Placentation refers to the arrangement of ovules within the ovary, specifically where the placentae (the tissues to which ovules are attached and from which they receive nutrients) are located. Different placentation types arise from differences in: the number of carpels forming the ovary, whether the carpels are fused or free, and whether septa (partitions) divide the ovary into multiple locules (chambers). Understanding placentation is important for plant identification and classification, as it is a consistent taxonomic character within plant families.

Six main types of placentation are recognised: Marginal: monocarpellary, unilocular ovary; ovules on margin (ventral suture); examples: Pea, Bean, Cassia (Fabaceae/Leguminosae family). Axile: polycarpellary, multilocular ovary (septa present dividing ovary); ovules on central axis; examples: Tomato, China rose, Lemon, Lily, Tulip. Parietal: polycarpellary, unilocular ovary (septa absent or incomplete); ovules on ovary wall/parietal wall; examples: Argemone, Mustard, Cucumber, Papaya, Violets. Free central: polycarpellary, unilocular ovary; ovules on free central column/axis not connected to ovary wall; septa absent; examples: Primrose, Dianthus (carnation), Stellaria. Basal: single ovule attached at the base of a unilocular ovary; examples: Sunflower, Marigold (Asteraceae/Compositae). Superficial: polycarpellary, multilocular; ovules on inner surface of locule walls on all faces; example: Nymphaea (water lily).

Marginal (Pea, Pisum sativum): The familiar pea pod clearly illustrates marginal placentation — open a pea pod and the peas (seeds) are arranged in two alternating rows along the margin of the single carpel. Axile (Tomato, Lycopersicon esculentum): Cut a tomato transversely and see the multiple locules (usually 2-6) with the central placenta bearing ovules/seeds. The septa (walls) between locules all meet at the central axis. Parietal (Argemone mexicana, prickly poppy): The ovary is formed from multiple carpels that fuse to form a single chamber; placentae are on the inner walls, each bearing multiple ovules. In mustard, the parietal placentae are connected by a false septum (replum), creating an apparently bilocular ovary. Free central (Primrose, Primula): The ovary is unilocular, with a central column (axis) arising from the base that bears multiple ovules but does not connect to the ovary wall in the middle — the axis is "free."

Placentation type is a reliable taxonomic character used for plant family identification. Leguminosae (Fabaceae): marginal placentation is diagnostic — all members have monocarpellary pods with marginal placentation. Solanaceae (tomato, potato, brinjal, tobacco, chilli): axile placentation is characteristic. Brassicaceae (mustard, cabbage, radish): parietal placentation with a characteristic false septum (replum) creating a siliqua or silicula fruit type. Cucurbitaceae (cucumber, pumpkin, watermelon): parietal placentation in a distinctive pepo fruit. Primulaceae (primrose, cyclamen): free central placentation characteristic of the family. Asteraceae/Compositae (sunflower, marigold, chrysanthemum): basal placentation with single ovule, producing achene-type fruits. Knowing these family-placentation associations helps rapidly identify plant families from ovary cross-sections.