Pteridophytes or Pteridophyta, in the broad interpretation of the term, are vascular plants (plants with xylem and phloem) that reproduce and disperse via spores. As they produce neither flowers nor seeds, they are referred to as Cryptogams. Pteridophyta is the most highly evolved group among the Cryptogams. Vascular tissue is present and the plant body is differentiated into root, stem and leaves.
Pteridophyta is predicted as an oldest Cormophyta. Cormophyta is plant having real roots, stems and leaves. It means that the roots, stems and leaves have vascular bundle so Pteridophyta are also known as tracheophyta. Pteridophyta living in water are known as hydrophyte, sticking in other plants are called epiphyte and living in residue or waste of other plants are known as saprophyte. Most Pteridophytes are terrestrial and grows in moist and shady places while some flourish well in open dry places. Pteridophyta do not produce seeds but they produce spores. Spores are produced by leaves, usually in under surface of leaves. The young leaves are rolled up. Stems of Pteridophytes are located under the soil called rhizome. The rhizome grows hair like roots called fibrous roots and also grows leaves stalk. There are stems of Pteridophytes that grow up the soil. Example – Fern. |
|
In certain Pteridophytes the sizes of leaves are different. Pteridophytes have little leaves (microphyll) and large leaves (macrophyll). Leaves that produce spores are called sporophyll and leaves that do not produce spores are called tropophyll. The function of tropophyll is to only photosynthesize. Spores that are produced by Pteridophyta are located in spore box which is known as sporangium. Groups of sporangium are contained in sorus. Sorus is protected by membrane called inducium.
Pteridophyta are widely known as Ferns. More than 12000 different species of ferns are distributed worldwide. Ferns are also some of Earth’s first land plants.
Pteridophytes consist of two separate class:-
1) Lycopodiopsida
2) Polypodiopsida
The Lycopodiopsida are a class of plants often loosely grouped as the Fern allies. Traditionally the group included not only the clubmosses and firmosses, but also the spikemosses and the quillworts.
The Polypodiopsida class of plants include Fern, horsetails, whisk fern, marattioid ferns and ophioglossoid ferns. It is often loosely grouped as Ferns. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided. Fronds in the largest species of ferns can reach some six metres in length!
Pteridophyte Life cycle
Just like with seed plants and mosses, the life cycle of pteridophytes involves alternation of generations. This means that a diploid generation (the sporophyte, which produces spores) is followed by a haploid generation (the gametophyte or prothallus, which produces gametes). Pteridophytes differ from mosses and seed plants in that both generations are independent and free-living, although the sporophyte is generally much larger and more conspicuous.
Pteridophyta have life cycle known as metagenesis with two generations, they are sporophyte and gametophyte generations.
- Sporophyte Generation. Sporophyte generation producing spores is aPteridophyta itself. So, the Pteridophyta that we see in daily life is sporophyte generation. This generation is longer than gametophyte. Sporophyte can grow and produce some bud in order to add new individual. It is called asexual reproduction. While the spores is exited from sporangium and spread by wind, if it fall in suitable place it will grow as new individual plant that called asprotalium.
- Gametophyte Generation. Gametophyte generation is producing gamet known as protalium. Protalium is talus formed have 1-2 cm approximately in size. This form like a heart that is usually lives in moist place. Unlike Bryophyta, gametophyte in Pteridophyta only lives on some weeks. Protalium formed antheridium as male gamet and archegonium as female gamet. Antheridium produce sperm while archegonium produce ovum. Fertilization between sperm and ovum formed zygote. Furthermore, zygote will grow as new Pteridophyta that have roots, stems, and leaves.
Differences between sporophyte and gametophyte generations in Pteridophyta
Based on kinds of spores that produced by Pteridophyta, it is divided into homospores, heterospores, and transitions fern.
- Homospores Fern. Homospores fern is only produce one kind of spore. It also known as monoceous fern, for example is Lycopodium sp (wire fern). This fern have stem like a wire that lives creeping on the other plant.
