Archegonium Fern, Antheridia &amp


Navigation überspringen



fill=”#FFFFFF” points=”26.6,39.43 42.93,30 26.6,20.57″/>












Über YouTube
Presse
Urheberrecht
Kontakt
YouTuber
Werbung
Entwickler
Impressum
NetzDG Transparenzbericht

Nutzungsbedingungen
Datenschutz
Richtlinien & Sicherheit
Neue Funktionen testen

Alle Preise inklusive MwSt.

© 2018 YouTube, LLC


Auf der Seite für rechtliche Unterstützung kannst du Inhaltsänderungen aus einem rechtlichen Grund beantragen.

Fern Structures and Reproduction

 

           
Ferns are
seedless, vascular plants. They contain two types of vascular tissue
that are needed
to move substances throughout the plant. Evolutionarily, this addition
of
vascular tissue to plants is what allowed ferns to grow up and out
rather than
just spreading along the ground. The more primitive mosses rely on
osmosis and
diffusion for material movement and need to stay in close contact with
the
ground. With the addition of vascular tissue, water, nutrients and food
could
now be transported throughout a taller plant. The first type of
vascular
tissue, xylem, is responsible for moving water and nutrients throughout
the
plant. As the xylem cells reach maturity they die, losing their
cellular
contents. The external cell walls remain intact. These cell walls are
stacked
end to end forming long tubes from the roots, through the stems, up to
the
leaves. As water vapor exits the leaves through the stomata, a process
known as
transpiration, a vacuum is created, pulling more water from the roots
up the
xylem tube. This vacuum/suction moves water throughout the entire
plant. The
stiff cell walls of the xylem also provide support for the fern plant
as it
grows taller. The other vascular tissue, phloem, is responsible for
moving
glucose throughout the plant. Phloem tissue is comprised of sieve
elements;
live cells which have lost their nuclei, and companion cells; which
seem to
�take care of� the sieve cells. To move food throughout the fern,
glucose is
pumped into the sieve elements (this process requires cellular energy).
Water
also moves into the phloem tissue via osmosis, creating a pressure that
�pushes� the glucose throughout the plant.

 Ferns
are only capable of primary growth i.e.
growing upward. They do not increase in diameter, a type of growth
known as
secondary growth. This primary growth occurs at the tips of the plant�s
shoots
and roots within areas called apical meristems. The apical meristems
contain meristematic
tissue which gives rise to all other types of plant tissue.

Ferns also contain
true roots,
stems and leaves. The fern leaves are considered to be megaphylls,
meaning they
have several vascular strands within them. Fern leaves are also known
as
fronds. When leaves first emerge they are often tightly coiled and
called
�fiddleheads� since they resemble the very top part of a fiddle
instrument. The
fronds arise from an underground stem known as a rhizome. Underground
roots are
attached to the rhizome and serve as an anchor for the plant along with
absorbing water and nutrients from the ground.

Modern ferns are
descended from
some of the oldest plants on Earth. They are believed to have arisen
between
420-360 million years ago. Their phylogeny is as follows:

Domain-Eukarya
(their cells contain nuclei)

Kingdom-Plantae (they
contain
chlorophyll for photosynthesis and cell walls)

Division-Pteridophyta
(also called
Polypodiophyta when used as a part of Tracheophyta or vascular plants)

Class-Pteridopsida

Order-Athyriales (one of
the
largest)

Further classification is based upon the fern�s
sporangia,
whether or not they are protected by a covering called an indusium, the
shape
of the fronds, and how the fronds unfold.

           
All
vascular plants feature an alternation of generations within their life
cycle:
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
producing
cells called sporogenous cells. These cells undergo meiosis to form
haploid
spores. The spores on most ferns are the same size and perform the same
function. Therefore ferns are known as homosporous plants. The
sporangia are
usually in clusters known as sori, found on the underside of the fern
leaves. Some
ferns have a covering over the sporangia known as an indusium. When the
spores
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
gametophyte
plant. A gametophyte is the plant that produces gametes. The fern
gametophyte
is a small (approximately 5 mm), bisexual, heart-shaped plant called a
prothallus. The prothallus is haploid, since it grew from a spore which
had
been formed by meiosis. It does not have any vascular tissue and uses
small
rhizoids to anchor it to the ground. On the underside of the prothallus
the sex
organs form. The female structure, called an archegonium, contains a
single
egg. The male structure, the antheridium, contains many flagellated
sperm. The
sperm are released from the antheridium and swim through a thin film of
water
to a nearby archegonium to fertilize the egg. Since the antheridium and
archegonium are on the same prothallus the fern has several strategies
to
prevent self-fertilization. These strategies will be discussed later in
this
paper. Once fertilization of the egg has occurred, a diploid zygote has
been
created. As the zygote grows into an embryo it remains attached to the
prothallus. The embryonic plant depends upon the prothallus for water
and
nutrients. As the embryo grows and develops into a mature diploid plant
the
prothallus dies. This mature plant is called the sporophyte generation
since it
produces spores. The sporophyte plant is the one most commonly
recognized as a
fern. The sporophyte then produces new spores as described above.
             Another
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).

