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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Hornworts
(Anthocerotophyta)
Representative
Hornwort
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.
Hornworts, 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 hornwort is
typically flat and leaflike.
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts within the cells of the thallus.
EXCHANGE
OF MATERIALS WITH THE ENVIRONMENT
Gases diffuse, by "osmosis", through the cell
walls of a bryophyte thallus.
In at least parts of the "sporophyte" (described
below) of hornworts, 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
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
Hornworts 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", enclosed in chambers
within the hornwort thallus. 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", embedded within the hornwort 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 (via an embryo stage)
into the mature, diploid "sporophyte" -- the sporophytes of a
hornwort are simply upright-growing, club-footed "horns"
(stalks), which for a while after the death of the gametophyte may survive
independently, photosynthesizing their own foods and absorbing their own
water and minerals.
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 and, upon germination,
develop into the young gametophytes of a new generation.
Green
Plants (Viridaeplantae)
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