Question

Describe the various types of placentation found in flowering plants.

Answer 1

Hint:Innermost floral whorl gynoecium contains one or more carpels. Carpels have three parts, stigma, style and ovary. Ovary may be single chambered or many chambered. It contains one or more ovules or megasporangia.

Complete answer:
Before discussing Placentation, we have to explain more about ovules and placenta.
Initially, ovules in the ovary are filled with a mass of cells called nucellus, which may be covered by one or more integuments. Ovules may be straight (orthotropous) in some cases and inverted (anatropous) in others. After fertilization, the ovary becomes the fruit and ovules become the seeds.
Each ovule is connected to the ovary by a short stalk called funicle. The point of attachment of funicles to ovule is called hilum. At the point of attachment of funicle to ovary, there is a cushion-like tissue called placenta from which ovules develop.

The arrangement of placenta and ovules within the ovary is called placentation. There are different kinds of placentation namely, marginal, parietal, axile, free central, basal and superficial placentation.
In marginal placentation, placenta bearing ovules develop along the junction of the two margins of the carpel. It occurs in monocarpellary or multicarpellary, apocarpous and unilocular ovaries. E.g.: Clitoria, Crotalaria, Pea.
In parietal placentation, placenta bearing ovules develop on the inner wall of the ovary, along the fused margins of carpels. It occurs in multicarpellary, syncarpous, unilocular or multilocular ovaries. The number of placentae is equal to the number of carpels. Eg: Cleome, Cucurbita, Mustard, Argemone etc.

In axile placentation, placenta bearing ovules develop from the central axis, where the partition wall or septa meet. It occurs in multicarpellary, syncarpous and multilocular ovaries. Eg: Hibiscus, Ixora, Tomato, Lemon.
In free central placentation, ovules are borne on a central column without any septa. It occurs in multicarpellary, syncarpous and unilocular ovaries. Eg: Primrose, Portulaca, Dianthus.

In basal placentation, placenta is seen at the base of the ovary. It occurs in monocarpellary polycarpellary and unilocular ovary with a single placenta at the base. Eg: Tridax, Marigold, Sunflower. In superficial placentation, Placenta are irregularly scattered along the inner surface of the partition wall. It occurs in multicarpellary, syncarpous and multilocular ovaries. Eg: Water lily.

Figure: Various types of placentation.

Additional information:
Single chambered or unilocular ovaries can be seen in Pea plants and many chambered or multilocular ovaries can be seen in Hibiscus.

Note:In carpels, ovary is a swollen basal part, style is the slender middle part and stigma is the flat terminal part. Stigma receives pollen grains during pollination and style holds the stigma in the correct position for this purpose. In many flowers, stigma secretes a sticky fluid which promotes pollen germination.