The arrangement of ovules within the ovary is known as placentation. Placenta is the point of attachment of ovules or future seed to the ovary.
Types of placentation;
Marginal: gynoecium is monocarpellary and unilocular; placenta bearing the ovules in a row develops along the junction of the two margins of the folded carpel, inner to the ovary wall e.g. pea, gram.
Parietal: ovary is compound formed by the fusion of two or more carpels with their adjacent margins. Carpels are united to form only one chamber. Placentae are seen on the inner surface of the ovary, at the junctions of the carpels, sometimes the unilocular ovary is found to be divided by the development of a false septum called as replum e.g. mustard, cucumber.
Axile: gynoecium is polycarpellary syncarpous, ovary is many chambered and the number of chambers corresponds to the number of carpels. The walls of the carpels in the centre of the ovary are united to form an axis, which bears the placentae e.g. China rose, tomato, Bhindi.
Free central: similar to axile placentation but ovary is one chambered, gynoecium is syncarpous and polycarpellary but unilocular as septae are absent. In the central part the placenta bears many ovules e.g. Dianthus, Primula.
Basal: The ovary is unilocular and the placentae bearing a single ovule develop at the base of the ovary e.g. sunflower.
Superficial: gynoecium is polycarpellary syncarpous and multilocular as in axile placentation in which entire inner walls of chambers are lined with placental tissue so that ovules develop all around e.g. water lily (Nymphaea).