placenta, in botany, the surface of the carpel (highly modified leaf) to which the ovules (potential seeds) are attached. The placenta is usually located in a region corresponding somewhat to the margins of a leaf but is actually submarginal in position. The placentation, or arrangement of ovules within the ovary, is frequently of taxonomic value. Placentation is usually submarginal in a simple pistil (female sex organ). In a compound pistil, two or more carpels are used in various ways, placentation being parietal, with carpels united by their adjacent margins and the ovules disposed along the inner ovary walls; axile, with carpels folded inward and the ovules along the central axis of the ovary; free central, derived from the axile, with a central column bearing the ovules; basal, with ovules positioned on a low column at the base of the ovary; or laminar, with ovules scattered over the inner surfaces of carpels.