Four kinds of modified leaves make up a complete flower: carpels and stamens (primary reproductive structures) and petals and sepals (secondary structures). The carpel is the female reproductive structure. It has a stigma, where the pollen becomes attached and germinates; a style, through which the pollen tube grows; and an ovary with one or more ovules. The egg cell that will unite with the sperm cell, delivered by the pollen tube, forms in the ovule. The stamen is the male structure; its filament supports an anther, in which the pollen is formed. The often brightly colored petals are important in attracting pollinators, and the often-leaf like sepals enclose the bud before the flower opens. The many species of flowering plants are usually distinguished from one another by the way these four basic flower parts are modified, although closely related species within a genus may have quite similar flowers.
If a flower lacks any of the four basic parts, it is called incomplete. If it lacks one of the essential reproductive parts (stamens or carpels), it is called imperfect. Thus, flowers that have both stamens and carpels but lack petals or sepals are perfect incomplete flowers. Imperfect flowers can be male or female. If male and female flowers occur on the same plant, the plant is called monoecious; if male and female flowers are on separate plants, it is dioecious. Maize, or corn, is a monoecious plant, with its tassels (stamens) at the top and its ears (carpels) on the stem below. Cottonwoods are dioecious-- the male trees produce pollen, and female trees produce seeds.