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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Whisk
"Ferns" (Psilotum) etc.
(Psilophyta)
Representatives
Psilotum
Tmesipteris
Biology
ENVIRONMENTS
Psilotum
is terrestrial, found mostly in Tropical regions. This
group of primitive "vascular" plants is rare in the modern
world.
OVERALL STRUCTURE
Cell walls, composed primarily of cellulose, give shape to individual cells.
Psilotum etc. are small plants composed of "rhizoids"
(root-like growths, without truly "vascular" tissue); a
branching "rhizome" (a horizontal underground stem); an
upright-growing, branching, angular stem (with truly vascular tissues, as
described below); and scale-like "microphylls" (mock leaves -- simply outgrowths from the stem, without vascular tissues).
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts, mostly within cells in the stems.
EXCHANGE OF MATERIALS WITH THE ENVIRONMENT
Water vapor and gases diffuse through the cell walls of the
stems (covered with a waxy "cuticle", to prevent
drying-out) and presumably also enter and exit the plant body via
"stomata" pores (each typically regulated by a pair of
"guard cells" in vascular plants).
Water with dissolved substances is absorbed by the rhizoids,
which also anchor the plant to the soil.
A fungus always grows with the rhizome of Psilotum,
presumably in a "symbiotic" relationship in which the fungus
helps make water and minerals available to the plant and the plant, in
turn, feeds the fungus (similar to the relationship of fungi and algae in lichens
or mycorhizzae).
INTERNAL TRANSPORT
The stem of Psilotum,
like all vascular plants, is composed of several layers. The
outermost layer is the "epidermis", which covers the
"cortex", composed of "parenchyma" cells (thin-walled,
undifferentiated cells, which conduct photosynthesis and store materials),
"sclerenchyma" cells (stiff cells, which support the stem), and
"endodermal" cells (which form the inner layer of the cortex and
filter materials penetrating inward). To the inside of the cortex,
forming the core of the stem, is the irregular-shaped "stele" --
composed of food-conducting "phloem" tissue surrounding a
central cylinder of water-conducting "xylem" tissue.
The
phloem tissue of Psilotum includes "sieve cells". Like
"sieve tube members", in the phloem of flowering
plants, the sieve
cells of Psilotum are alive; but unlike the sieve tube members of
flowering plants, the sieve cells of Psilotum are not connected
end-to-end via "sieve plates" and are not accompanied by
"companion cells".
The xylem tissue of Psilotum
includes "tracheid" cells, with spiral or ladder-like
wall-thickenings. Like "vessel elements", in the xylem of
flowering plants, the tracheids of Psilotum are not alive; but unlike
the vessel elements of flowering plants, which are perforated on their end
walls, forming long, continuous, water-conducting vessels, the tracheids
of Psilotum are perforated with "pits" on their side
walls, through which water flows.
DEVELOPMENTAL CONTROL
The growth and development of Psilotum etc. are
under genetic and undoubtedly hormonal control.
ASEXUAL REPRODUCTION
Psilotum etc. can probably reproduce asexually, via
fragmented body parts.
SEXUAL REPRODUCTION
As
in other plants, there is an "alternation of generations" in the
life cycle, between "diploid" forms (with both sets of
chromosomes) and "haploid" forms (with just one set of
chromosomes).
The diploid "sporophyte"
(the typical plant body) is dominant. It produces tiny, bulbous
"sporangia" in between the stem and the scale-like
"leaves". From the
sporangia are produced haploid "meiospores", via
"meiosis" (cell division that cuts the number of chromosomes in
half). After being dispersed, the meiospores germinate into
irregularly shaped, underground haploid "gametophytes" (which,
like the rhizoids of the sporophytes, are associated with a fungus).
Within the gametophytes are borne "gametangia": The
male "antheridia" and the female "archegonia".
From the antheridia are produced sperms (the male gametes), which use
their flagella to swim through soil moisture or other environmental water
to an egg (the female gamete), inside an archegonium, within which
fertilization takes place and the embryonic (diploid) sporophyte will
develop.
Green
Plants (Viridaeplantae)
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