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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Conquest of Land Page 2

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Structure of Foliage Leaf and Vegetative Reproduction in Cycas


  • Foliage Leaf: Compound, distinct leaf stalk. The basal portion of leaf stalk shows spines instead of leaflets. This is xerophytic adaptation. Single leaflet is called 'pinna' which is shiny green and leathery. It is 'sessile'. Young leaf shows rolled up condition called 'circinate ptyxis'.
  • Anatomy of Leaflet: Internally, the leaflet shows thick layer of cuticle on both epidermal layers. Stomata are present in sunken pits. They are present only on lower epidermis. Sclerenchymatous hypodermis prevents trapping of excess heat. Mesophyll is differentiated into spongy and palisade tissue. As the lateral veins in the leaflet are absent, 'transfusion tissue' is present in the middle layer of lamina for conduction. The vascular bundle is with bundle sheath, conjoint, collateral and open.
  • Vegetative Reproduction: It is by means of small adventitious buds produced at the base of stem called 'bulbils'.




Structure of Male Cone and Dispersal of Spores in Cycas


  • Male Cone: Present at the tip of the stem of male plant. It has a central axis on which 'microsporophylls' are compactly arranged in an acropetal succession. Each microsporophyll is triangular, woody, wedge-shaped structure with a narrow prolongation called 'apophysis'. Surface of microsporophyll bears many 'pollen sacs', i.e., 'microsporangia' which form a group of 3 to 6 called 'sorus'. Microsporangium contains microspore mother cells which on meiosis produce 'microspores'.
  • Dispersal and Structure of Microspore: Microspores, i.e., pollen grains are dispersed by wind. Each one has an outer covering called 'exine' and 'inner' called 'intine'. At the time of dispersal, it shows three cells namely 'tube cell', 'generative cell' and 'prothallial cell'. This is called male gametophyte.




Structure of Female Cone of Cycas


  • Female Cone: A group of 'megasporophylls' is produced at the apex of mature female plant. The megasporophylls are arranged loosely in a cluster. Each megasporophyll shows an upper leafy broad part and a lower stalk-like part. Ovules are naked and are produced on the lower stalk-like part.
  • Mature Ovule: It is erect ('Orthotropous'), sessile and is with central mass of tissues called 'nucellus'. Integument is three-layered with outer and inner soft and fleshy layers while middle has a stony layer. The narrow opening 'micropyle' continues as micropyle canal which ends in 'pollen chamber'. The functional megaspore produces female gametophyte. 2-6 'archegonia' are present which lie in 'archegonial chamber'. Archegonium has a layer of jacket cells, short neck cells, large egg cell or female gamete.




Pollination and Fertilization in Cycas


  • Pollination is by wind. A pollination drop is produced at the tip of ovule. The pollen grains get trapped in it. Pollen grains settle in the pollen chamber of ovule and further development is carried out. Pollen tube is produced which carries two spermatozoids with it.
  • Spermatozoids are motile with cilia and are top-shaped. They enter the archegonia and one of the spermatozoids fuses with the egg cell to form diploid 'zygote'.
  • Zygote undergoes repeated divisions to form an embryo. Ovule becomes the seed and integuments form seed coat.
  • The embryo contains two cotyledons, radicle and plumule. The seed germinates by 'hypogeal' method to give rise to new plant.




Alternation of Generations in Cycas


  • In the life cycle of cycas, two distinct generations are observed, namely, sporophytic and gametophytic.
  • Here plant itself is a dominant sporophyte while the gametophyte is much reduced and is dependent on sporophyte.
  • Two different types of spores are produced, i.e., microspores and megaspores which give rise to male and female gametophytes respectively.
  • Thus, sporophytic generation gives rise to gametophytic generation and gametophytic generation gives rise to sporophytic generation.
  • As both these stages alternate with each other, the process is called alternation of generations.



Characters of Angiosperms


  • Angiosperms are the flowering, vascular plants where ovules are present inside the ovary.
  • Flowers are with essential and non-essential whorls.
  • Double fertilization is a distinguishing character of angiosperms.
  • Xylem is with vessels, which helps in more efficient conduction of water.
  • The phloem is with sieve tubes and companion cells.
  • Seven-celled, eight-nucleated embryo sac is the female gametophyte.
  • Angiosperms are further divided into two classes, namely, Dicotyledonae (Dicots) and Monocotyledonae (Monocots).
  • Dicots and monocots show differences with respect to root system, stem, leaf venation, flower structure, number of cotyledons.






