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Fern Structures and Reproduction
seedless, vascular plants. They contain two types of vascular tissue
that are needed
to move substances throughout the plant. Evolutionarily, this addition
vascular tissue to plants is what allowed ferns to grow up and out
just spreading along the ground. The more primitive mosses rely on
diffusion for material movement and need to stay in close contact with
ground. With the addition of vascular tissue, water, nutrients and food
now be transported throughout a taller plant. The first type of
tissue, xylem, is responsible for moving water and nutrients throughout
plant. As the xylem cells reach maturity they die, losing their
contents. The external cell walls remain intact. These cell walls are
end to end forming long tubes from the roots, through the stems, up to
leaves. As water vapor exits the leaves through the stomata, a process
transpiration, a vacuum is created, pulling more water from the roots
xylem tube. This vacuum/suction moves water throughout the entire
stiff cell walls of the xylem also provide support for the fern plant
grows taller. The other vascular tissue, phloem, is responsible for
glucose throughout the plant. Phloem tissue is comprised of sieve
live cells which have lost their nuclei, and companion cells; which
�take care of� the sieve cells. To move food throughout the fern,
pumped into the sieve elements (this process requires cellular energy).
also moves into the phloem tissue via osmosis, creating a pressure that
�pushes� the glucose throughout the plant.
are only capable of primary growth i.e.
growing upward. They do not increase in diameter, a type of growth
secondary growth. This primary growth occurs at the tips of the plant�s
and roots within areas called apical meristems. The apical meristems
tissue which gives rise to all other types of plant tissue.
Ferns also contain
stems and leaves. The fern leaves are considered to be megaphylls,
have several vascular strands within them. Fern leaves are also known
fronds. When leaves first emerge they are often tightly coiled and
�fiddleheads� since they resemble the very top part of a fiddle
fronds arise from an underground stem known as a rhizome. Underground
attached to the rhizome and serve as an anchor for the plant along with
absorbing water and nutrients from the ground.
Modern ferns are
some of the oldest plants on Earth. They are believed to have arisen
420-360 million years ago. Their phylogeny is as follows:
(their cells contain nuclei)
chlorophyll for photosynthesis and cell walls)
Polypodiophyta when used as a part of Tracheophyta or vascular plants)
Order-Athyriales (one of
Further classification is based upon the fern�s
whether or not they are protected by a covering called an indusium, the
of the fronds, and how the fronds unfold.
vascular plants feature an alternation of generations within their life
the sporophyte generation and the gametophyte generation. In ferns, the
multicellular sporophyte is what is commonly recognized as a fern
plant. On the
underside of the fronds are sporangia. Within the sporangia are spore
cells called sporogenous cells. These cells undergo meiosis to form
spores. The spores on most ferns are the same size and perform the same
function. Therefore ferns are known as homosporous plants. The
usually in clusters known as sori, found on the underside of the fern
ferns have a covering over the sporangia known as an indusium. When the
are mature, they are released from the sporangia. If a spore lands on a
suitable site, it will germinate and grow via mitosis into a mature
plant. A gametophyte is the plant that produces gametes. The fern
is a small (approximately 5 mm), bisexual, heart-shaped plant called a
prothallus. The prothallus is haploid, since it grew from a spore which
been formed by meiosis. It does not have any vascular tissue and uses
rhizoids to anchor it to the ground. On the underside of the prothallus
organs form. The female structure, called an archegonium, contains a
egg. The male structure, the antheridium, contains many flagellated
sperm are released from the antheridium and swim through a thin film of
to a nearby archegonium to fertilize the egg. Since the antheridium and
archegonium are on the same prothallus the fern has several strategies
prevent self-fertilization. These strategies will be discussed later in
paper. Once fertilization of the egg has occurred, a diploid zygote has
created. As the zygote grows into an embryo it remains attached to the
prothallus. The embryonic plant depends upon the prothallus for water
nutrients. As the embryo grows and develops into a mature diploid plant
prothallus dies. This mature plant is called the sporophyte generation
produces spores. The sporophyte plant is the one most commonly
recognized as a
fern. The sporophyte then produces new spores as described above.
method of reproduction ferns use is clonal spreading. Underground
rhizomes grow and sprout new sporophyte plants. Huge clonal colonies of
ferns have been found that are made up of thousands of individual
clonal plants called ramets (Klekowski, 2003). This extensove clonal
spread is especially adaptive for the sporophyte phase. Their long life
span (several years) and extensive competition for space with other
plants allows ferns to quickly become the dominant
understory plant in newly disturbed areas
of the forest. However, aproximately 10% of fern species do have
gametophytes that reproduce vegetatively using an asexual bud-like
structure known as a gemmae (Haig, 2006).
archegonia and antheridia on the same gametophyte, one would assume a
level of inbreeding. If intragametophytic selfing occurred, where sperm
fertilize an egg on the same gametophyte plant, the resulting
become 100% homozygous in only one generation, losing all genetic
necessary for future evolution to occur. Any lethal recessive gene
the species would very quickly die out. Intergametophytic selfing,
from one gametophyte fertilized the egg on another gametophyte both
from the same sporophyte, would also quickly result in
fact, this does not happen. Homosporous pteridophyte species are not all highly inbred or evolutionarily
stagnant (Haufler, 2002). There are several mechanisms ferns use to
self-fertilization and the resulting homozygosity. Intergametophytic
sperm fertilizing an egg on a different gametophyte where both plants
from separate sporophytes, occurs. This is similar to pollen from one
landing on the stigma of another separate lily plant several meters
fern gametophytes produce a pheromone called antheridiogen. This
causes neighboring immature gametophytes to produce only antheridia.
