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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Horsetails
(Sphenopsida, or
Equisetopsida)
Representatives
Existing
Horsetails, including Scouring Rush (Equisetum species)
(Extinct)
Coal Age Forest
Trees
Biology
ENVIRONMENTS
Horsetails
live on land, from swamps to sand dunes.
OVERALL STRUCTURE
Cell walls, giving shape to individual cells, are composed
primarily of cellulose, typically impregnated with the abrasive mineral
silica (American pioneers used "scouring rushes" as natural
scouring pads).
Horsetails are typically small- to mid-sized plants.
A "rhizome" (a horizontal, underground stem) produces roots, at
"nodes" (joints), and also produces jointed, ridged, sometimes
branched, upright stems. The nodes of the upright stems bear whorls
of scale-like leaves or smaller stems. The stems are
hollow, except for a cross plate at each node, just above which is
some "meristematic" tissue (in which cell-division takes place) --
it is easy to pull horsetails apart at these fragile junctures.
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts, almost entirely within the cells in the stems.
EXCHANGE OF MATERIALS WITH THE ENVIRONMENT
Water vapor and gases flow especially through
"stomata" pores (each regulated by a pair of "guard
cells"). A waxy
"cuticle" helps prevent water loss through the shoots.
Water with dissolved substances is absorbed especially
through the roots.
INTERNAL TRANSPORT
The
stems of horsetails, like those of other "vascular" plants, are
composed of several layers.
The
outermost layer of a stem is the "epidermis", which covers the
"cortex", composed of "sclerenchyma" tissue (of stiff
cells, which form a continuous layer under the epidermis as well as the
ridges that support the stem), "parenchyma" cells (thin-walled,
undifferentiated cells, which conduct photosynthesis and store materials),
air "canals", and "endodermal cells" (of various
arrangements). In the very center of the stem is a large central air
canal, ringed by bundles of vascular tissue. A vascular bundle is
typically covered by a (presumably filtering) layer of endodermal cells,
of the cortex, and is composed of an air canal surrounded by
food-conducting "phloem" tissue, to the outside, and
water-conducting "xylem" tissue, to the inside.
DEVELOPMENTAL CONTROL
The growth and development of horsetails is under genetic
and undoubtedly hormonal control.
ASEXUAL REPRODUCTION
Horsetails can reproduce asexually, via vegetative body
parts.
SEXUAL REPRODUCTION
In
horsetails, 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 at the ends of its
shoots "strobili" cones, composed of groups of "sporangiophores"
-- stalked, capped, highly modified leaves bearing "sporangia"
on the undersides of the caps, facing in towards the shoot. Within the sporangia,
"spore mother cells" each produce four haploid "meiospores",
via "meiosis" (cell division that cuts the number of chromosomes
in half). All the meiospores are alike, for any given species of
horsetail: Modern species of horsetails are "homosporous";
extinct species of horsetails were "heterosporous" (with distinctly
different, male and female meiospores).
After
being released, the meiospores disperse, as by means of flagella-like
outgrowths of the hard outer wall, sensitive to changes in humidity.
Each surviving meiospore eventually germinates into a tiny,
independent-living, liverwort-like haploid "gametophyte", in
which are embedded the "gametangia", which produce the
"gametes": Male "antheridia" produce sperms;
female "archegonia" produce eggs.
The spiral sperms swim from
the antheridia through environmental water to the archegonia and fertilize
the eggs. The (diploid)
"zygote" (fertilized egg) develops, through an embryo stage,
into a new (diploid) sporophyte.
Green
Plants (Viridaeplantae)
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