Adiantum

Occurrence

Adiantum is a common fern. It is found in the plains of the Punjab. It grows in shady places. It is found on moist walls or rocky places. The common specie of this genus is Adiantum Capillus-Veneris.

General structure

The vegetative plant body is a sporophyte. It is differentiated into stem, leaves and roots.

1.  Rhizome: 

The stem is underground rhizome. Rhizome is closely covered by scales called palea. The older parts of the rhizome boar numerous bases of the old leaves. Rhizome develops numerous branched adventitious roots.

2. Leaves:

Adiantum has large bipinnately compound leaves. The main axis of the leaf is called the radius. The leaflets of the first order are called pinnae and leaflets of the second order are called as pinnules. Each leaflet is green and triangular. It has broader end towards the apex. The broader end is divided into three or four small lobes. These lobes are reflexed back. These reflexed apical lobes bear the sporangia on their underside. The young leaves are coiled inward in the embryonic state. It is called circinnate vernation.

Internal Structure

Internal structure of Rhizome

1.  Epidermis: The rhizome is covered by a single layered epidermis. It is without any stomata.

2.  Cortex: It is present inner to the epidermis. It is mainly formed of parenchymatous cells. The peripheral region of the cortex has one or two layers sclerenchymatous cells.

3.  The stele in the rhizome is dictyostetic. It has four or five meristele or bundles. They are arranged in a ring. The central part of the stele or pith. Each meristele is surrounded by its own endodermis. The stele becomes solenostelic due to presence of a single leaf gap at a particular level.

Metaxylem is present in form of plate in the centre of each meristele. It has one, two or three protoxylem groups on the side or on the ends. The xylem is surrounded by a narrow layer of phloem. Sometime a single layer of parenchymatous cells is present between the xylem and the phloem.

4.  Pericycle and endodermis: A single layered pericycle is presented outer to the phloem. The pericycle is surrounded by a single layered endodermis. Its cells have casparian strips on their radial walls.

Internal structure of Leaf: 

Both the surfaces of leaf are covered by epidermis. Stomata are present in the lower epidermis. The epidermal cells have chloroplast. An undifferentiated layer of mesephyll cells is present between both epidermises. The rachis is composed of a single layered, cortex, endodermis and stele.

Internal Structure of the Root:

The root is composed of epidermis, cortex and the stele. In the older roots the inner most cells of the cortex become sclerenchymatous.

Sporangia:

The sporangia are borne on the under side of the reflected lobes of the oinnae. The reflexed lobe of the leaf forms a covering or false indusium over son. The groups of sporangia are called soil (sing sonis).

Each sporangium has a stalk. The sporangia are subglobose or ovate in outline. The wall of each sporangium is formed of a single layer of cells. A vertical row of cells along the narrow side form annulus. Annulus has thick walled cells. The cells on the opposite side of the annulus have thin walls. These cells fonn stomium. Each sporangium produces about 48 – 64 spores. The mature sporangia become thy. The outer thin walls of the annulus cells contracts. It exerts a force on the stomium cells. The wall of the sporangium ruptures at this point and release spores.

Development of the Sporangium

1.  Each sporangium develops from a single superficial cell. This cell enlarges to form an outgrowth. This cell cuts off a small cell at the base. It itself becomes the sporangial initial.

2.  The sporangial initial divides transversely to form lower cell and upper cell. The lower cell is the stalk cell. The upper cell is the sporangial cell.

3.  The stalk cell divides to form two cell thick stalk. The sporangial cell divides and cut off three cells at the periphery. It itself becomes tetrahedral central cell.

4.  This central cell cuts off another peripheral cell at the tip. The four peripheral cells thus produced. They become jacket initials. These cells divide to produce a single layered wall. The central tetrahedral cell is the archesporial initial.

5.  The archesporial initial cuts off another set of small peripheral cells. This second set of peripheral cells divides to .produces tapetum around the archesporium. The archesporial cells undergo three or four divisions producing 12-16 spore mother cells. The . tapetal cells provide nourishment to the developing spores.

6.  Each spore mother cell undergoes meiosis and gives rise to four spores. Adiantum is homosporous.

7.  The mature spores are brownish in colour. Each spore has a three layered wall. The outermost layer is perenium or epispore. The inner most is endosporium or intine. The middle layer is thicker and it called exine or exosporium.

