Placentae (placentas) are swellings of tissue on which the ovules are attached.
Placentae can be scarcely noticeable (just the part of the ovary to which ovules are attached) or greatly enlarged. In watermelon, Citrullus lanatus, and many other members of the family Cucurbitaceae the flesh of the fruit is largely composed of massively enlarged placental tissue. [The eating of watermelon is therefore placenta-eating, placentophagy].
- the location of placentae (placentation) is an important taxonomic character, and tends to be characteristic of plant families of parts of plant families
- Simple pistils often have marginal placentation. The placenta runs along the margins of the carpel. There is one locule.
- Amborella (most "basal" of the "basal angiosperms") has a single ovule borne medially at the top of the adaxial side of the sac.
- Compound pistils can have several distinctive types of placentation: axile, parietal, free central, basal.
Types of placentation:
- Axile: usually one locule per carpel; one placenta per carpel, located centrally
- Parietal: has the placentas on the inner wall of the ovary. The carpels are fused to each other along their margins, but individual carpels are not closed, so there is only one locule. In the Brassicaceae there are two “false” locules because the placentas have grown together to form the replum.
- Free central: one locule with the placenta on a central stalk arising from the locule base
- Basal: one locule with the placenta on the floor of the locule