- Heterospores Fern. Heterospores fern produce two kinds of spores. The small spores called microspores and produce antheridium as male gamet, while large spores called macrospores and produce archegonium as female gamet. For examples are Selaginella sp and Marsilea sp.
- Transitions Fern. Transitions fern produce spores that have same in size but is differenced into male spores (+) and female spores (-). Example is Equisetum debile(tail horse fern).
The sexuality of pteridophyte gametophytes can be classified as follows:
· Dioicous: each individual gametophyte is either male (producing antheridia and hence sperm) or female (producing archegonia and hence egg cells).
· Monoicous: each individual gametophyte produces both antheridia and archegonia and can function both as a male and as a female.
Protandrous: the antheridia mature before the archegonia (male first, then female).
Protogynous: the archegonia mature before the antheridia (female first, then male).
The sporophyte generation in ferns and their allies is long-lived compared to the gametophyte. It is mostly larger and structurally more complex in that it has a vascular system of xylem and phloem, often associated structural supporting tissue and is mostly organized into highly specialized stems, leaves and roots. The aerial parts are protected by a waxy cuticle and are thus able to withstand greater degrees of exposure than the gametophyte; stomata are present allowing the exchange of gases between internal chambers and the external environment. The sporophyte has a double set of chromosomes (i.e. diploid) and is the asexual part of the life cycle.
At maturity the sporophyte develops specialized structures of varying complexity on the leaves called sporangia in which the spores are produced; a number of sporangia may be aggregated into structures called sori. Spores consist of a single cell surrounded by a durable cell wall; they are produced by meiotic divison in which the number of chromosomes is halved and are hence haploid. Spores are minute, non-motile and often produced in large numbers; they are shed by rupturing of the sporangial wall and can be dispersed long distances by wind or water. When mature and under appropriate conditions the spores are dispersed widely into the environment, and if they land in a suitable place germinate to create the next generation of gametophytes, completing the cycle.
Most pteridophytes produce spores of only one size (homosporous), but a significant number of species produce spores of two distinct sizes (heterosporous): minute microspores and relatively very large megaspores. Microspores produce the male antheridia and megaspores produce the female antheridia.
In addition to the sexual gametophyte-sporophyte life cycle, some of pteridophytes have developed various vegetative means of propagation to increase the extent and number of their population. This is advantageous where seasons are unreliable or the environment is otherwise not conducive to gametophyte production, and in some cases it may just be advantageous to generate numerous genetically identical individuals.
A number of quite unrelated species produce vegetative buds or bulbils that are capable of producing roots and new plants. These bulbils are leaf derive structures and are produced in leaf axils or at various places on the leaf surface. Plants may chain along as successive bulbils take root, establish themselves and produce fronds and bulbils of their own. Proliferating bulbils can be found in Asplenium, Camptosorus, Diplazium, Polystichum,Ampelopteris, Huperzia and a number of other genera.
Some species with long creeping rhizomes can extend over large areas and it may not be obvious that plants on one side of a population were physically derived from plants on another. Large brakes or open areas can be covered by scrambling ferns with creeping underground rhizomes in this manner; Gleicheniaceae genera (Gleichenia, Sticherus, Dicranopteris) and bracken (Pteridium) are good examples. Similarly populations of climbing epiphytes may be increased in extent and mass as rhizomes branch, break and regrow; examples include genera of Polypodiaceae (Pyrrosia, Lemmaphyllum, Microsorum). The floating ferns Salvina and Azolla are striking examples of propagation by stem fragmentation.
Proliferation is possible from root stolons, tubers and similar structures. This occurs in some species of Nephrolepis and Blechnum.
More of survival strategies than a means of propagation, some ferns of arid areas are able to dry out almost completely without actually dying. Crisp and brittle to touch, they resuscitate rapidly when rains come and continue their growth. A number of species of Cheilanthes behave this way. Commonly called resurrection ferns.