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Flowers arise from the apical portion of a stem in a region called the receptacle. They may be borne directly on a main stem axis or rachis (sessile) or on a slender stalk or stem called a pedicel. They usually consist of four whorls of parts that develop in the following series, from the outer whorl to the inner: sepals, petals, stamens, and carpels. These whorls of parts may develop in discrete cycles, or be more or less continuous and spirally arranged. If all four whorls of floral parts are present, the flower is complete; if one or more whorls is missing, the flower is incomplete. The symmetry of flowers usually can be defined as radial (actinomorphic) or bilateral (zygomorphic). In a few groups the flowers are termed irregular (asymmetric) when the petals or sepals are dissimilar in form or orientation. Floral diversity is achieved by numerous variation patterns resulting from changes in 1) symmetry, 2) numbers of each floral part, and 3) degree of fusion of the parts. If like parts are fused, they are connate; if unlike parts are fused, they are adnate. Prefixes such as gamo-, sym-, and syn- denote connation, as in gamopetaly and sympetaly (fusion of petals to each other) and syncarpy (fusion of carpels). The prefix epi- refers to adnation, as in epipetalous stamens (stamens fused to the petals). In floral morphology, the prefixes poly- and apo- represent the lack of fusion, as in polypetaly (separate petals) and apocarpy (separate carpels). The two outer whorls of the flower, sepals and petals, are sometimes called accessory in that they do not produce the sex cells so are not directly involved in the sexual life cycle. These two parts are known collectively as the perianth. The two inner whorls, stamens and carpels, are the essential parts of the flower because they produce the sex cells or gametes that are part of sexual reproduction. When stamens and carpels are both present, the flower is perfect or bisexual; if either of these two is missing, the flower is imperfect or unisexual. For example, a flower that lacks only stamens is imperfect and incomplete, whereas a flower that lacks only petals is perfect, but incomplete. The sepals usually resemble small, greenish leaves and comprise the outermost whorl of the flower. The sepals are known collectively as the calyx and appear to serve a protective function for the inner parts of the flower.