About Placentation

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About Placentation

The mode of arrangement of ovules or ovule bearing cushions on the inner wall of ovary is called placentation. As a rule, the origin of an ovule or a group of ovules determines the position of the placenta. It is of following types:

Marginal: In this type, a placenta develops on the inner junction of margins of carpel of a monocarpellary, unilocular and superior ovary, e.g., members of family Leguminosae.

Axile: In this type, placentae develop in the axils of carpels of polycarpellary, syncarpous, multilocular and superior ovary, e.g., members of families Solanaceae, Malvaceae, Labiatae, Liliaceae, etc.

Parietal: In this type, placentae develop on the inner junctions or margins of carpels of bicarpellary or tricarpellary, syncarpous, unilocular and superior or inferior ovary. But later on due to formation of replum unilocular ovary becomes falsely bilocular (e.g., members of family Brassicaceae) and trilocular (e.g., members of family Cucurbitaceae).

Central: In this type, placentae develop at the centre of carpels of polycarpellary, syncarpous, multilocular and superior ovary, e.g.. Citrus.

Free central: It is similar to central placentation but placentae remain free from inner wall of ovary. It may arise in two ways:

By the dissolution of the margins of carpels (true walls) of a polycarpellary, syncarpous, multilocular and superior ovary so that it becomes falsely unilocular, e.g., members of family Caryophyllaceae.

By the elongation and penetration of thalamus into ovary upto its centre and then bearing placentae, e.g., members of family Primulaceae.

Basal: In this type placentae develop at the base of carpels of a bicarpellary, syncarpous, unilocular and inferior ovary, e.g., members of family Asteraceae. Basal placentation is characterized by the presence of single ovule.

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About Placentation

About Placentation

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