NEET Biology Notes Plant Kingdom Pteridophytes
- The pteridophytes include horsetails and ferns. They are used for medicinal purposes and as soil binders and frequently grown as ornamental plants. They are first terrestrial plants having vascular tissues, i.e., xylem and phloem.
- Pteridophytes are found in cool, damp and shady places and flourish well in sandy soil conditions.
- In pteridophytes, the main plant body is a sporophyte, which is differentiated into true root, stem and leaves. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases, sporophylls may form distinct compact structures called strobili or cones (Selaginella and Equisetum).
- Ferns or pteridophytes are primitive vascular plants without seeds.
- The gametophytes bear male and female sex organs called antheridia and archegonia respectively. In majority of pteridophytes, all the spores are of similar kinds, such plants are called homosporous.
- Pteridophytes, which produce two kinds of spores, macro (large) and micro (small) spores are called heterosporous, e.g. Selaginella and Salvinia.
- The development of zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit and considered as an important step in evolution.The four main classes of pteridophytes are
- Psilopsida (e.g. Psilotum]
- Lycopsida (e.g. Selaginella and Lycopodium)
- Sphenopsida (e.g. Equisetum)
- Pteropsida (e.g. Dryopteris, Pteris and Adiantum).Pteridophytes have vascularised root, stem and leaves but they could not achieve desired success in establishing under land conditions due to the following reasons :
- Requirement of moist, wet place for germination of spore.
- Susceptibility of prothallus (gametophytes) to desiccation.
- In Adiantum (maiden hair fern) the main body is sporophyte (diploid). It is differentiated into root, stem and leaves. The stem is perennial, dichotomously branched and thickly covered with scales called paleae.
- The primary root is short lived and is replaced by adventitious roots. The leaves are compound and arise alternately or spirally on the rhizome. The leaflets am
called pinnules. The midrib is absent. The veins spread in fan-like manner in the lamina of leaflet. The rachis of leaf is black in colour.
- Vegetative reproduction occurs by fragmentation of rhizome and growth of adventitious buds. The latter occur at, the leaf tip of Adiantum caudatum. Adiantum is known as walking fern because of its ability to form new plants, whenever the leaf tip happens to come in contact with soil.
- A fertile leaf bearing sporangia, is called sporophyll. In Adiantum, any vegetative leaf can develop sporangia and become a sporophyll. The margin of pinnule blade is folded towards the lower side to form the false indusium. It covers the groups of sporangia called sori (sing, sorus).
- The sori are sub-marginal in position! These may be continuous in the form of coenosorus or in the form of small and separate sori. Each sorus may show different developmental stages of sporangium. On maturity of spores, the indusium shrivels and exposes the sporangia. Annulus dries up, shortens and breaks the sporangial jacket in the region of stomium.
- The spores are thrown out like a catapult. They are dispersed by wind.
- After reaching a suitable substratum, each spore produces gametophyte or prothallus. Prothallus is thin flat cordate, independent green thalloid gametophyte that lives in wet and shady places. It is attached to the substratum by means of unicellular rhizoids. The rhizoids bearing ventral side also possess antheridia and archegonia.
- Archegonia develop behind the apical notch in an area called apical cushion. Each archegonium is a flask-shaped structure with an embedded venter and protruding neck. Venter encloses a venter canal cell and an egg or oosphere. Antheridia occur scattered among rhizoids.
- They are sessile and hemispherical. Each antheridium has a jacket of 3-cells and 32-48 androcytes. An androcyte produces a single multiflagellate sperm or spermatozoid.
- Cross fertilisation generally occurs due to the protandry of prothallus. A thin film of water is required for spermatozoid to swim up to archegonia. Sperms are attracted to archegonia by malic acid. The sperm and egg on fusion forms a diploid structure known as zygote.
- Zygote undergoes divisions and produces an embryo that has a foot for obtaining nourishment from prothallus, a stem apex, a root and the first leaf. Embryo grows in size and becomes an independent fern plant. Life history is diplohaplontic with heteromorphic or heterologous alternation of generations.
Heterospory is the production of spores of two different size and two different development patterns small spores are called microscopes (males spores) and larger spores as megaspores (female spores). Heterospory is an expression of sex determining process of the plants. It is most important development in the vascular plants because it leads to the seed development.