Ferns
are a group of highly developed cryptogams and are widely distributed all over
the world. They are shade and moisture-loving plants and, therefore, grow
abundantly in cool shaded, moist places both in the hills and in the plains.
They
belong to the division called Pteridophyta.
Some
live as epiphytes, growing on other trees, for example Platycerium on the palm
trees.
A
common and widely distributed fern is Dryopteris.
Dryopteriss
a land plant that grows in wet soil and under the shade of trees.
It
consists of roots, stems, and leaves.
The
Pteridophyta (ferns) are the most advanced of the cryptogamic plants.
Ferns
are mostly perennial herbs with stems often in the form of a rhizome, by which
they commonly reproduce vegetatively. The stem is stout, erect and aerial.
Roots are adventitous (fibrous) growing in clusters, from the rhizomes.
Leaves
are commonly pinnately compound and consist of two parts. The frond (leafy
portion) and the stipe (stalk). The life-cycle of a fern is complete in two
phases sporophytic and gametophytic.
The
fern plant is the sporophyte and this is followed by another small green flat
structure called the prothallus, which is the gametophyte.
In
this article, you should be able to draw and label the sporophyte of a fern, list
at least ten characteristics of ferns, describe the structural features of a fern
leaf (frond), summaries the process of reproduction in Ferns, list the
biological and economic importance of ferns and list at least six differences between mosses
and ferns.
Characteristics of Ferns
Persistent
vegetative plant body is the sporophyte, which is differentiated into stem,
roots and leaves The stem, roots and leaves all have conducting tissues making
up the vascular system The conducting system is made up of xylem, tracheids and
phloem which transport water, mineral salts and food.
The
leaves are called fronds.
Fronds
are large and compound pinnate.
They
vary greatly in size and the life-cycle shows alternation of generations with
the sporophyte being dominant over the gametophyte generation.
The
sporophyte generation is diploid (2n) producing spores and is independent of
the gametophyte generation at maturity.
The
gametophyte generation is haploid (n) and produces the gametes that are
generally smaller and different in shape from the sporophyte generation. It is
called the prothallus.
The
male reproductive structure is the antheridium, while the female structure is
the archegonium. Water is required to transport the antherozoids to the
archegonium for fertilisation.
The
sporophyte reproduces asexually while the gametophyte reproduces sexually e.g.
of ferns include Nephrolepis, Adiantum, Polypodium, Dryopteris, Pteris.
The
stem is a rhizome, which bears numerous adventitous roots on the lower surface
in contact with the soil.
The
stem is covered with numerous small brown scales called ramenta. The stem is
non-branching Mature leaves bear groups of sporangia (abaxial) surface of the
frond (leaf).
Each
sore encloses many sporangia that bear numerous spores.
Vegetative Structure of Ferns
Has
a true stem called a rhizome that lies horizontally underground and is covered
with brown scale-like structures called ramenta.
Leaves
and roots arise from buds on the rhizome.
It has adventitious roots with conducting tissue, which carry water from the ground to the leaves.
They also help to hold the plant firmly to the ground.
Young
leaves arising from the rhizome are covered with ramenta and are rolled into
tight spirals. They have circinate venation.
Mature
leaves (fronds) are large and-contpicuous.
They
are divided into the leafy frond and stalk or stipe. The stipe is covered with
ramenta and attaches the frond to the rhizome.
The leafy frond has a main axis called rachis. The rachis bears leaflets called pinnae, which may at times be subdivided into pinnules.
Fertile fronds are
called sporophylls. These bear son (sin. sorus), which are packets of spores
occuring on the underside of the fronds.
Fronds
are green and photosynthetic.
Nutrition in Ferns
Ferns
obtain their food through the process of photosynthesis as in other higher
plants. This is so because their leaves contain chlorophyll which is required
for photosynthesis. Besides the roots of the, plant absorb water and mineral
salts from the soil.
Reproduction in Ferns
Spores
develop within the sporangia; when they mature they are released and dispersed
when sporangia dehisce.
The
indusium dries up and shrinks; The walls of the sporangium dries out and
tension builds up in the annulus, which curls back, flinging the spores out of
the sporangium.
Under
favourable conditions of moisture and temperature each spore germinates into a
very small, thin, heart-shaped green structure, called the Prothallus.
The
margin of the prothallus is very thin and single-layered while the central part
is comparatively thick A Antheridium. A, a young one with antherozoid mother
cells; B, a mature one after bursting; and C, an antherozoid C many layered.
Unicellular hairy Rhizoids grow out from the under surface of the prothallus
and anchor the prothallus to the soil.
The
prothallus is delicate and lacks a cuticle and is prone to dessication, so it
survives only in damp conditions.
The Antheridium
The
antheridium is a spherical or oval body consisting of a wall/jacket with 1 or 2
cap cells at the apex, It also consists of a number of antherozoid mother cells
(20 - 50) which develop into antherozoids. Antherozoids are spirally coiled
multinucleated.
The Archegonium
Both
archegonia and antheridia are borne on the lower surface of the prothallus
(gametophyte). The archegonium is a flask-shaped organ. It is the female
reproductive organ. The swollen basal portion is called the venter while the
slender upper tube-like portion is the neck. The venter enclose a single large
egg cell or ovum (the female gamete) when mature. The venter is linked to the
neck through a canal (ventral canal) which extends into the neck canal
Fertilization
At maturity the archegonium secretes malic acid and mucilage to attract antherozoids. In the presence of a film of water the antheridium bursts to release the ciliated antherozoids.
Antherozoid swims through the venter to
fertilise (fuse) the egg forming a DIPLOID zygote. The diploid zygote is called
an Oospores.
The
oospore divides rapidly to form an embryo.
Which
later develops into the sporophytic generation. This sporophyte is initially
depended on the prothallus until it develops roots.
Alternation of Generations
The
fern plant during its life history passes through two stages or GENERATIONS
The
plant itself which is dominant is the sporophyte.
The
leaves of the sporophyte bears the spores and are called the sporophylls.
The
spores themselves are Haploid and are produced through reduction division
(meiosis).
When
the spores disperse and germinate they give rise to the Gametophyte or the
prothallus
This
gametophyte produces the sex organs (archegonia and antheridia) which contain
the gametes (Ovum and antheroids).
The
fertilisation of the ovum by the antherozoid produces a zygote which becomes
the oospore that gives rise to the sporophyte or the fern plant. The fern plant
is now DIPLOID as a result of fertilisation of the ovum by antherozoid.
Thus,
the two generations (sporophyte and gametophyte) alternate regularly with each
other This phenomenon is referred as Alternation of Generations.
Biological and Economic Importance of Ferns
They
are sometimes used for decoration as ornamental plants
They
are also of evolutionary importance and form links between mosses and
gymnosperms.
They
are the first land plants with true roots.
They
are eaten by animals Fossil ferns contribute to the formation of coalbeds,
petroleum products and natural gas.