allows many more sperm to be produced for possible cross-fertilization
egg. Archegonia and antheridia mature at different rates to prevent
intragametophytic selfing. One of the most interesting strategies used
to prevent evolutionary stagnancy is polyploidy. Over 95% of ferns have
found to be polyploid (Haufler, 2002). This results from a process
allopolyploidy. Allopolyploidy occurs when two haploid sets of
chromosomes have come from two different species of ferns. The result
is that during meiosis, the homologous chromosomes cannot match up
because they are somewhat dissimilar. When intergametophytic selfing
occurs, the chromosomes
similar enough that a sporophyte zygote can form and grow. However,
sporophyte tries to produce spores via meiosis, the chromosomes are not
enough to pair up as needed. They are homoeologous, not homologous.
inability to undergo meiosis and produce spores renders the plant
However, sometimes the chromosomes replicate creating polyploid
homologous chromosomes can now pair up and separate during meiosis,
spores. These now diploid spores are released, germinate and grow into
gametophytes. The diploid gametes self-fertilize creating healthy
plants. Although this self-fertilization may sound like the
selfing described above, it is not. The polyploidy that was created
chromosomes doubled before pairing provides genetic variation within
to prevent homozygosity. The presence of homoeologous chromosomes also
prevent lethal recessive gene expression. With extra dominant copies of
fewer lethal genes are expressed. Studies have also shown that
homoeologous chromosomes will pair up with a homologous chromosome
meiosis, creating new gene combinations.
today�s common ferns have descended from some of the oldest plants on
their reproductive cycle and genetic variability are complex.
Glen, Rene. �Pteridophytes: the mysterious plants
no seeds.� Lantern 42. 1 (1993): 48-51.
Haig, David and Wilczek, Amity. “Sexual conflict and alternation of
haploid and diploid generations.” Philosophical Transactions of the
Royal Society B: Biological Sciences 361. 1466 (2006): 335-343.
Haufler, Christopher H. �Homospory 2002: An
Progress in Pteridophyte Genetics and Evolutionary Biology.� Bioscience
52. 12 (2002): 1081-1094.
Klekowski, Edward. “Plant clonality, mutation, diplontic selection and
mutational meltdown.” Biological
Journal of the Linnean Society 79. 1 (2003): 61.
Krogh. Biology: A Guide to the
Solomon, Berg, Martin. Biology Belmont:
Brooks/Cole-Thomson Learning, 2005.
- The Editors of Encyclopaedia Britannica
Archegonium, the female reproductive organ in ferns and mosses . An archegonium also occurs in some gymnosperms , e.g., cycads and conifers. A flask-shaped structure, it consists of a neck, with one or more layers of cells, and a swollen base—the venter—which contains the egg. Neck-canal cells, located above the egg, disappear as the archegonium matures, thus producing a passage for entry of the sperm. The sperm are produced in the corresponding male reproductive organ, the antheridium .
Learn More in these related Britannica articles:
- plant: Definition of the categoryFemale gametangia are called archegonia; male gametangia, antheridia. At maturity, archegonia each contain one egg, and antheridia produce many sperm cells. Because the egg is retained and fertilized within the archegonium, the early stages of the developing sporophyte are protected and nourished by the gametophytic tissue. The young undifferentiated…
- plant: Homosporous life histories…motile flagellate sperm, and each archegonium (female gametangium) forms one nonmotile egg. Fusion of an egg and a sperm (syngamy) creates a zygote and restores the 2
nploidy level. Various mechanisms prevent the fusion of eggs and sperm from a bisexual gametophyte (inbreeding). For example, the sex organs may mature…
- plant development: Preparatory events…singly in flask-shaped structures (archegonia) that also are either stalked or sunken in the gametophyte. The neck of the flask is closed by neck canal cells, which later break down to permit the entry of the male gamete. The egg itself lies in the basal part, or venter, of…
- fern: Sexual reproductionThe egg-producing organ, the archegonium, contains one gamete (sex cell), which is always located in the lower, more or less dilated portion of the archegonium, the venter. The upper part of the archegonium, the neck, consists of four rows of cells containing central neck cells. The uppermost of the…
- plant reproductive system: The cellular basis…cellular jacket is called an archegonium, although, like an oogonium, it produces eggs. In most of the plants dealt with in this article, the eggs are produced in archegonia and the sperms in antheridia with surface layers of sterile cells.…
More About Archegonium
9 references found in Britannica articles
- In bryophyte: Reproduction and life cycle
- In plant: Definition of the category
- In fern: Sexual reproduction
- In plant: Homosporous life histories
- In plant reproductive system: The cellular basis
- vascular plants
- In plant development: Preparatory events
- In conifer: Gametophyte phase
- In cycadophyte: Gametophyte phase
- In gymnosperm: General features
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