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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Conquest of Land Page 1

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Introduction of Vascular Plants


  • Among the kingdom 'plantae', algae represent the most primitive aquatic plants.
  • In the due course of time, due to evolutionary changes, the plants started shifting on land. Bryophytes are the first plants to come over land but they grow in damp and shady
    places. Bryophytes are called amphibians of plant kingdom. They show characters of terrestrial as well as aquatic plants.
  • Pteridophytes, gymnosperms and angiosperms became fully terrestrial because these plants showed adaptations, suitable for their survival.






Adaptations for Terrestrial Habitat



  • The root system was very well developed. Well developed stem, branching
    pattern and leaves for proper photosynthesis.
  • Since these plants are large in size, some tissues are required which help the
    plant to give support. Such sclerenchymatous mechanical tissues are present
    in the terrestrial plants.
  • Conducting tissues like xylem and phloem are present to conduct water and prepared food to different parts of the plant body. Thus, these plants are 'vascular plants'.
  • In order to prevent excessive loss of water from the plant body, a thin layer of cuticle is present. But in order to release excess water, special openings
    called stomata are present.
  • Out of the two phases in the life cycle, i.e., gametophyte and sporophyte,
    in case of terrestrial plants, sporophyte became the dominant phase which
    is diploid.
  • Sporophyte produces two types of spores, i.e., microspores and megaspores
    which give rise to male and female gametes respectively.
  • After fertilisation, ovule develops into seed. Thus, angiosperms and gymnosperms
    are called 'spermatophyta'.




Characteristics of Pteridophytes


  • Pteridophytes are vascular cryptogams, i.e., vascular non-flowering plants. Here, sporophyte is independent and dominant.
  • The gametophyte is reduced, simple but photosynthetic, i.e., independent.
  • Sporangia are produced on the leaves. These sporangia produce haploid spores.
  • Male sex organs called antheridia and female sex organs called archegonia are produced on the gametophyte.
  • Fusion of male and female gametes needs water as a medium.






Nephrolepis-Root System, Stem and Vegetative Propagation



  • Nephrolepis is a common tropical fern (pteridophyte).
  • The sporophyte is differentiated into root, stem and leaves.
  • The roots are of adventitious type. The stem is underground, covered with ramenta, and is called 'caudex'. Leaves are compound.
  • 'Stolons' are small branches given out from caudex which help in vegetative reproduction. They produce 'spurs', i.e., bud-like structures which give rise to a new plant.






Structure of Leaf of Nephrolepis



  • The young leaf remains in coiled form and is called 'circinate ptyxis'.
  • The compound leaf shows central axis. The lower part of the axis is dark brown and does not bear leaflets. It is called 'stipe' while the upper 3/4th part which bears alternately arranged leaflets is called 'rachis'.
  • Each leaflet is sessile, lanceolate, auricled. It shows forked venation.






Dorsal and Ventral Surface of Leaflet of Nephrolepis



  • Dorsal or Adaxial Surface of Leaflet:
    It shows a row of white dot like glands called 'hydathodes' which are present at the ends of fork along the margin of leaflet. They release water with calcium carbonate salt dissolved in it. The water evaporates and calcium carbonate salts remain in the form of white dots. Thus, also called 'chalk glands'. The hydathodes are also called 'water stomata'. The process by which water is released through hydathodes is called 'guttation'.
  • Ventral or Abaxial Surface of Leaflet:
    It shows small kidney-shaped 'Sori' only on the upper forked veins along the margin. Each sorus is covered by an 'indusium' and contains many 'sporangia' which bear spores.






Structure and Dehiscence of Sporangium of Nephrolepis



  • Sporangium: It is a biconvex capsule having multicellular stalk. Jacket shows single layer of cells. The cells are of two types:
    a) Annulus: These are thick walled hygroscopic cells.
    b) Transverse Cells: These are thin-walled, slightly elongated, transversely placed cells which form 'stomium' during dehiscence. Inside sporangium are 16 spore mother cells each of which undergoes meiosis, thus, producing 64 identical spores (Homosporous condition).
  • Dehiscence of Sporangium: It takes place in dry weather. Annulus cells lose water, shrink and the stomium opens. The annulus goes back carrying spores attached to it. It comes back to its original position being elastic, thus the spores are released with a force.






