The carpel is the gross morphological part of the flower concerned with female sporogenesis and gametogenesis and is the homologue of the stamen. In its simplest form, the carpel is a leaflike organ composed of the ovary, style, and stigma, although various kinds of fusions between carpels have produced a complex organ in the modern flower. As is well known, along its margins the ovary encloses ovules, which become seeds in the mature fruit. In its role of sheltering the progenitors of seeds, the carpel determines the extent of physical and chemical influences that regulate the transformation of ovules into seeds. Tissues constituting the wall of the carpel serve as supporting structures when the ovary becomes the fruit.

The ovule is functionally a megasporangium because it defines the structural unit of the carpel in which sporogenesis and subsequent differentiation of the female gamete, or the egg, take place. In the context of the reproductive biology of angiosperms, the functional unit of the plant that produces the egg for fusion with the sperm is the megagametophyte or the female gametophyte. The megagametophyte is a highly reduced group of cells dependent upon the sporophytic plant for its nurture and nutrition; it is generally equated with the embryo sac.