Phylum filicinophyta (ferns)

The filicinophyta are also known aspteridophyta; commonly called the ‘ferns’. They are usually restricted to dampshady habitats. Few ferns are capable of growing in full sunlight, althoughbracken (pteridium) is a common exception. Ferns are common in tropical rainforests, where temperature, light and humidity are favourable.

Ferns are vascular plants, meaning thatthey contain vascular tissue. Vasculartissue is made up of xylem and phloem. These tissues are concerned withtranslocation (transport) of water and nutrients round the plant body.

Xylem helps carries mainly water andmineral salts, whereas phloem carries mainly organic solutes in solution suchas sugars.

Vascular tissue is a major evolutionaryadvance compared with the simple conducting cells of some bryophytes and algae.It is found only in the sporophyte generation, and is one reason why thesporophyte generation becomes conspicuous in all vascular plants.

Vascular tissue has two importantproperties. First, it forms a transport system, conducting food and wateraround the multicellular body, thus allowing the development of large, complexbodies. Secondly, these bodies can be supported because xylem, apart from beingconducting tissue, contains lignified cells of great strength and rigidity.

Another lignified tissue, sclerenchyma,also develops in vascular plants and supplements the mechanical role of xylem

The characteristics of the filicinophytaare summarised in the table below.

General characteristics of the filicinophyta (ferns)

General characteristics of ferns
Alternation of generations in which the sporophyte is dominantGametophyte is reduced to a small, simple prothallusVascular tissue present (xylem and phloem) in sporophyte: sporophyte therefore has true roots, stems and leavesLeaves relatively large and called frondsSpores produced in sporangia which are usually in clusters called soriE.g. Dryopteris filix-mas (male fern), Pteridium (bracken)

The sporophyte generation possesses trueroots, stems and leaves. Roots penetrate the sol with the result that water anddissolved nutrients can be obtained more easily. Xylem conducts it to otherparts of the plant.

Once plant bodies could achieve supportabove the ground, there must have been competition for light, so there would bea tendency for taller forms to evolve.

Ferns and tree-ferns were the dominantvegetation for about 70 million years, from the Devonian to the Permian eras.After this conifers and, later, flowering plants largely replaced them.

Despite these advances in adapting to aland environment, which are associated with the sporophyte generation, in fernsthere remains a major problem with the gametophyte. It is called a prothallus, and produces sperm whichmust swim to reach the female gametes, as is the case in bryophytes.

The male fern (Dryopteris filix-mas)

Dryopterisfilix-mas is probably the most common British fernand is found in damp woods, hedgerows and other shady places throughout thecountry. The fronds (leaves) of the sporophyte may reach a metre or more inheight and grow from a thick horizontal stem, or rhizome. This bearsadventitious roots.

Branches from the main stem may eventuallybreak away and give rise to separate plants, a form of vegetative reproduction.

The bases of the fronds are covered withdry brown scales called ramenta thatprotect the young leaves from frost or drought. The young leaves from show acharacteristic tightly rolled structure. The ramenta gradually become smallerand less dense up the main axis of the frond. This axis is called the rachis, and the leaflets either sideare called the pinnae. The small rounded subdivisions of the pinnae are called pinnules.

Spores are produced during late summer instructures called sporangia. Sporangiadevelop in clusters called sori onthe undersides of pinnules. Each sorus has a protective covering called an indusium. Inside each sporangiumdiploid spore mother cells divide by meiosis to produce haploid spores. Whenmature, the indusium shrivels and drops off, and the exposed sporangium wallsbegin to dry out. Eventually the wall ruptures, and spores are catapulted fromthe sporangium.

The spores germinate to form thegametophyte generation. The gametophyte is a thin heart-shaped plate of cellsabout 1cm in diameter. It is green and photosynthetic and is anchored byunicellular rhizoids to the soil. This delicate prothallus lacks a cuticles andis prone to drying out, so can only serve in damp conditions.

The gametophyte (prothallus) producessimple antheridia and archegonia on its lower surface. These sex organs protectthe gametes within them. Gametes are produced by mitosis of gamete mothercells, the antheridia producing sperm and each archegonium an ovum, as in thebryophytes. Each sperm has flagella. When ripe, and conditions are wet, eachantheridium releases it sperm, which swim through a film of water towards thearchegonia. The product of fertilisation is a diploid zygote. Note thatfertilisation is still dependent on water as in the bryophytes.

The zygote grows into the sporophytegeneration. The young embryo absorbs nutrients from the gametophyte until itsown roots and leaves can take over the role of nutrition. The gametophyte stillwithers and dies.

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