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 is imperfect.
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 inflorescence.
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. Baby’s 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.
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