Biology Notes on Fern

The below mentioned article provides biology notes on Fern.

Adiantum (Maiden Hair Fern) and Dryopteris (Male Shield Fern) are the two common ferns that are found in moist shady places in tropical, subtropical and temperate areas. Adiantum is also observed to occur under bridges, water courses and walls of old well. Plant body is a perennial independently living evergreen sporophyte having vascular tissues.

It is differentiated into stem, leaves and roots. Roots are adventitious. Stem is an underground dark brown rhizome. The large aerial leaves or fronds develop acropetally in spirals from the upper surface of the rhizome. The young leaves show circinate ptyxis. Persistent leaf bases of the dead leaves are found in older parts of rhizome.

Petioles are shining, smooth and blackish in Adian­tum (hence Maiden Hair Fern).Leaf base possesses an adven­titious bud in case of Dryopteris. Lamina is pinnately divided. It is unipinnate in Adiantum caudatum, bipinnate in Dryopteris (incompletely or completely) and A. capillus-ve- neris and tripinnate in A. cuneatum.

Venation is furcate or dichotomously divided. In Dryopteris, a leaflet receives a single midrib which gives rise to lateral veins showing dichotomous divisions. Venation is, therefore, furcate and pinnate. In Adiantum, a leaflet receives a number of veins which spread like a fan, dividing dichotomously in the way.

The young leaves, young parts of rhizome, petiole and rachis of mature leaves are covered over by brown to black scales called paleae or ramenta.The fern multiplies vegetatively by fragmentation of rhizome and development of adventitious buds. In Dryopteris the adventitious bud present at leaf base separates and grows into a new plant.

In Adiantum caudatum and a few other species, adventitious buds develop at leaf tips. When such leaf tips happen to touch the soil, they form new plants, the process helps in spreading the fern over a large area. Because of this reason A. caudatum is also called walking fern.

Leaflets of a mature leaf bear yellowish brown spots having clusters of sporangia called sori. The leaflets and leaves having sori or sporangia are called sporophylls. In Adiantum, the sori are borne submarginally at the distal end on the under surface of the leaflets. The margin of the leaflet is reflexed to cover sorus. The reflexed margin is known as false indusium.

In Dryopteris, the sori develop in two rows, one on either side of the midrib. Each row contains 4-6 sori except in smaller leaflets which may have 1-2 sori or can be sterile. Each sorus is covered by a membranous sheath of its own. This covering is called true indusium. The covered sori of Dryopteris are kidney-shaped in outline. This has given the name of male shield fern to Dryopteris.

A sorus consists of parenchymatous cushion or placenta. The placenta bears a number of stalked biconvex sporangia. In each sporangium there is a single layered jacket that encloses 12-16 diploid spore mother cells. A marginal row of jacket cells are differentially thickened to form annulus. The remaining marginal cells constitute stomium.

The diploid spore mother cells divide meiotically to form haploid spores. With the maturity of spores, the indusium shrivels. The exposed sporangia dehisce in the region of stomium due to differential contraction of annulus. The spores are thrown away. They are dispersed by air currents.

After falling on a suitable soil, each spore germinates to form a flat cordate, green, thalloid, non vascular, free living, inconspicuous, small, multicellular, independent gameto­phyte called prothallus. The prothallus is monoecious, that is, it bears both the types of sex organs, male antheridia and female archegonia. They are borne ventrally.

Antheridia occur in the area of rhizoids while archegonia are borne behind the apical notch in an area called apical cushion. Antheridium is hemispherical in outline. It has a 3-celled jacket and about 32 sperm mother cells. The sperms are multiflagellate (= multiciliate) and spirally twisted.

Archegonium is flask-shaped in outline. It has an embedded venter and a projecting neck. Neck has 4-rowed wall which encloses a single binucleate neck canal cell. The venter contains a single venter canal cell and an oosphere. In the mature state, the venter canal cell and the neck canal cells gelatinise.

Sperms are attracted (chemotaxy) to the opened arche­gonia by malic acid present in their mucilage. A sperm fuses with an oosphere to form a diploid oospore. The oospore gives rise to an embryo which grows to form the fern plant. The prothallus dies meanwhile.

Salvinia (Fig. 3.17):

It is an aquatic fern with both annual (e.g., S. nutans) and perennial species (e.g., S. molesta). It is called sorrow of Kashmir. The plant body consists of a floating stem bearing two rows of large green hairy leaves on the upper surface and highly branched leaf roots on the lower surface. The roots act as balancers. Hair protects leaves from wetting at places.

The plant bears sporocarps sympodially. Sporocarps are sporangia bearing bodies. It is heterosporous.