Structure of Gametophyte (Prothallus) of Nephrolepis



  • The spore germinates to produce independent, autotrophic, haploid gametophyte, i.e., 'Prothallus'.
  • Each prothallus is heart shaped, multicellular, having a notch towards anterior broad end.
  • Below the notch is a multicellular cushion like structure where archegonia are present (female reproductive structures).
  • Except for the cushion, the prothallus is single layer in thickness.
  • The cells contain chlorophyll.
  • Towards posterior side are rhizoids and among rhizoids are the male reproductive structures called 'antheridia'.






Sex Organs in Nephrolepis



  • Antheridium: Each antheridium is somewhat globular in structure having a wall of three jacket cells; viz. first ring cell, second ring cell and cap cell or lid cell.
  • The male gametes called 'antherozoids' or 'spermatozoids' are produced within the antheridium.
  • The antherozoids are motile, cork screw-shaped, with flagella. They come in the outside water when the lid cell of antheridium breaks open.
  • Archegonium: Inverted flask like structure. Two main parts namely broad, lower 'Venter' and narrow long 'neck'.
  • The venter is protected by prothallial cells while neck has its own neck cells.
  • Inside the neck region is a single, binucleate 'neck canal cell', and at the tip of the neck are 'lid cells'. Venter contains a large 'egg'.
  • Between venter and neck is a cell called 'venter canal cell'.









  • In Nephrolepis, the antheridia mature before archegonia. This condition is called protandry. Cross fertilisation. Water is essential for fertilisation.
  • The antherozoids are attracted by the presence of certain specific organic chemical substances like malic acid which is produced in the archegonium. Such chemical attraction is called 'chemotaxis'.
  • The antherozoid fertilizes the egg to form diploid 'zygote' or 'ospore' which then develops into an embryo.
  • In the course of time, embryo gives rise to independent sporophyte.






Alternation of Generations of Nephrolepis



  • In the entire life cycle, there are two distinct generations called sporophytic and gametophytic generation.
  • Both of them alternate with each other in the life cycle, i.e., sporophyte produces gametophyte and gametophyte gives rise to sporophyte. Thus, the process is called alternation of generations.
  • Sporophyte is asexual generation which produces spores. Gametophyte is sexual generation which produces male and female gametes.
  • Fusion of gametes results in formation of sporophyte.




Characteristics of Gymnosperms


  • Gymnosperms are seed-bearing, vascular plants.
  • The plant itself is a sporophyte which is differentiated into root, stem and leaves.
  • Tap root system sometimes shows mycorrhiza or coralloid roots.
  • The aerial, erect stem is mostly unbranched.
  • Leaves are of two types, namely, foliage and scale leaves.
  • The sporophyte is dioecious, i.e., male and female plants are separate.
  • Vegetative reproduction by bulbils.
  • The ovules are naked. Wind pollination is observed.
  • Xylem consists of tracheids, companion cells are absent in phloem.
  • Cycas or sago-palm is one of the examples of gymnosperms.









  • Cycas is a palm-like small tree representing the sporophytic generation. It is differentiated into root, stem and leaf.
  • Cycas possesses a tap root system with a main root bearing a number of lateral roots.
  • Some of lateral roots of Cycas which grow near the soil surface become negatively geotropic and branch dichotomously to form coral like roots. These coral-like roots are called coralloid roots.
  • The coralloid roots harbour blue green algae like Nostoc and Anabaena and certain bacteria which fix the atmospheric nitrogen and make it available to the Cycas plant, while Cycas plant provides them shelter and food (carbohydrates). Thus coralloid roots of Cycas illustrate symbiosis.
  • The leaflet of Cycas shows a number of xerophytic characters such as thick cuticle, sclerenchymatous hypodermis, restriction of stomata to the lower surface and well developed conducting and mechanical tissues.
  • There are two types of reproductive leaves in Cycas, viz., microsporophyll and megasporophyll.
  • The microsporophyll bears microsporangia on its lower surface. The microsporangia are oval in shape and contain numerous microspores or pollen grains.
  • The megasporophyll consists of a lower fertile portion and an upper sterile portion. The fertile portion bears ovules.
  • The ovule of Cycas is largest in the plant kingdom. It consists of a massive nucellus covered and protected by thick covering called integument.
  • Inside the ovule 2 to 5 archegonia develop. Each archegonium has a venter consisting of an egg nucleus and a ventral canal nucleus and a neck consisting of two neck cells.
  • Pollination in Cycas occurs with the help of wind.
  • A mucilaginous substance which is secreted by the ovule in the form of a drop, is called pollination drop.
  • The pollination drop plays an important role in catching the pollen grains coming along with the wind.








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