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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Liverworts
(Hepaticophyta, or
Marchantiomorpha)
Representatives
Liverworts
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
on or 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.
Liverworts, like other bryophytes, are small plants with
mock rootlets ("rhizoids") and mock leaves -- 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).
The "thallus" (main body) of a liverwort is
typically flat and liver-, leaf-, or ribbon-shaped; but it may look like a
leafy shoot.
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts, typically concentrated within the cells in the upper surface of the body.
EXCHANGE
OF MATERIALS WITH THE ENVIRONMENT
Gases diffuse, by "osmosis", through the cell
walls of a bryophyte thallus.
In liverworts, gases pass in and out through simple pores
and air spaces within the upper portion of the thallus.
Water with dissolved substances is absorbed through the
rhizoids, typically on the lower surface of the plant body.
INTERNAL TRANSPORT
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.
Food is stored within cells in the lower surface of a
liverwort thallus.
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"), found in small "cups"
on liverworts.
SEXUAL REPRODUCTION
Some liverworts are "monoecious" -- each plant
producing both male and female "gametes" (eggs and sperms) -- but other liverworts 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) produce capsule-like "antheridia", typically borne
within a groove or on a round disk atop a stalk on the upperside of the
liverwort thallus. The antheridia each produce many sperms, which
typically use their propeller-like flagella to swim through environmental
water (such as a mucilage exuded by the antheridia of a liverwort).
One of the sperms may reach a single, non-motile egg, which is produced
inside a vase-like "archegonium", typically borne within a
groove or on a star-shaped disk atop a stalk on the upperside of a
liverwort thallus. 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 (through an embryo
stage) into the mature, diploid "sporophyte" -- typically a
capsule embedded within the female gametophyte, upon which it is dependent for water and nutrition.
Within the sporophyte's jacket of sterile cells, each of
numerous diploid "spore mother cells" produces four haploid
"meiospores", via "meiosis" (cell division that cuts
in half the number of chromosomes). The meiospores are released -- either actively (as in response to changing conditions of humidity) or
passively (by the death of the gametophyte) -- and may be spread by the
wind. Upon germination, the meiospores develop into
the young gametophytes of a new generation.
Green
Plants (Viridaeplantae)
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