Adiantum - Pteridophytes

Adiantum

Division – Pteropsida

Class - Leptosporangiopsida

Order – Filicales

Family – Polypodiaceae

Genus – Adiantum

Adiantum is commonly known as ‘Maiden hair fern’ or ‘Walking fern’. They are distributed in the tropical and temperate regions of the world. Some of the Indian species include Adiantum capillus-veneris, Adiantum pedatum, Adiantum caudatum and Adiantum venustum. The sporophyte is differentiated into rhizome, roots and leaves Figure 2.31.

External features

Rhizome

The rhizome is a perennial, subterranean dichotomously branched structure and is creeping in A. capillus-veneris or may be erect as in A. caudatum. It is covered with persistent leaf bases and hairy outgrowths called ramenta.

Root

The roots are adventitious and arise from the rhizome.

Leaf

The leaves are also called fronds and are pinnately compound (unipinnate- A. caudatum, bipinnate- A. capillus- veneris) the young leaves are circinately coiled. The petiole is long, black and shiny. The venation is free and dichotomous in all the species. The vein spread in a fan-like manner in the lamina. The leaves bear marginal sori which are covered by a false indusium.

Internal structure

Root

The root is differentiated into epidermis, cortex and central vascular cylinder.

The epidermis is the outermost layer and bears unicellular root hairs. The cortex is divided into outer wide parenchymatous and inner narrow sclerenchymatous layer. The stele is simple and possesses a central core of xylem in diarch condition with phloem on either side of it (Figure 2.32).

Rhizome

The rhizome in transverse section shows a single layered epidermis covered by cuticle. Some epidermal cells bear multicellular hairs. The Epidermis is followed by two to three layered hypodermis made up of sclerenchyma tissue. A parenchymatous ground tissue is present. The young rhizomes have amphiphloic siphonostele. The older rhizomes have solenostele or dictyostele (Figure 2.33).

Petiole

The petiole in T.S. shows a single layered epidermis with thick cuticle. Epidermis is followed by a sclerenchymatous hypodermis which provides mechanical support. There is an extensive parenchymatous ground tissue. The central region possesses a single large horse shoe shaped stele. Xylem forms central core surrounded by phloem (Figure 2.34).

Pinnule

The Pinnule shows upper and lower epidermis. The cells contain chloroplasts. Stomata are confined to lower epidermis. The mesophyll is not differentiated into palisade and spongy parenchyma. The vascular bundle is surrounded by sclerenchymatous bundle sheath.

Reproduction

Adiantum is homosporous. The reproduction takes place by the production of spores. The spores are produced in sporangia. A group of sporangia forms sori. The sori are marginal but the reflex margins of the pinna form a protective membranous structure called false indusium (Figure 2.35). The development of sporangium is of leptosporangiate type.

The sorus does not show any definite sequence hence fall under mixed type.

A mature sporangium bears a multicellular stalk and a spherical or elliptical single layered structure called capsule. The capsule contains haploid spores. The wall of the capsule is differentiated into thick walled annulus and thin walled stomium. On maturity the sporangium bursts and spores are released. The spores germinate and undergo repeated division to produce a prothallus. The prothallus is flat, green and heart shaped. It is monoecious and represents the gametophytic phase. Sex organs called antheridia and archegonia develop on the prothallus. Antheridia release multiflagellate antherozoids which swim in water and reach the egg of the archegonium to accomplish fertilization. 

The fertilization results in zygote(2n) and it represents the first cell of sporophytic generation. The zygote develops into embryo which further differentiates into sporophyte. Thus Adiantum shows alternation of generation (Figure 2.36).