence of their evolution and their provenance must remain
speculative. The earliest fossil remnants are of liverworts from the Upper
Devonian Period when the majority of the vascular land plants first appear (400
million years ago). Moss first appears in the known fossil record in the
Permian Period some 100 million years later. However, there are vestigial
structural aspects of moss physiology that support the primordial hypothesis. A
logical progression would have been for the green algae of the oceans to
establish a terrestrial beachhead. The gametophyte and the sporophyte then
evolved independently with erect stems and green leaves and eventually merged
to a single entity. This is suggested by the fact that the sporophyte has
chlorophyll and plastids (food storage bodies) and that the gametophyte has
stomata (breathing pores) on its leaves; these would indicate that they were
both at one time independent entities. The protonema which is the key to the
life cycle grows in a branching and threadlike manner that is very similar to
the growth of green algae, its likely progenitor.