Download The Bryophytes comprise three groups of plants, the Hepaticae or

Notes on mosses
The Bryophytes comprise three groups of plants, the Hepaticae or Liverworts, the
Anthocerotae or Hornworts and the Musci or Mosses. The life cycles of these plants, like
those of all land plants, exhibit a regular alternation between two morphologically and
physiologically distinct generations, the haploid gametophyte and the diploid
sporophyte with double the haploid number of chromosomes. However, bryophytes
differ from all other plants in three important ways:
Botanically, mosses are non-vascular plants. Many people think of mosses as living only
in moist woods however mosses can be found growing on bare rocks, sand dunes and
even glaciers
They are small (a few centimeters tall) herbaceous (non-woody) plants that absorb
water and nutrients mainly through their leaves and harvest carbon dioxide and sunlight
to create food by photosynthesis. They differ from vascular plants in lacking waterbearing xylem tracheids or vessels. However they have water and food transporting cells
known as hydroids and leptoids respectively. These cells are analogous to the
tracheids and vessels of vascular plants and they closely resembles transport cells found
in the fossils of the earliest land plants.
The dominant phase of the moss life cycle is the gametophyte (haploid). The small
green plant you find as a moss in the woods is the haploid gametophyte. Moss
gametophytes are either erect or extensively branched prostrate plants that consist of
an axis (commonly called a stem) bearing spirally arranged leaf-like appendages (usually
referred to as leaves); more often than not, they are anchored to the substratum by
branched multicellular filaments called rhizoids (do not confuse with rhizomes)
Although variable in
shape, moss leaves
usually consist of a
single cell layer and
are traversed by a
midrib that is
always more than
one cell in
thickness; it may
extend from the
base of the leaf to
the tip or beyond,
or may terminate some distance from
the tip. The margins of the leaves are often toothed, the teeth pointed or rounded.
The haploid gametophyte
is the long-lived or
dominant generation and
is, therefore, the plant
most likely to be
encountered in the field.
The body of the plant that
one sees is the
gametophyte (haploid).
The sporophyte, unlike
other plants, does not
become a free living and
independent plant, on the
contrary, it develops from
the gametophyte.
The life of the new moss
begins with the
germination of a spore.
The spores are produced in
the sporangium (spore
producing structures)
which in the case of
mosses is the capsule.
Inside the capsule, Meiosis
takes place and spores are
produced (n).
Once the spores are
released and the
conditions of moisture and temperature are ideal, the spores germinates producing first
a basic tissue structure the protonema and later the protonema develop buds that grow
into a leafy female gametophyte and a leafy male gametophyte (1). Each of these have
rhizoids, etc. In the female leafy gametophyte, the plant will produce female
gametangia (gamete producing structures) called archegonia. Inside the archegonia we
find an egg cell. The leafy male gametophyte produces male gametangia called
antheridia, which originate sperm cells. The sperm cells are released and swim (2),
enter the female archegonia and fertilize the egg cell. The fertilized egg cell is the
sporophyte. First a single cell zygote (2n) and later develops into the sporophyte.
The sporophytes of mosses, like those of the liverworts, consist of a foot, seta and
capsule and remain permanently attached to the gametophytes. They are borne at the
tips of erect gametophytes. Here, in the capsule Meiosis will take place one more time
closing the cycle.
Summary and Major points for Bryophytes
1. The bryophyte gametophyte, rather than the sporophyte, is the larger, long-lived or
dominant generation.
2. The bryophyte sporophyte, unlike that of other plants, does not become a free-living
and independent plant; instead, it remains permanently attached to the gametophyte.
3. The bryophytes lack xylem and phloem, the specialized tissues used by all other
plants for the transport of water and food materials (primarily sugar).
References: I thank Dr. Livija Kent from UMASS Amherst for her detail information and
knowledge about mosses and her willingness to share it.