Hibiscus - Vegetative Parts of the Plant



  • Hibiscus belongs to dicotyledonae - The plant is a sporophyte which is divided into root, stem, leaves and flowers.
  • Along with taproot system, the stem shows branching.
  • Leaves are simple, petiolate, stipulate and are arranged in an alternate fashion. Each leaf has acute apex and toothed-margin called 'serrate' margin. The leaf shows reticulate venation.






Structure of Hibiscus Flower



  • Flower has a long stalk called 'pedicel'. Hibiscus flower is complete, bisexual with radial symmetry. Since the ovary is superior, the flower is hypogynous.
  • The non-essential whorls of Hibiscus are calyx and colourful corolla. Calyx is made up of five green sepals. In addition to calyx and corolla, there is one more whorl found at the base of calyx called 'epicalyx' which is made up of small green 'bracteoles'.
  • Androecium: It is the male reproductive structure made up of stamens. In Hibiscus, there are many stamens which are with small filaments and 'kidney-shaped' anthers called 'reniform' anthers. Here, filaments of all stamens are united to form a long, hollow tube called 'staminal tube' through which the style passes. This condition is called monadelphous condition.
  • Gynoecium: It is the female reproductive part of flower made up of 'carpels' or 'pistil'. Each carpel in Hibiscus has a basal ovary, long style and pentafid, 'capitate' stigma.






Types of Reproduction in Hibiscus



  • Vegetative Reproduction: This takes place with the help of pieces of stem, cut and planted. Each stem piece develops its own root system and forms a new plant.
  • Sexual Reproduction: Here, male and female gametes are produced by androecium and gynoecium.






Structure of Male and Female Gametes in Hibiscus



  • Male Gametes: In the pollen sac of anther, there are microspore mother cells, which undergo meiosis and produce microspores or pollen grains. Hibiscus microspore has 'spiny exine' and thin 'intine'. It has two cells, namely, 'tube cell' and 'generative cell'. Tube cell produces pollen tube while generative cell produces two male gametes.
  • Female Gametes: These are formed inside the ovule. The nucellus of ovule contains megaspore mother cell which undergoes meiosis to form megaspores. Only one functional megaspore gives rise to 'embryo sac' by successive divisions. The egg cell, i.e., female gamete is present inside embryo sac.






Structure of Ovule in Hibiscus



  • It is inverted, i.e., 'anatropous'.
  • It has a stalk called 'funicle'.
  • The 'nucellus' is surrounded by two 'integuments' which leave a gap called 'micropyle'.
  • The embryo sac is female gametophyte which contains an 'egg' cell and two 'synergids' towards micropylar end, three 'antipodal' cells towards chalazal end and a 'secondary nucleus' in the centre.






Pollination and Fertilization in Hibiscus



  • Hibiscus flowers being attractive, pollination is by insects. The pollen grains come in contact with the stigmatic surface and they produce pollen tube. The generative cell gives rise to two male gametes.
  • Fertilization: The pollen tube carrying two male gametes passes through the style and reaches the ovary. It then enters the ovule through micropyle. The tip of pollen tube ruptures and two male gametes are released in the embryo sac. Out of two male gametes, one fuses with the egg to form zygote while other fuses with the secondary nucleus to form endosperm. This is called 'double fertilization'.






Post Fertilization Changes and Alternation of Generations in Hibiscus



  • After fertilization, ovary develops into a fruit, ovules form seeds, two integuments form two seed coats called testa and tegmen. The zygote forms embryo consisting of two cotyledons, radicle and plumule. As the endosperm is used by developing embryo, the seed is 'non-endospermic'.
  • Alternation of Generations: In the entire life cycle, the diploid sporophyte alternates with haploid gametophyte. Two types of gametophytes are produced due to microspores and megaspores. Fusion of gametes is fertilization which maintains the diploid condition. It is the beginning of sporophytic generation.



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