|
The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Club Mosses & Scale
Trees
(Lycopsida)
Representatives
Club
Mosses, including Ground Pine
Prehistoric
Scale Trees, Known as Fossils
Biology
ENVIRONMENTS
Club
mosses are found on land, especially in warm, wet, shady areas. Many
club mosses live in the Tropics, but some live in the Arctic or even in
Temperate deserts.
OVERALL
STRUCTURE
Cell
walls, composed primarily of cellulose, give shape to individual cells.
Club
mosses are typically small plants that look like mosses. Unlike
mosses but like higher plants, club mosses are truly "vascular"
plants (Please see below), with true roots, leaves, and stems (the stems both
horizontal and vertical and often branching).
ENERGY
CAPTURE
Light-energy
is captured, for photosynthesis, by chloroplasts, especially within the
cells in the leaves.
EXCHANGE
OF MATERIALS WITH THE ENVIRONMENT
Water
vapor and gases flow especially through "stomata" pores (each
regulated by a pair of "guard cells") in the leaves.
A waxy "cuticle"
helps prevent water loss from shoots in at least some species.
Water
with dissolved substances is absorbed especially through the roots.
INTERNAL
TRANSPORT
The
stem of club mosses, like
other vascular plants, is composed of several layers (similar to
those in the root).
The
outermost layer of the stem is the "epidermis", which covers the
"cortex", composed of "parenchyma" cells (thin-walled,
undifferentiated cells, often widely separated by air spaces, 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, filter
substances penetrating inward, and sometimes help support the stem).
To the inside of the cortex, forming the inner core of the stem or root,
is the sometimes branching "stele", composed of a cylinder of
food-conducting "phloem" tissue surrounding intertwining strands
or a solid core of water-conducting "xylem" tissue.
The
phloem tissue of club mosses typically includes living "sieve
cells". Like "sieve tube members", in the phloem of flowering
plants, the sieve cells of club mosses are alive. Unlike the
sieve tube members of flowering plants, the sieve cells of club mosses are
not connected end-to-end by perforated "sieve plates", forming
"sieve tubes", and are not accompanied by "companion
cells".
The
xylem of club mosses includes non-living "tracheids"
(communicating via "pit pairs" in their side walls) and
sometimes non-living "vessel elements" (connected via
perforations in their end walls into water-conducting
"vessels").
There
is no "pith" in the center of the stem or root of club
mosses: All in all, the vascular system of club mosses is much like
that in the roots of flowering plants.
DEVELOPMENTAL
CONTROL
The
growth and development of club mosses is under genetic and undoubtedly
hormonal control.
ASEXUAL
REPRODUCTION
Club
mosses can reproduce asexually, by means of vegetative body parts.
In
addition, multicellular "bulbils" (not unicellular spores) can generate new plants asexually in some species.
SEXUAL
REPRODUCTION
In
club mosses, 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 "sporangia" within the "axils" (the angles
at the base) of "sporophylls" (specialized leaves at the ends of
shoots). In some species, the sporophylls are modified and grouped
together as a "strobilus" (a cone -- hence the name
"ground pine").
The
diploid sporangia produce haploid "meiospores", via
"meiosis" (cell division that cuts the number of chromosomes in
half).
Some
club mosses are "homosporous":
The sporangia are all alike, producing meiospores
that are all alike. After
being dispersed, these meiospores germinate above-ground or under-ground into
the independent, yet inconspicous, irregular-shaped haploid
"gametophytes" (which may grow with a fungus, as does Psilotum).
Produced within the gametophytes are "gametangia", which will
produce the "gametes" (the male sperms and the female
eggs). The gametophytes may be "monoecious" -- each plant
producing both male and
female gametangia -- or "dioecious" -- with separate sexes (that
is, with male plants and female plants).
The sperms use their flagella to swim through environmental water from the
antheridium to an egg in an archegonium, in which fertilization occurs and
from which the embryonic (diploid) sporophyte grows.
Other
club mosses are "heterosporous":
There are male and female sporangia as well as male and female meiospores, produced by the
appropriate sporangia.
A large
"megasporangium" produces a single surviving "megaspore
mother cell", which produces four large haploid
"megaspores"; whereas a small "microsporangium" produces
many surviving "microspore mother cells", each of which produces four small haploid "microspores".
Enclosed within the wall of each megaspore (which may not separate from the
strobilus for quite some time) a round, multicellular female gametophyte is
produced. Within portions of the gametophyte that stick out of the
wall of the megaspore (which will eventually burst), vase-like archegonia develop, in each of which is
an egg. Enclosed within the wall of
each microspore (which soon separates from the strobilus) a round, multicellular male
gametophyte is produced, almost all of which develops into a
capsule-like antheridium, whose outer layer of sterile cells enclose many
developing, flagellated sperms. After the microspore wall and antheridium capsule burst, the sperms swim
out through
environmental water to the archegonia, where they can each fertilize an
egg. The (diploid)
"zygote" (fertilized egg) produces not only the embryonic sporophyte but also an attached
"suspensor", which, by growing, pushes the developing embryo
down into the nourishing female gametophyte.
Some
extinct relatives of club mosses, the scale trees, bore true seeds -- the female gametophyte, with a
dormant sporophyte embryo within, was retained within the cone of the
mother sporophyte (similar to a female, seed-bearing cone in a modern conifer).
Green
Plants (Viridaeplantae)
|
