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.
Some flowers have
only one carpel, others have two or a few, and still others
have many. Several
carpels in a single flower
may be separate or fused together. If fused, they may be
joined only at the ovaries or along their entire length. The
ovary
may contain one to many ovules, and these may be arranged
in various
ways. Frequently, the ovaries are attached to the receptacle
(the end of the stem, or peduncle, that supports the flower
parts) at the same level as the other flower parts, in which
case the
ovary is said to be hypogynous. In some cases the other flower
parts are attached above the ovary, which is then said to
be epigynous. In the rose family and some of its relatives,
the
stamens, petals, and sepals are attached around the ring
of a cup with the ovaries at the bottom of the cup.
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.
