Placentation

Placentation, the arrangement of the ovules on the soft spongy tissue known as the placentas. In a few cases a single ovule, or a placenta bearing several ovules, appears to be a direct prolongation of the floral axis, independent of the carpellary leaves which may form an ovary round it. This is termed axial placentation. It occurs in the solitary gymnospermous ovule of the yew (q.v.); in the similarly solitary terminal ovules of the reed-mace (Typha), the rhubarb tribe (Polygonaceae), and the peppers (Piperaceae); and in that in the inferior ovary of the walnut tribe (Juglandeae). In the Composite (q.v.) one ovule arises from the base of the inferior ovary; but it is lateral to the axis, the apex of which is visible beside it, so that the ovule is homologous to a leaf, as also in the gymnospermous Ginkgo and in the Primrose family, where the placenta bears several lateral ovules. In a few cases, as in water-lilies, poppies, etc., the ovules are produced all over the inner surface of the carpellary leaves. They are then termed superficial, and are generally somewhat rudimentary in structure, being probably homologous to trichomes. In the majority of flowering-plants the placentation is marginal, the ovules corresponding to lobes or leaflets of the carpellary leaf. In most one-chambered ovaries the margins of the carpellary leaves form the spongy placentas and bear the ovules, often in double rows, each row belonging to one leaf-margin. This is termed parietal placentation. In Cruciferse (q.v.) we have the exceptional case of parietal placentation in a two-chambered ovary, the margins of the two carpellary leaves splitting, and one half growing inwards so as to form the partition or repluin, while the other half bears the ovules. Many-chambered ovaries are so mainly from the infolding of the margins of carpellary leaves which form the septa or dissepiments between the chambers (loculi). In poppies and cucumbers this infolding is only partial; but in lilies, Iris, etc., these septa unite to form a central placenta which has been termed axile. In the pinks and other Caryophyllacese the septa are only distinguishable at the base of the ovarian cavity in the young stage, and this placentation, like that of the primrose family, has been described as free central.