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Classification of Plants- Botany

Classification of Plants

 

 

 

 

Pteridophyta

 

 

  • Introduction : The pteridophytes (Gk. Pteron = feather and phyton = plants ; means plants with feather like fronds or ferns). They are flowerless, seedless, spore producing vascular plant which have successfully invaded the land. Pteridophytes represent an intermediate position between bryophytes and spermatophytes (Gymnosperm and Angiosperm). It is classified by Carolus Linnaeus (1754) under the class cryptogamia. They are also called vascular cryptogams. The group has a long fossil history. It is the most primitive group that flourished in Devonian and Carboniferous periods of Palaeozoic Era.
  • Habitat : The plants of pteridophytes are mostly terrestrial. They prefer shady habitats. Some species of Selaginella and Adiantum are xerophytes. A fern, Acrostichum aureum is a halophyte. Some species e.g., Selaginella oregana, Psilotum flacidum, Lycopodium squarrosum and ferns like Asplenium nidus, Pleopeltis sp. are epiphytes. Marsilea occurs as a terrestrial, amphibious as well as an aquatic plant. There are true aquatics ferns like Salvinia, Azolla and Ceratopteris.
  • Sporophytic plant body
    • The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true leaves.
    • The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing apex.
    • The stems are usually herbaceous (except in some woody ferns) and branched monopodially or dichotomously.
    • The leaves may be small microphyllous (e.g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).
    • All the vegetative parts possess vascular tissues (i.e., xylem and phloem) organized in definite groups or steles. Secondary growth does not occur in most of the living pteridophytes (except in Isoetes).

 

  • Apical growth : The pteridophyte generally possess a single apical cell with three cutting faces in the shoot apex. The root tip also has a single apical cell but with four cutting faces.
  • Spore producing organs : The plants of pteridophytes are sporophytes. They reproduce asexually by forming spores in sporangia. They are homosporous but a few plants are heterosporous also e.g., Isoetes, Selaginella, Marsilea, Regnellidium, Pilularia, Azolla and Salvinia. In Selaginella the sporangia are borne in relation to sporophylls which constitute a strobilus. In Equisetum they are borne on sporangiophores which constitute a cone. In ferns the sporangia are borne in sori on the sporophylls. The sori are of three types:
    • Simple sorus : Here all the sporangia mature at the same time.
    • Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger stages.
    • Mixed sorus : It shows mixed arrangement of younger and older sporangia.

    In Marsilea, Azolla, Salvinia etc. the sori are present in a box like structure called sporocarp. The sorus may be naked or covered by an inducium. The inducia may be true or false. A true inducium is a specially developed structure whereas a false inducium is formed by curving of the sporophyll margin.

 

  • Sporangium : The sporangia are generally stalked structures. Each sporangium is distinguishable into a jacket enclosing a mass of sporogenous tissue. The sporangial jacket may be 2-4 layered. The innermost wall layer is the tapetum. No tapetum is formed in Psilotum and Tmesipteris. It is a nutritive layer which degenerates at maturity of the sporangium. The sporangial jacket in some ferns shows distinctions of annulus and stomium. On the basis of development the sporangia have been classified by Goebel, 1881 into two categories as under :

 

    • Eusporangiate type : Such a sporangium develops from a group of superficial initials. They divide periclinally into outer and inner components. The outer cells form the wall whereas the inner cells give rise to sporogenous tissue.
    • Leptosporangiate type : Such a sporangium arises from a single superficial initial. It divides periclinally into outer and inner components. While the inner cell forms the stalk, the outer gives rise to sporagnium proper. In Marattia alata, the sporangia in a sorus may fuse to form a synangium.
    • Spore : The plants may be homosporous, i.e., produce only one type of spores (e.g., Lycopodium, Pteridium) or heterosporous i.e., produce two different types of spores, smaller microspores and larger – megaspores (e.g., Selaginella, Marsilea etc.). The spore germination is homosporous pteridophytes may be bipolar (e.g., Lycopodium, Equisetum) or tripolar (e.g., Hymenophyllum) or amorphous (e.g., Angiopteris).

    The spores germinate to produce haploid gametophyte, called prothallus. The homosporous pteridophytes produce bisexual (monoecious) gametophytes whereas heterosporous one produce unisexual (dioecious) gametophytes.

  • Sex organs : The archegonia and antheridia are generally of embedded type. The archegonium consists of neck which usually projects from the surface of the prothallus. It contains 1-2 neck canal cells. There is no venter. The egg and the ventral canal cell remain surrounded by the cells of prothallus. The antheridia are generally sessile. They have a 1-cell thick jacket enclosing a mass of androgonial cells. They form the androcytes which are metamorphosed into biflagellate (Lycopodium, Selaginella) or multiflagellate (Equisetum, ferns) antherozoids.

 

 

 

Sub Topics of Classification of Plants

 


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