01 Reproduction in Lower and Higher Plants - part 04 - Structure of Anatropous ovule and Megasporogenesis

01 Reproduction in Lower and Higher Plants - part 04 - Structure of Anatropous ovule and Megasporogenesis


Structure of Anatropous ovule : 

  • Each ovule develops inside the ovary and is attached to the placenta by a small stalk called funiculus. 
  • The place of attachment of funiculus with the main body of ovule, is called hilum. 
  • In angiosperms, the most common type of ovule is anatropous in which micropyle is directed downwards and is present adjacent to the funiculus (funicle). 
  • The ovule consists of central parenchymatous tissue, the nucellus which is surrounded usually by two protective coverings called integumentsviz. Outer and an inner integument.
  • A narrow opening at the apex of the ovule is called micropyle. 
  • Chalaza is the base of ovule directly opposite to micropyle. 
  • Embryo sac (female gametophyte) is oval multicellular structure embedded in the nucellus.

Megasporogenesis : 

  • It is the process of formation of haploid megaspores from diploidmegaspore mother cell (MMC). 
  • Megaspore mother cell becomes distinguished in the nucellus, more or less in the centre but towards micropylar end of ovule.

Development of female gametophyte : 

  • Megaspore mother cell undergoes meiosis to form linear tetrad of haploid cells i.e. megaspore. 
  • Upper three megaspores abort and lowest one towards centre of nucellus remains functional. 
  • It acts as the first cell of female gametophyte. 
  • Generally one megaspore towards centre is functional megaspore. 
  • It is infact the first cell of female gametophyte. It undergoes three successive, free nuclear mitotic divisions. 
  • Thus total eight nuclei are formed, four of which are located at each pole. 
  • One nucleus from each pole migrates towards the centre and are called polar nuclei. 
  • Three nuclei towards micropylar end constitute egg apparatus. It consists of large central, haploid egg cell and two supporting haploid synergid cells.
  •  Synergid shows hair like projections called filiform apparatus, which guide the pollen tube towards the egg.
  • Antipodal cells are group of three cells present at the chalazal end. 
  • The two haploid polar nuclei of large central cell fuse to form diploid secondary nucleus or definitive nucleus, just prior to fertilization. 
  • This seven celled and eight nucleated structure is called an embryo sac. 
  • This method of embryo sac development from a single megaspore is described as monosporic development. 
  • In angiosperms, the development of female gametophyte is endosporous i.e. within the megaspore. 
  • Female gametophyte is colourless, endosporic and is concealed in the ovule enclosed by ovary. 

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