Adiantum is widely distributed fern with 200 known species. It grows in mild climate. Moist and shady places are liked by Adiantum as habitat.
Morpholigical features of Adiantum (External Structure)
Body of Adiantum can be divided into root, stem and leaves.
The stem of Adiantum is underground, so it is called as rhizome. It does not grow deep inside the soil. Its rhizome grows horizontally near the soil surface. Scales, called palea covered the surface of rhizome.
Leaves of Adiantum are called fronds. These leaves are large, about 4-6 inches in length and are bipinnately compound. Leaflets of first order are called pinnae and leaflets of second order are called pinnules. Main axis of leaf on which leaflets are produced is rachis. Rachis is of black color and shiny. Due to this characteristics color and shine of rachis, Adiantum is also known as Maiden hair fern.
Roots are produced from under side of rhizome. These are adventitious roots.
Anatomy of Adiantum (Internal structure of Adiantum)
Anatomy of Rhizome
Rhizome is wavy in outline. Outermost layer of rhizome is epidermis that is composed of thin walled cells. Outer to this, thick cuticle is present. Beneath the epidermis, well developed cortex is present. Cortex is formed by parenchyma cells with intercellular spaces. In some species of Adiantum, beneath the epidermis, there is a layer of sclerenchyma cells (hypodermis). In other species, sclerenchyma cells are scattered among the parenchyma cells in cortex region. Internal to cortex there is well developed stele. An outer endodermis is present just beneath the cortex. Then there is outer phloem beneath the outer endodermis. Inside the outer phloem, xylem tissue is present. Then there is inner phloem and inner endodermis respectively. Centre of rhizome is occupied by pith.
Anatomy of Adiantum Leaf (Internal structure of Adiantum leaf)
Transverse section of petiole of leaf is almost round in outline. Outermost layer is epidermis. Next to epidermis is hypodermis, composed of sclerenchyma cells. Then there is gound tissue of parenchyma cells and xylem and phloem.
Leafblade is covered with epidermis from both sides, upper epidermis and lower epidermis. Stomata are present in lower epidermis. Epidermis also contains chloroplast and is covered with thin cuticle layer. Mesophyll cells are limited in extent, therefore epidermis performs major role in photosynthesis. Centre of leaf is occupied by xylem and phloem. Phloem faces the lower epidermis and xylem faces the upper epidermis. Completer vascular tissue is surrounded by sheath of sclerenchyma cells.
Anatomy of Adiantum Root (Internal structure of Adiantum Root)
Outermost layer is epidermis that surrounds cortex region. Cortex contains sclerenchyma cells. Beneath the cortex well developed thick walled cells form endodermis. Endodermis also contain casparian strip, which checks the movement of water and minerals. Stele is protostele (without pith). Just outer to stele and beneath the endodermis there is a layer of parenchyma cells called pericycle.
Reproduction of Adiantum (Life Cycle)
Life cycle of Adiantum contains two generations i.e. sporophyte and gametophyte. Both of these generations are independent.
Sporophyte of Adiantum produces vegetative leaves at start. At later stages, fertile leaves also start producing along with vegetative leaves. Fertile leaves produce sori on their underside. Sori are group of sporangia. These sori are covered with a flap of tisse called false indusium.
A mature sporangium is flattene, spherical or ellipsoidal. It consists of a stalk and upper swollen portion called capsule. Capsule is covered with single layered wall. Wall consists of two portions Annulus and Stomium. Annulus portion contains cells with thick radial and inner tangential cell walls. Stomium consists of cell with thin cell walls. This is the site for bursting of sporangia. Inside sporangia, spores are produce by meiosis of spore mother cells. Many spores are produce inside sporangia. Spore wall contains two layers exine and intine.
When spores get mature, the wall of sporangia burst. Sporangium becomes dry, so the cells of annulus region contracts which exerts pressure on stomium cells. Stomium is weak region of wall of sporangia, so sporangia get burst from this region. Bursting of sporangia caused the dispersal of spores. After falling on suitable place, spore germinates. During germination, exine of spore bursts and intine elongate into a tube like structure. The apical portion of tube give rise to new generation of Adiantum the gametophyte generation.
Gametophyte of Adiantum:
Gametophyte of Adiantum has heart like shape. It has a notch, where growing point reside. Gametophyte of Adiantum is many cell thick from centre and only one cell thick at margins. Rhizoids are produce from underside of Gametophyte for anchorage and absorption of water and nutrients. Gametophyte contains chloroplast, so carried out photosynthesis. Gametophyte is independent. Two kinds of organs antheridia and archegonia are produce on gametophyte.
Archegonia is flask shaped structure with two portions i.e. ventre and neck. Ventre contain egg while neck contains neck canal cells. Antheridia are globose structures, in which many antherozoids are produced. Antherozoids when get mature has two flagella for movement in water.
Antherozoids after releasing from antheridia travel through water chemotactically towards archegonia. Antherozoids fertilize the egg inside archegonia. Resultant zygote develops into embryo. Embryo starts divisions to form sporophyte. Sporophyte remains dependent on gametophyte at start but soon it becomes independent.