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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
True Ferns,
Including
Tree Ferns
(Pterophyta)
Representatives
Ferns
Biology
ENVIRONMENTS
Ferns live on land (sometimes attached to other plants), especially in
warm, humid, shady areas.
OVERALL STRUCTURE
Cell walls, composed primarily of cellulose, give shape to individual cells.
Ferns are small- to large-sized plants. They have true roots;
most have "rhizomes" (horizontal, underground stems); some (tree
ferns) have tree-like stems; and almost all have large, feathery leaves ("fronds"). A frond -- typically borne on a "rachis" (a long, rigid
"petiole" leafstalk) and divided into "pinnae" leaflets
(each of which may be further divided) -- grows by unrolling from its tip
(which is "meristematic", active in cell-division).
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 the shoots.
Water with dissolved substances is absorbed especially by
the roots.
INTERNAL TRANSPORT
The stem of ferns, like other "vascular" plants,
is composed of several different layers.
The
outermost layer is the "epidermis", which covers the "cortex",
composed of "parenchyma" cells (thin-walled, undifferentiated
cells, which store materials). Embedded within the cortex in some
species are "sclerenchyma" cells (stiff cells, which support the
stem), and embedded with the cortex of all ferns are one to several "vascular
bundles", each encircled by an "endodermis" (of cortex
cells, which filter substances entering the vascular bundles). Each
vascular bundle contains a cylinder of food-conducting "phloem" tissue
surrounding a central cylinder of water-conducting "xylem"
tissue.
The phloem tissue of ferns includes "sieve
cells". Like "sieve tube members", in the phloem of flowering
plants, the sieve cells of ferns are alive; but unlike the sieve
tube members of flowering plants, the sieve cells of ferns are not connected
end-to-end via perforated "sieve plates" into "sieve
tubes" and are not accompanied by "companion cells".
The
xylem tissue of ferns typically includes "tracheids"
(communicating via "pit pairs" in their side walls) and only
occasionally includes likewise non-living "vessel elements" (connected
via perforations in their end walls into water-conducting
"vessels").
DEVELOPMENTAL CONTROL
The growth and development of ferns is under genetic and
undoubtedly hormonal control.
ASEXUAL REPRODUCTION
Ferns can reproduce asexually, via vegetative body parts or,
in some species, via specialized buds from
leaves.
SEXUAL REPRODUCTION
In
ferns, like 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 on the underside or edges of specialized leaves (which may or may not
look like vegetative leaves) "sporangia", sometimes grouped together as
"sori", each sorus sometimes covered by a cap-like "indusium".
Within a sporangium, "spore mother cells" produce haploid
"meiospores", by "meiosis" (cell division that cuts
the number of chromosomes in half). The meiospores are typically similar to one another
-- almost
all ferns are "homosporous".
After reaching the soil, the spores germinate into small, heart-shaped,
liverwort-like "gametophytes",
called "prothallia". Each (haploid) prothallus bears on its underside not only rhizoids but also
the "gametangia" of both sexes (Ferns are "monoecious",
not with separate sexes): Capsule-like "antheridia" and vase-like
"archegonia". The
gametangia produce the "gametes", sperms and eggs. The
spiral sperms swim from their antheridia through environmental
water, one sperm then fertilizing the single egg within an archegonium. The diploid
"zygote" (fertilized egg) develops, through an embryo stage, into the
sporophyte, whose leaves are at first undivided.
Green
Plants (Viridaeplantae)
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