Flowers

• Structures of a flower (source: Noah Elhardt via Wikimedia Commons).
  
  The structures we call flowers are made up of external and internal parts. The perianth, or outer part of a flower, is composed of the calyx (a ring of sepals--usually green, leaf-like structures that protect the inner parts of the flower) and the corolla, or "crown," of petals (the white or brightly-colored parts of the flower that are intended to attract pollinators). Inside the petals are the internal male and female reproductive parts of a flower: the stamens and the pistils.
  
  

Stamens

• The male reproductive structures are called stamens, and are composed of anthers and filaments. Anthers are the swollen parts at the end of the filaments, or stalks, that support them, and they contain the pollen that is used in reproduction. Attracted by the petals, pollinators visit flowers and when they land, the pollen grains attached to the anthers rub off on their feet and other body structures, to be carried away to the next flower, thus "pollinating" it. Male reproductive structures are often easily recognizable in the middle of flowers as the bright yellow stalks sticking up.
  
  

Pistils

• The female parts of a flower are usually less showy and are called the pistils. Pistils are composed of a bulbous ovary at the base, which tapers up into a slender style, and then opens up into the stigma. The stigma is the site at which pollinators such as bees deposit pollen from male flower parts, thus fertilizing the ovary and causing a fruit to develop.
  
  

Parts

• Flowers are sometimes classified based upon the presence or absence of their four basic parts: calyx, corolla, stamens and pistils. "Complete" flowers have all of the parts listed above; "incomplete" flowers are missing at least one of them. If a flower has both stamens and pistils, it is called "perfect"; if it lacks one or the other, it is "imperfect." "Apetalous" flowers lack petals and "naked" flowers have neither petals nor sepals. "Staminate" flowers have only stamens and "pistillate" flowers only pistils.
  
  

Symmetry

• Flowers are also categorized according to the symmetry--or lack thereof--of their parts. "Regular" (also called "actinomorphic") flowers can be cut into four parts that all look the same. "Irregular" ("zygomorphic") flowers are not arranged symmetrically. There are three types of irregular flowers: "Papilionaceous" flowers have five petals of three different types (example: sweet pea); "labiate" flowers have petals that are fused into tubes divided into a three-plus-two arrangement (example: snapdragon); "orchidaceous" flowers have three upper sepals and three fused petals forming a "lip" (example: orchid).