Prothallus or Gametophyte

Development of prothellus: The spore falls on a suitable place. It swells very much thus the two outer walls burst at the pointed end. The intine grows out forming a short tube. A colourless cell is cut off at the base of this tube. This cell produces first rhizoid which grows down into the soil. The tip cell divides to produces a short filament of cells.

The cells of the filament develop .chlorophyll. They become green. The apical cell divides to form a wedge-shaped apical cell. This apical cell divides to form a flat plate of cells. These cells divides to form a heart shaped prothallus.

Prothellus becomes several celled thick in the middle. But it remains single layered at the margins. Several cells on the ventral side of the prothallus produce rhizoids. Fthizoids absorb water and fix the protballus. All the cells of the prothallus are green. Each cell contains a single disc shaped chloroplast. The prothallus of Adiantum is completely independent. It can manufacture its own food.

Sex Organs

Adiantum is monoecit as. The sex organs are borne on the ventral side of the prothallus. The antheridia are produced earlier than the archegonia. The antheridia are located in the middle part of the prothallus among the rhizoids. But the archegonia are present near the apical notch.

Antheridium

Each antheridium is a rounded structure. It projects above the surface of the prothallus. The wall of the antheridium ik formed of three cells. The basal cell is funnel-shaped. If forms the lower half of the wall. The upper cell is ring like. It forms the upper half of the wall. The apical cell forms lid. Each mature antheridium contains about 30-50 androcytes or antherozoid mother cells. The mucilaginous walls of the androcytes swell. It pushes the lid cell upward. Thus androcytes come out of antheridium. The nucleus of androcyte changes into antherozoid. Antherozoids have spiral band and numerous flagella. The antherozoids come out of the mother cell walls.

Development of Antheridium

1. Each antheridium develops from a single superficial cell. This cell divides to form upper central cell and lower first ring cell.

2.  The central cells divide to form outer jacket cell and primary androgonial cell.

3.  The jacket cell divides to form a cover cell and second ring cell. The first and second ring cells and cover cells form the wall of antheridium. The primary androgonial cell undergoes several divisions to produce a mass of about 30-50 androcytes or antherozoid mother cells. Each androcyte changes into antherozoid.

Archegonia

Neck of each archegonium protrudes above the surface of the prothallus. Neck is curved backward towards the posterior side of the prothallus. Venter is embedded in the tissue of the prothallus. Its wall is not distinct from the surrounding tissue. Neck is formed of four vertical rows of cells. Each mature archegonium contains a large egg or oospbere at the base and a small ventral canal cell. The ventral canal cell and the neck canal cell disintegrate. They form mucilaginous mass. It comes out through the neck of the archegonium.

Development of the Archegonium

1.  Each archegonium develops from a single superficial cell near the growing apex. This cell enlarges to form a small basal cell and large upper cell.

2.  The basal cell divides to form venter. The upper larger yell by two transverse divisions produce a row of three cells.

3. The upper cell is the primary cover cell. It divides to form four neck initials. Neck initial divides to form neck. The middle cell is the neck canal initial. It forms neck canal cell. The lower most cell is the primary ventral cell. It divides to produce an oosphere and a ventral canal cell.

Fertilization:

The antheridia and archegonia of the same thallus mature at different times. So cross fertilization takes place. The antherozoids are chemotactically attracted towards archegonia. Several antherozoids enter the archegonium but only one of them fuses with the oosphere to form oospore.

Development of the Embryo:                 •

1.  The oospore increases in size. It divides to produce eight cells or octants. The four upper cells become epibasal cells. The four lower cells become hypobasal cells.

2.  The two epibasal cells divide to form first leaf or cotyledon. The other two epibasal cells divide to form stem. The two hypobasal cells give rise to the first root. The other two hypobasal cells produce the foot. Foot penetrates into the tissue of the protballus.

3.  The primary root grows for sometime under the prothallus. It then enters the soil and absorbs nutrients. The cotyledonary leaves are simple. Stem apex produce more leaves. Stem apex grows horizontally in the soil forming the rhizome. Prothallus ultimately disappears.

Alternation of Generation:

Adiantum shows a regular alternation of sporophytic and gametophytic generations. Both generations are independent. Sporophyte produces the haploid spores by meiosis. The spores germinate to form haploid prothallus or gametophyte. Prothellus is moroecious. It produces antheridia and archegonia. The union of antherozoid and oosphere produces diploid oospore. Oospore germinates to form diploid sporophyte.

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