           
With
archegonia and antheridia on the same gametophyte, one would assume a
very high
level of inbreeding. If intragametophytic selfing occurred, where sperm
fertilize an egg on the same gametophyte plant, the resulting
sporophyte would
become 100% homozygous in only one generation, losing all genetic
variability
necessary for future evolution to occur. Any lethal recessive gene
would be
expressed and
the species would very quickly die out. Intergametophytic selfing,
where sperm
from one gametophyte fertilized the egg on another gametophyte both
having come
from the same sporophyte, would also quickly result in
homozygosity. 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
prevent
self-fertilization and the resulting homozygosity. Intergametophytic
crossing,
sperm fertilizing an egg on a different gametophyte where both plants
arose
from separate sporophytes, occurs. This is similar to pollen from one
lily
landing on the stigma of another separate lily plant several meters
away. Some
fern gametophytes produce a pheromone called antheridiogen. This
pheromone
causes neighboring immature gametophytes to produce only antheridia.
This
allows many more sperm to be produced for possible cross-fertilization
of the
egg. Archegonia and antheridia mature at different rates to prevent
intragametophytic selfing. One of the most interesting strategies used
by ferns
to prevent evolutionary stagnancy is polyploidy. Over 95% of ferns have
been
found to be polyploid (Haufler, 2002). This results from a process
known as
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
are
similar enough that a sporophyte zygote can form and grow. However,
when the
sporophyte tries to produce spores via meiosis, the chromosomes are not
similar
enough to pair up as needed. They are homoeologous, not homologous.
This
inability to undergo meiosis and produce spores renders the plant
infertile.
However, sometimes the chromosomes replicate creating polyploid
individuals. The
homologous chromosomes can now pair up and separate during meiosis,
creating
spores. These now diploid spores are released, germinate and grow into
healthy
gametophytes. The diploid gametes self-fertilize creating healthy
sporophyte
plants. Although this self-fertilization may sound like the
intragametophytic
selfing described above, it is not. The polyploidy that was created
when the
chromosomes doubled before pairing provides genetic variation within
the plant
to prevent homozygosity. The presence of homoeologous chromosomes also
helps
prevent lethal recessive gene expression. With extra dominant copies of
genes
available,
fewer lethal genes are expressed. Studies have also shown that
occasionally
homoeologous chromosomes will pair up with a homologous chromosome
during
meiosis, creating new gene combinations.

           
Although
today�s common ferns have descended from some of the oldest plants on
Earth,
their reproductive cycle and genetic variability are complex.

 

 

 

Resources:

Glen, Rene. �Pteridophytes: the mysterious plants
that have
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
Odyssey of
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 Natural World Upper Saddle
River
: Prentice Hall, 2005.

 

Solomon, Berg, Martin. Biology Belmont:
Brooks/Cole-Thomson Learning, 2005.

 

 

 

 

 

 

           


250 thistle logo

ENCYCLOPÆDIA BRITANNICA

250 thistle logo

START YOUR FREE TRIAL

Advertisement

Log In · Join

  • Demystified
  • Quizzes
  • Galleries
  • Lists
  • On This Day
  • Biographies
  • Newsletters

START YOUR FREE TRIAL

Advertisement

Search Britannica

What are you looking for?
Browse popular topics:
  • Ted Bundy
  • Titanic
  • Las Posadas
  • Robert the Bruce
  • Boxing

Bring fact-checked results to the top of your browser search.
Learn More.

chrome store logo

Archegonium

plant anatomy
Written By:

  • The Editors of Encyclopaedia Britannica
See Article History

Alternative Titles:
archegonia, oogonia

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:

  • Weeping willow (Salix babylonica).

    plant: Definition of the category
    Female 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…
  • Weeping willow (Salix babylonica).

    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 2n ploidy level. Various mechanisms prevent the fusion of eggs and sperm from a bisexual gametophyte (inbreeding). For example, the sex organs may mature…
  • The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.

    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…
  • Tree fern (Cyathea medullaris).

    fern: Sexual reproduction
    The 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…
  • how flowering plants reproduce

    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.…

ADDITIONAL MEDIA

  • Fern gametophytes and associated structures.

More About Archegonium

9 references found in Britannica articles

Assorted References

    • bryophytes
      • In bryophyte: Reproduction and life cycle
      • In plant: Definition of the category
    • ferns
      • In fern: Sexual reproduction
    • gametangia
      • In plant: Homosporous life histories
      • In plant reproductive system: The cellular basis
    • vascular plants
      • In plant development: Preparatory events

    gymnosperms

      • conifers
        • In conifer: Gametophyte phase
      • cycadophytes
        • In cycadophyte: Gametophyte phase
      • pollination
        • In gymnosperm: General features

      Article History

      Article Contributors


      Feedback

      Corrections? Updates? Help us improve this article!
      Contact our editors with your feedback.