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MOSSES, members of the group
of plants called bryophytes, are among the first plants to colonise
bare surfaces, creating pockets of humus and detritus and providing
a foothold for higher plants to begin to grow. Although comparatively
primitive plants, mosses are adapted to life in a wide range
of environments ranging from permanently wet water margins to
exposed rooftops.
Mosses represent the link between
thallophytes (flat, stemless and rootless plants such as liverwort)
and cormophytes (plants with distinct leaves, stems and roots).
They are thought by some authorities to be the common ancestor
that all higher plants evolved from, their spores having been
discovered in older geological strata than any other land plant.
Structure
Mosses have leaves and
stems, although the latter are relatively simple structures when
compared to higher plants. The leaves may be large and held away
from the stem, or small and tightly pressed against it, like
the foliage of Chinese juniper. Mosses have no rots as such.
The tiny root-like filaments that grow downward from each plant
are called rhizoids. Their prime purpose is to anchor the moss
to the surface on which it is growing, although some species
do absorb moisture through their rhizoids (endohydric mosses).
However, most mosses of interest and use to bonsai growers are
able to take up moisture through the surface of all parts of
the plant (ectohydric mosses).
Most mosses have no cuticle,
or protective layer of skin-like cells, to reduce water loss
through evaporation. Many species are able to withstand total
desiccation for many months, becoming bright green again within
minutes of contact with water. Those species which are adapted
to living on rocks, where they are bone dry for most of the time,
often sport fine hairs on the tips of the leaves. These reflect
much of the sun's heat and reduce air-flow over the leaves. Such
mosses have very short rhizoids, designed to grip hard, dry surfaces
and are almost impossible to establish on the porous, soft surface
in a bonsai container. They will survive reasonable well for
a time, but are very easily dislodged and gradually deteriorate.
Other species, such as sphagnum,
have a margin of specially modified empty cells which line the
margin of the leaves. These cells are able to absorb and hold
an enormous amount of water, which accounts for their essential
role in the creation and maintenance of peat bogs. This property
also explains the use of sphagnum for horticultural purposes
such as hanging baskets and air layering.
Sexual reproduction
For relatively simple
plants, the sexual reproduction of mosses is a complex process,
relying on the existence of a film of water for the successful
completion of its initial stage at least. Male and female organs
are borne on separate stems which may be on the same plant
(monoecious) or on separate plants (dioecious). The male organ
releases a very large number of cells, called antherozoids, which
each have two long filaments which move in a way that literally
causes the antherozoids to swim through the water until they
reach the female organs. Once they have reached their target,
the antherozoids pierce the neck of the female organ and fuses
with the egg cell inside. The egg cells and antherozoids are
collectively known as gametes, and the generation of the moss
plant that bears them is known as a gametophyte.
The fertilised egg (zygote) develops
into a new, spore producing generation of plant called a sporophyte.
Sporophytes remain attached to the tops of the gametophytes and
are normally parasitic upon them. This generation grows into
the familiar spore capsules held high on stiff, slender stalks.
The spore capsules are triggered to release their spores by the
presence of sufficient atmospheric moisture.
On germination, the spores produce
extensive, green, branched filaments (protonema) which can persist
from a few days to many months before developing buds from which
new plants grow.
Vegetative
reproduction
Most mosses can grow
from small pieces of protonema, stem or even from sections of
individual leaves. However, don't believe that, just because
one species reproduces well from crumbled dry fragments, other
species will do the same. Only your own experience can tell you
which species will oblige.
In vegetative reproduction, new
plants generally grow from bud-like structures called gemmae.
These may be borne on the protonema, the stems or, in some cases,
the leaves. Some mosses also have other specialised methods of
vegetative reproduction bulbils borne on various parts
of the plant or small tuber-like structures on the rhizoids.
When attempting vegetative reproduction,
it is essential to ensure that the moss fragments are kept moist
and shaded until the new plants are established. Sufficient shade
can usually be provided by scattering a thin layer of crushed
brick, pumice or Biosorb on the surface of the soil after applying
the crumbled moss fragments.
Moss grows almost all year round
when conditions are right. Although most useful mosses are highly
active during spring, the approaching hot, dry weather makes
establishment of new plants difficult, relying as they do on
being closely packed together for efficient water retention.
By far the best time of year to establish new moss in autumn,
when the cooler, shadier weather and the constant presence of
atmospheric moisture create ideal conditions for moss culture.
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