. Explain the term Placentation

Best Answer

The ovules are attached on ovary walls on one or more cushion called placenta. The arrangement of ovule within ovary wall is known as placentation. It is of following types -

i. Marginal: Marginal placentation is found in unilocular ovary. The placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows. E.g., Leguminosae. 

ii. Parietal: This type of placentation is found in unilocular syncarpus ovary. In it the ovule develops on the innerwall of the ovary or on peripheral part. Ovary becomes bi or multilocular due to the formation a false septum. E.g., Cucurbita, Argemone, and Cruciferae family (Mustard). 

iii. Axile: It is found in multicarpellary syncarpous gynoecium. The fusion margin of the carpels grows inward and meet in the centre of the ovary. Thus an axis forms in the centre of  the ovary and ovary becomes multichambered. The ovules are borne at the central axis. Number of these chambers are equal to the number of carpels. E.g., Potato, China rose, Onion, Lemon, Orange, Tomato. 

iv. Free central: This type of placentation is found in syncarpous gynoecium. In it, the ovary is unilocular and the ovules are borne on the axis in the centre of the ovary. Septum are absent in ovary and placentation is axile in beginning. After sometime walls of chamber destroy and only ovulated central axis is left. E.g., Primrose, Dianthus (Caryophyllaceae) 

v. Superficial - This type of placentation is found in multicarpellary syncarpous gynoecium. The ovules are attached on the walls of locule. E.g., Nymphea (Water lily)

vi. Basal:  The ovary  is unilocular  and a  single  ovule  is  borne  at  the  base  of  ovary. E.g., Marigold, Sunflower (Asteraceae family), wheat.