|
The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Mosses
(Bryophyta)
Representatives
Mosses
Note: Liverworts,
hornworts, and mosses were traditionally
grouped together, as "bryophytes"
(a term now restricted to mosses).
Biology
ENVIRONMENTS
Most
bryophytes live in moist habitats on land, although many actually live
in the water. Certain prehistoric bryophytes were the first truly
successful terrestrial plants -- the first "macroscopic" (that
is, larger than microscopic) forms of life to evolve structures and
functions allowing them to thrive for at least part of their life cycle
out of the water.
OVERALL STRUCTURE
Cell walls, composed primarily of "cellulose" (a
"polysaccharide", a chain-like molecule made of sugars), give
shape to individual cells.
Mosses are small plants with mock rootlets ("rhizoids"),
often-branching mock stems, and mock leaves -- all of these organs are
termed "mock" because they lack true "vascular"
(conductive) tissue (as described below) and instead consist of masses of
"parenchyma" tissue (composed of loose-fitting, thin-walled,
typically undifferentiated cells).
Anchored by their rhizoids, groups of mosses often form
large mats, as on rocks or trees.
The body of a moss consists of two different forms: A
"protonema" (composed of branching filaments) and -- growing
vertically or horizontally from buds on the protonema -- the familiar "leafy"
shoots.
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts within the cells of the moss "leaves" etc.
EXCHANGE
OF MATERIALS WITH THE ENVIRONMENT
Gases diffuse, by "osmosis", through the
"stems" and "leaves", covered with a waxy
"cuticle" to prevent drying-out.
In at least portions of the "sporophyte" (described
below) of mosses, water vapor and gases flow through
"stomata" pores (each presumably regulated by a pair of
"guard cells", as in higher plants).
Water with dissolved substances is absorbed through the
rhizoids, typically on the lower surface of the plant body.
INTERNAL TRANSPORT
Although the stem of mosses contains a "central
cylinder" with some simple water-conducting cells and some other
cells similar to the food-conducting "sieve tube members" of flowering
plants, bryophytes have no fully developed "xylem" or
"phloem" tissue, as in higher plants:
Materials typically diffuse, by osmosis, between the cells in the
small plant body.
DEVELOPMENTAL CONTROL
The growth and development of bryophytes is under genetic
and presumably hormonal control.
ASEXUAL REPRODUCTION
Bryophytes reproduce asexually, via fragments of body parts
or via specialized, multicellular "gemmae" bodies (not
unicellular asexual "spores").
SEXUAL REPRODUCTION
Many mosses are "monoecious" -- each plant producing both male and female "gametes" (eggs
and sperms) -- but some other mosses are "dioecious" (with
separate sexes -- that is, with male and female plants).
As
in other plants, there is an "alternation of generations" in the
life cycle, between "haploid" forms (with just one set of
chromosomes) and "diploid" forms (with both sets of
chromosomes).
The haploid "gametophytes" (the typical plant
bodies) of mosses produce capsule-like "antheridia", borne on
often flower-like stalked heads. The
antheridia produce sperms, which typically use their propeller-like
flagella to swim through environmental water (typically a watery film
covering the plants). One of the sperms may reach a single,
non-motile egg, which is
produced inside a vase-like "archegonium", borne on a
"leafy" stalked head. Unlike any
multicellular "gametangia" (gamete-bearing structures) of "thallophytes"
(algae and other lower plants), the male and female gametangia of
bryophytes and higher plants include an outer layer of "sterile"
(non-reproductive) cells, covering the gamete(s) within.
The "zygote" (fertilized egg) develops within the
archegonium of the female gametophyte and develops (via an embryo stage)
into the mature, diploid "sporophyte" -- typically a capsule
atop a stalk attached (by a "club foot") to the female gametophyte, upon which it is
dependent for water and nutrition.
Within the sporophyte's jacket of sterile cells, numerous diploid
"spore mother cells" each produce four haploid "meiospores",
via "meiosis" (cell
division that cuts in half the number of chromosomes). The
meiospores are released -- often actively (as in response to changing
conditions of humidity) -- and are then typically spread by the wind. Upon germination, the
meiospores develop into a filamentous protonema, from which arise the
"leafy" shoots of a new generation of moss.
Green
Plants (Viridaeplantae)
|
