Ferns: Characteristics, Vegetative Structure, Nutrition, Reproduction and Biological and Economic Importance

 

Ferns: Characteristics, Vegetative Structure, Nutrition, Reproduction and Biological and Economic Importance

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.

Post a Comment

0 Comments