Flower variations in structure
Like virtually all forms in nature, flowers display many variations in their structure. Most flowers have all four whorls—pistil, stamens, petals, and sepals. Botanists call these complete flowers. But some flowers are incomplete, meaning they lack one or more whorls. Incomplete flowers are most common in plants whose pollen is dispersed by the wind or water. Since these flowers do not need to attract pollinators, most have no petals, and some even lack sepals. Certain wind-pollinated flowers do have small sepals and petals that create eddies in the wind, directing pollen to swirl around and settle on the flower. In still other flowers, the petals and sepals are fused into structures called a floral tube.
Flowers that lack either stamens or a pistil are said to be imperfect. The petal-like rays on the edge of a sunflower, for example, are actually tiny, imperfect flowers that lack stamens. Imperfect flowers can still function in sexual reproduction. A flower that lacks a pistil but has stamens produces pollen, and a flower with a pistil but no stamens provides ovules and can develop into fruits and seeds. Flowers that have only stamens are termed staminate, and flowers that have only a pistil are called pistillate.
Although a single flower can be either staminate or pistillate, a plant species must have both to reproduce sexually. In some species with imperfect flowers, the staminate and pistillate flowers occur on the same plant. Such plants, known as monoecious species, include corn. The tassel at the top of the corn plant consists of hundreds of tiny staminate flowers, and the ears, which are located laterally on the stem, contain clusters of pistillate flowers. The silks of corn are very long styles leading to the ovaries, which, when ripe, form the kernels of corn. In dioecious species—such as date, willow, and hemp—staminate and pistillate flowers are found on different plants. A date tree, for example, will develop male or female flowers but not both. In dioecious species, at least two plants, one bearing staminate flowers and one bearing pistillate flowers, are needed for pollination and fertilization.
Other variations are found in the types of stems that support flowers. In some species, flowers are attached to only one main stem, called the peduncle. In others, flowers are attached to smaller stems, called pedicels, that branch from the peduncle. The peduncle and pedicels orient a flower so that its pollinator can reach it. In the morning glory, for example, pedicels hold the flowers in a horizontal position. This enables their hummingbird pollinators to feed since they do not crawl into the flower as other pollinators do, but hover near the flower and lick the nectar with their long tongues. Scientists assign specific terms to the different flower and stem arrangements to assist in the precise identification of a flower. A plant with just one flower at the tip of the peduncle—a tulip, for example—is termed solitary. In a spike, such as sage, flowers are attached to the sides of the peduncle.
Sometimes flowers are grouped together in a cluster called an inflorescence. In an indeterminate inflorescence, the lower flowers bloom first, and blooming proceeds over a period of days from the bottom to the top of the peduncle or pedicels. As long as light, water, temperature, and nutrients are favorable, the tip of the peduncle or pedicel continues to add new buds. There are several types of indeterminate inflorescences. These include the raceme, formed by a series of pedicels that emerge from the peduncle, as in snapdragons and lupines; and the panicle, in which the series of pedicels branches and rebranches, as in lilac.
In determinate inflorescences, called cymes, the peduncle is capped by a flower bud, which prevents the stem from elongating and adding more flowers. However, new flower buds appear on side pedicels that form below the central flower, and the flowers bloom from the top to the bottom of the pedicels. Flowers that bloom in cymes include chickweed and phlox.
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