The filicinophyta are also known as pteridophyta; commonly called the ‘ferns’. They are usually restricted to damp shady habitats. Few ferns are capable of growing in full sunlight, although bracken (pteridium) is a common exception. Ferns are common in tropical rain forests, where temperature, light and humidity are favourable.
Ferns are vascular plants, meaning that they contain vascular tissue. Vascular tissue is made up of xylem and phloem. These tissues are concerned with translocation (transport) of water and nutrients round the plant body.
Xylem helps carries mainly water and mineral salts, whereas phloem carries mainly organic solutes in solution such as sugars.
Vascular tissue is a major evolutionary advance compared with the simple conducting cells of some bryophytes and algae. It is found only in the sporophyte generation, and is one reason why the sporophyte generation becomes conspicuous in all vascular plants.
Vascular tissue has two important properties. First, it forms a transport system, conducting food and water around the multicellular body, thus allowing the development of large, complex bodies. Secondly, these bodies can be supported because xylem, apart from being conducting 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 filicinophyta are 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 true roots, stems and leaves. Roots penetrate the sol with the result that water and dissolved nutrients can be obtained more easily. Xylem conducts it to other parts of the plant.
Once plant bodies could achieve support above the ground, there must have been competition for light, so there would be a tendency for taller forms to evolve.
Ferns and tree-ferns were the dominant vegetation 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 a land environment, which are associated with the sporophyte generation, in ferns there remains a major problem with the gametophyte. It is called a prothallus, and produces sperm which must swim to reach the female gametes, as is the case in bryophytes.
The male fern (Dryopteris filix-mas)
Dryopteris filix-mas is probably the most common British fern and is found in damp woods, hedgerows and other shady places throughout the country. The fronds (leaves) of the sporophyte may reach a metre or more in height and grow from a thick horizontal stem, or rhizome. This bears adventitious roots.
Branches from the main stem may eventually break away and give rise to separate plants, a form of vegetative reproduction.
The bases of the fronds are covered with dry brown scales called ramenta that protect the young leaves from frost or drought. The young leaves from show a characteristic tightly rolled structure. The ramenta gradually become smaller and less dense up the main axis of the frond. This axis is called the rachis, and the leaflets either side are called the pinnae. The small rounded subdivisions of the pinnae are called pinnules.
Spores are produced during late summer in structures called sporangia. Sporangia develop in clusters called sori on the undersides of pinnules. Each sorus has a protective covering called an indusium. Inside each sporangium diploid spore mother cells divide by meiosis to produce haploid spores. When mature, the indusium shrivels and drops off, and the exposed sporangium walls begin to dry out. Eventually the wall ruptures, and spores are catapulted from the sporangium.
The spores germinate to form the gametophyte generation. The gametophyte is a thin heart-shaped plate of cells about 1cm in diameter. It is green and photosynthetic and is anchored by unicellular rhizoids to the soil. This delicate prothallus lacks a cuticles and is prone to drying out, so can only serve in damp conditions.
The gametophyte (prothallus) produces simple antheridia and archegonia on its lower surface. These sex organs protect the gametes within them. Gametes are produced by mitosis of gamete mother cells, the antheridia producing sperm and each archegonium an ovum, as in the bryophytes. Each sperm has flagella. When ripe, and conditions are wet, each antheridium releases it sperm, which swim through a film of water towards the archegonia. The product of fertilisation is a diploid zygote. Note that fertilisation is still dependent on water as in the bryophytes.
The zygote grows into the sporophyte generation. The young embryo absorbs nutrients from the gametophyte until its own roots and leaves can take over the role of nutrition. The gametophyte still withers and dies.