Part – 6: Female Reproductive Structures
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This is the Part 6 of the Lecture Notes Series in Cycas. Here we will continue the discussion on Sexual Reproduction in Cycas. In the previous post, we discussed the Male Reproductive Structures of Cycas. Here we will see the structure and organization of Female Reproductive Structures. The female reproductive system of Cycas consists of Megasporophyll, Megasporangium aka Ovule, and Megaspore ask Embryo-sac.
Cycas Female Cone
♣ NO definite female cone or strobilus is produced in Cycas (no cone axis).
♣ Female strobilus in Cycas is an aggregation of spirally arranged megasporophylls.
♣ The megasporophylls of Cycas resemble foliage leaves.
♣ Megasporophyll of Cycas is similar to the seed-bearing fronts (leaves) of extinct seed ferns- Pteridopsermales or Progymnosperms.
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Megasporophyll of Cycas
♣ Megasporophylls are leafy with thick rachis and they contain megasporangia.
♣ The megasporangia are better-called OVULES.
♣ The ovules developed on the margins of megasporophyll on both sides.
♣ The number of ovules per sporophyll varies (one to many).
Ovule of Cycas
♣ Cycas ovule is simple, sessile and Orthotropous
♣ Orthotropous Ovule: A condition of having the ovule straight and upright with the micropyle at the apex. (a primitive type of ovule).
Ovules are ovoid or spherical shaped.
♣ Cycas ovule is with single integument which is differentiated into three layers-Outer and inner fleshy, middle stony layers.
♣ Ovules in Cycas are well provided with vascular supply.
♣ The integument surrounds a mass of tissue called the nucellus.
Nucellus
♣ Nucellus: The central and chief part of a plant ovule that consists of a mass of parenchymatous cells and also encloses the female gametophyte.
♣ The nucellus remains fused with the inner integument of the ovule except at the apex.
♣ At the apex, the integument forms a long micropyle.
♣ Micropyle: a small opening in the surface of an ovule.
♣ In the apical portion of the nucellus forms a nucellar beak.
♣ The nucellar beak force grows into the micropyle.
♣ In the micropylar region, a few cells of the nucellus disintegrate to form a chamber called the Pollen Chamber.
♣ A single conspicuous cell of the nucellus (deeply situated) forms the megaspore mother cell.
♣ Megaspore mother cell is diploid and represents the last stage of sporophytic generation.
Formation of Megaspore in Cycas
♣ In the beginning, all the cells of the nucellus are similar.
♣ Then, a cell deeply placed cell in the nucellus becomes prominent and acts as the megaspore mother cell.
♣ Megaspore mother cell undergo meiosis to form four cells (tetrad).
♣ These four cells are arranged linearly- one above the other (linear tetrad).
♣ The upper three megaspores degenerate and they have a nutritive function.
♣ The basal cell in the tetrad is the functional megaspore.
♣ Megaspore represents the first stage of female gametophytic generation.
♣ The functional megaspore enlarges.
♣ The functional megaspore of Gymnosperms is better called as Embryo-sac.
In the next post, we will see the formation of the Male and Female Gametophytic Generation in Cycas.
<<< Cyas Part – 5 —- Part – 7 >>>
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