The sole function of the flower, which is generally
the showiest part of the plant, is sexual reproduction. Its
attractiveness and fragrance have not evolved to please man but to
ensure the continuance of the plant species. Fragrance and color
are devices to attract pollinators that play an important role in
the reproductive process.
Parts of the Flower
As the reproductive part of the plant the flower contains the
male pollen and/or the female ovule plus accessory parts such as
petals, sepals, and nectar glands.
The pistil is the female part of the plant. It
is generally shaped like a bowling pin and located in the center
of the flower. It consists of the stigma, style, and ovary. The
stigma is located at the top, and is connected to the ovary by the
style. The ovary contains the eggs which reside in the ovules.
After the egg is fertilized the ovule develops into a seed.
The stamen is the male reproductive organ. It
consists of a pollen sac (anther) and a long supporting filament.
This filament holds the anther in position so the pollen it
contains may be disbursed by wind or carried to the stigma by
insects, birds or bats.
Sepals are small green, leaflike structures on
the base of the flower which protect the flower bud. The sepals
collectively are called the calyx.
Petals are highly colored portions of the flower.
They may contain perfume as well as nectar glands. The petals
collectively are called the corolla. The number of petals on a
flower is often used in the identification of plant families and
genera. Flowers of dicots typically have sepals and/or petals
in multiples of four or five. Monocots typically have these floral
parts in multiples of three.
Types of Flowers
If a flower has a stamen, pistils, petals, and sepals, it is
called a complete flower. If one of these parts is
missing, the flower is designated incomplete. If a flower
contains functional stamens and pistils, it is called a perfect
flower. (Stamen and pistils are considered the essential parts of
a flower.) If either of the essential parts is lacking, the flower
Pistillate (female) flowers are those which
possess a functional pistil(s) but lack stamens. Staminate
(male) flowers contain stamens but no pistils. Because
cross-fertilization combines different genetic material and
produces stronger seed, cross-pollinated plants are usually more
successful than self-pollinated plants. Consequently, more plants
reproduce by cross-pollination than self-pollination.
As previously mentioned, there are plants which bear
only male flowers (staminate plants) or bear only female flowers
(pistillate plants). Species in which the sexes are separated into
staminate and pistillate plants are called dioecious. Most
holly trees and pistachio trees are dioecious; therefore, to
obtain berries, it is necessary to have female and male trees.
Monoecious plants are those which have separate male and
female flowers on the same plant. Corn plants and pecan trees are
examples. Some plants bear only male flowers at the beginning of
the growing season, but later develop flowers of both sexes;
examples are cucumbers and squash.
How Seeds Form
Pollination is the transfer of pollen from an anther to a stigma.
This may occur by wind or by pollinators. Wind-pollinated flowers
lack showy floral parts and nectar since they don't need to
attract a pollinator. Flowers are brightly colored or patterned
and contain a fragrance or nectar when they must attract insects,
animals, or birds. In the process of searching for nectar these
pollinators will transfer pollen from flower to flower.
The stigma contains a chemical which stimulates the
pollen, causing it to grow a long tube down the inside of the
style to the ovules inside the ovary. The sperm is released by the
pollen grain and fertilization typically occurs. Fertilization is
the union of the male sperm nucleus (from the pollen grain) and
the female egg (in the ovule). If fertilization is successful, the
ovule will develop into a seed.
Types of Inflorescences
Some plants bear only one flower per stem and are called solitary
flowers. Other plants produce an inflorescence, a term
which refers to a cluster of flowers and how they are arranged on
a floral stem. Most inflorescences may be classified into two
groups, racemes and cymes.
In the racemose group, the florets, which are
individual flowers in an inflorescence, bloom from the bottom of
the stem and progress toward the top. Some examples of racemose
inflorescence include spike, raceme, corymb, umbel, and head. A
spike is an inflorescence in which many stemless florets are
attached to an elongated flower stem or peduncle, an example being
gladiolus. A raceme is similar to a spike except the florets are
borne on small stems attached to the peduncle. An example of a
raceme inflorescence is the snapdragon. A corymb is made up of
florets whose stalks and pedicels are arranged at random along the
peduncle in such a way that the florets create a flat, round top.
Yarrow has a corymb inflorescence. An umbel is similar except that
the pedicels all arise from one point on the peduncle. Dill has an
umbel inflorescence. A head or composite inflorescence is made up
of numerous stemless florets which is characteristic of daisy
In the cyme group, the top floret opens first
and blooms downward along the peduncle. A dischasium cyme has
florets opposite each other along the peduncle. Babys breath
inflorescence is an example. A helicoid cyme is one in which the
lower florets are all on the same side of the peduncle, examples
being freesia and statice inflorescences. A scorpioid cyme is one
in which the florets are alternate to each other along the
peduncle. Examples are tomato and potato inflorescences.