General Aspects of Pteridophyta

The plant kingdom is divided into two major groups cryptogamic and phanerogamae. Cryptogame is flowerless and seedless plants while phanerogamae are flowering and seed-bearing plants.

The word cryptogams is a synthesis of two Greek terms kruptos meaning ‘hidden’ and games meaning ‘wedded’. This single term encompasses all plants that reproduce using spores and, do not produce seeds. The algae, fungi, bryophytes, and pteridophytes are all cryptogams. The Pteridophyta are treated as vascular cryptogams as they have a well-developed conducting system. The lower cryptogams (algae, fungi, and Bryophyta) have no conducting system.

What are pteridophytes?

The term Pteridophyta also has a Greek origin. Perron means a “feather” and the name was originally given to this group because of their pinnate or feather-like fronds. Vascular cryptogams are, therefore, an assemblage of seedless vascular plants that have successfully invaded the land and reproduced using spores.

Features of pteridophytes:

• They are the first land plants on earth: It is thought that life started in the oceans and gradually adapted to dry land over millions of years of evolution. The pteridophytes were among the first plants to exist on land truly.
• They are seedless vascular plants: Pteridophytes do not have seeds and reproduce by spores. Vascular tissues are present, but xylem vessels and phloem companion cells are absent.
• The plant body of these organisms has true roots, stems, and leaves: The plant body is divided into the stem, roots, and leaves.
• Spores develop in sporangia: The structures in which spores are produced are known as sporangium. They are both homosporous (producing only one type of spore) and heterosporous (producing several types of spores) (meaning: two kinds of spores are produced.)
• In these plants, sporangia are produced in groups on sporophylls: Sporophylls are leaves that have sporangia on them. To cover the fragile growing sections, the tip of the leaves curl inwards.
• They have multicellular sex organs: Male sex organs are referred to as antheridia, while female sex organs are referred to as archegonia.
• These plants show true alternations of generation: In pteridophytes, the sporophyte and gametophyte generations are observed. The primary plant body is the diploid sporophyte.

Classification of Pteridophyta:

1) Sub-Division – Psilopsida

• These are the oldest known vascular plants. Most of them have become extinct (e.g., Rhynia, Horneophyton). Only two living species, Psilotum and Tmesipteris, are now available.
• The plant body is very simple and does not show much differentiation.
• Dichotomously branched rhizome takes the place of roots.
• Sporangia are directly borne on the stem (i.e., cauline). Either terminal or lateral.

2) Subdivision Lycopsida

• The plant body shows differentiation into roots, stems, and leaves.
• Leaves are microphyllous (small) having a single unbranched vein in the midrib region.
• Sporangia are borne in the axil of the fertile leaves.
• Sporophyll forms compact strobili (singular strobilus).

3) Subdivision Sphenopsida

• Plant body shows differentiation into nodes and internodes like higher vascular plants.
• Leaves microphyllous, and arises in whorls at each node.
• Sporangia develop on sporangiophores which form compact cones at the apex of fertile branches (e.g., Equisetum).

4) Subdivision Pteropsida (Ferns)

• Plant body shows much advancement towards higher vascular plants and is well-differentiated into roots, stem, and leaves.
• Leaves also show great advancement, and are megaphyllous (large) and pinnately compound.
• Sporangia develop on the ventral surface of the sporophylls, and are usually aggregated into sori (e.g., Dryopteris, Pteris, Pteridium, Polypodium, etc.)

The life cycle of Pteridophyta:

Pteridophytes show alternation of generations. Their life cycle is similar to seed-bearing plants, however, the pteridophytes differ from mosses and seed plants as both haploid gametophyte and diploid sporophyte generations are independent and free-living. The sexuality of pteridophytic gametophytes can be classified as follows:

• Dioicous: the individual gametophyte is either a male producing antheridia and sperm or a female producing archegonia and egg cells.
• Monoicous: every individual gametophyte may produce both antheridia and archegonia and it can function both as a male as well as a female.
• Protandrous: the antheridia matures before the archegonia.
• Protogynous: the archegonia matures before the antheridia.

Reproduction in pteridophytes:

Pteridophytes show a true alternation of generations. Here, the dominant sporophyte produces spores through meiosis. The gametophyte generation forms gametes by mitosis. The spores are

produced by the sporangia in the spore mother cells. These spores germinate and give rise to gametophytes.

These gametophytes are free-living, multicellular, and photosynthetic. They are called the prothallus. Generally, the gametophytes require damp and cool places to grow, due to their dependence on water. For this reason, the growth of pteridophytes is confined to certain geographical areas.

Antheridia is the name of the male sex organs and archegonia is the name of female sex organs. Antherozoids are the male gametes, which are released by the antheridia. Antherozoids can get transferred to the archegonia which are the female sex organs, only in the presence of water. Once the fusion of the gametes occurs, a zygote is formed. This zygote produces the sporophyte, after division.

When the spores of the plants are similar then these plants are called homosporous plants. Heterosporous plants are the ones that have two different kinds of spores. They are the megaspores and the microspores. In these heterosporous plants, the megaspores and microspores germinate and give female and male gametophytes respectively.

General characteristics of pteridophytes:

1. 1. Members of this group are the most primitive living vascular plants.

2. The main plant body is a sporophyte. It is differentiated into roots, stems, and leaves.

9. Plants (sporophyte) reproduce by spores formed in sporangia. Sporangia develop either on the ventral surface or in the axil of leaves.
10. Plants may be homosporous or heterosporous. The development of sporangium may be of eusporangiate or leptosporangiate.
11. Spores develop into a multi-cellular gametophytic body called prothallus – a thalloid structure. It is independent and chlorophyllous. It bears male reproductive organs called antheridia and female reproductive organs called archegonia. Reproductive organs are multicellular and jacketed.
12. Water is essential to effect fertilization as male reproductive units are flagellated and motile.
13. Seed formation never takes place in living pteridophytes.
14. An embryo develops in situ after fertilization and the sporophyte remains attached to the gametophyte till the development of the root.
15. Plants show clear alternation of generations. The sporophyte and gametophyte are independent of each other.

Conclusion:

Pteridophyta is one of the older groups of plants present in the Plant kingdom. They have evolved much earlier than the angiosperms. They are one of the very first “true” plants to adapt to life on land.

The primary characteristics of Pteridophytes are as follows: They are seedless, vascular plants that show true alternation of generations. Furthermore, the sporophyte has true roots, stems, and leaves. They reproduce by spores, which are developed in sporangia. They may be homosporous or heterosporous.