Bachelor’s Degree Programme (BSCG) – Plant Anatomy and Embryology Assig. Answer | BBYCT – 135 (Q-5)
5. a) List various types of ovules and describe them with outline diagrams.
b) Define apomixis and list its types. Discuss various causes of apomixis and its importance.
a) In Angiosperms, six different types of ovules can be formed. These are:
1. Orthotropous ovule: ‘The micropyle, chalaza and funicle lie in one straight line.
Example: family Polygonaceae, Piperaceae and ovule of most gymnosperms.
2. Anatropous (Inverted) ovule: The body of the ovule is completely inverted so that micropyle and hilum come to lie very close to each other. 82% of families of angiosperms contain this type of ovule.
Example: plants belonging to gamopetalae sub-class according to Bentham and Hooker classification.
3. Campylotropous: When the ovule is curved, the micropyle is directed towards chalaza. Chalaza is situated at right angles to funicle.
Example: Members of Cruciferae and Leguminosae family.
4. Amphibious or Transverse Ovule: The ovule curvature is more pronounced and embryo sac becomes horse-shoe shaped.
Example: Families Alismataceae, Butomaceae.
5. Hemianatropous Ovule: When nucellus and integuments lie more or less at right angles to the funicle.
Example: Families Ranunculaceae and Primulaceae.
6. Circinotropous ovule: The funicle is exceptionally long and forms a complete circle around the ovules, which is free from it except for a small area at the end of funicle. Example: Opuntia and other members of families Cactaceae and Plumbaginaceae.
b) Apomixis is a type of reproduction in which sexual organs of related structures take part but seeds are formed without union of gametes.
Types of Apomixis –
i) Recurrent Apomixis,
ii) Non-Recurrent Apomixis,
iii) Adventitious Embryony or Nucellar Embryony,
iv) Vegetative apomixis or bulbils
Causes: i)Can’t control accumulation of deleterious genetic mutations
ii) Usually restricted to narrow ecological niches Lack ability to adapt to changing
Importance: i) Assured reproduction in the absence of pollinators, such as in extreme environments
ii) Maternal energy not wasted in unfit offspring (cost of meiosis)
iii) Some apomictic plants (but not all) avoid the male energy cost of producing pollen