Question

Discuss types of placentation.

Answer 1

When we are discussing the formation, position, and structure of the placenta, we are talking about placentation. Plants have several different types of placentation which essentially refers to the type of attachment of the ovules inside the ovaries.

Complete answer:
There are 6 main types of placentation. These are marginal, parietal, axile, free central, basal, and superficial. Some of the names are self-explanatory, but let us see each.

Marginal placentation is shown by the leguminosae and fabaceae families. In this kind, the ovules attach along the junction of the carpel margins, and are found in unilocular, monocarpellary ovaries.
When placentation is Axile, it means that there is a central column on which the placentae are arranged, and the ovules are formed by dividing partitions that extend out from this column. This is seen in the hibiscus plants.

Parietal placentation is found in plants like cucumber which have unilocular, multicarpellary ovaries which are syncarpous. The placentae are along the wall of the ovary.

The fourth kind is free-central placentation. As the name suggests, the central column is not connected to the wall of the ovary, and no locules are formed. Free-central type of placentation is found in plants of the dianthus and primrose families.

Next is basal placentation. In this the placenta is at the base of the ovary, and there is only a single ovule. Inflorescences like the sunflower and marigolds show basal placentation.

Finally, we have superficial placentation which is commonly found in the brassica and Argemone families. Here the ovules are arranged along the inner surface of the partition walls, and the ovaries are multilocular.

Note: The kind of placentation that a plant has with the kind of arrangement of the ovules will decide the arrangement of the seeds within the fruit. For instance in citrus fruits like the orange, we can see the partitions and the arrangement of the seeds in axile positions when the segments are cut into.