Flowers are the modified shoots bearing modified leaves that serve as the sexual reproductive organs of angiosperms. This strategy for reproduction has been so successful that angiosperms now dominate the plant world, and accordingly there are many variations on the basic structure of a flower.
Flowers are organs of sexual reproduction produced by the angiosperms (phylum Anthophyta), the largest phylum of photosynthetic organisms, with roughly 250,000 species. This large number represents a great diversity of flower types, but all flowers have some common structural elements.
Flower Parts
Flowers are modified shoots bearing modified leaves. In the typical flower, the modified leaves can be grouped into four sets based on appearance and function: sepals, petals, stamens, and pistils. The sepals and petals are lowermost on the shoot toward the sides of the flower. The stamens and pistils are at the tip of the shoot at the inside.
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While sepals and petals are easy to see, stamens and pistils are often visible only when the flower is closely examined. Two other important parts are the pedicel, a stalk on which flowers are frequently borne, and the top of the pedicel, called the receptacle, to which the other flower parts are typically attached.
Of the four main parts, the sepals are generally the most leaflike and generally are attached to the bottom of the receptacle. Sepals protect the immature flower during the bud stage.
Flowers typically have three to eight sepals, depending on the species. Collectively, the sepals in a flower are called the calyx. Above the sepals are the petals. Although flattened like the sepal, each petal is usually soft and colored—white, yellow, pink, blue, purple, orange, maroon, or even brown.
Petals attract insects, hummingbirds, bats, or other animals, aiding the reproductive process.Usually, the number of petals in a flower will be the same as the number of sepals. Collectively, the petals in a flower are called the corolla.
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| Flower structure |
The stamens, located inside the petals, are composed of a small anther (ball-shaped, egg-shaped, or tubular) and a threadlike filament connecting the anther to the rest of the flower. The anther, in turn, is composed of two or four tiny chambers, within which powdery pollen grains are produced and stored.
Each grain of pollen contains the immature sperm of the plant. Thus, the stamens function as the male part of the flower in sexual plant reproduction, and they may number from one to dozens. The term androecium refers collectively to all the stamens within a flower.
The pistils form the final set of parts. Each pistil is often shaped like a vase, although the shape varies. The ovary, the base of the pistil, is swollen and hollow.
The wall of the ovary, called the pericarp, is typically green, and the hollow space in the ovary is called the locule. Within the locule are one or more tiny globular ovules, each containing an egg nucleus and thus functioning as the female structure in sexual reproduction.
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In addition to the ovary, the pistil is typically composed of two or more parts: the style, a slender neck like structure above the ovary, and the stigma, a swollen area at the top of the style that traps pollen grains with minute hairs covered by a sticky, sugary film. While most flowers have only one pistil, many have several pistils, attached to the receptacle. The pistils within a flower are collectively called the gynoecium.
Functions of Flower Parts
Flowers and their parts function to achieve sexual reproduction, including pollination and seed formation. After pollination is finished, the flower begins the process of seed and finally fruit formation. During pollination, pollen grains are released from the anther and carried to the stigma, either by animals (such as insects, birds, and bats) or by wind.
Animals, attracted by the flower’s colors or aromas, visit flowers to obtain food—either the pollen itself or the nectar, a sugary liquid produced by small glands called nectaries at the base of the flower. The animal brushes up against the anthers, which deposit pollen on the animal’s body.
The animal transfers the pollen to the stigma of either the same flower (self-pollination) or a second flower (cross-pollination). During wind pollination, the anthers release their pollen, which is then borne by air currents. Some of the grains are deposited on a stigma of the same or another flower.
Each pollen grain germinates and produces a slender thread of protoplasm that grows downward through the style and into the ovary. This thread, the pollen tube, contains the sperm and grows toward an ovule, where it deposits its sperm.
The sperm then fuses with the egg, achieving fertilization as the first cell of the new generation is produced. The ovule matures to form a seed. At the same time, the surrounding ovary enlarges greatly, becoming a fruit as other parts of the flower recede and die off.
Because the stamens and pistils are intimately involved in reproduction, botanists refer to these as essential parts. The sepals and petals are termed nonessential parts, though in fact they remain important.
The sepals and petals are sometimes called the perianth because they are found on the periphery of the anthers. A complete flower is one that has all four sets of parts. A perfect flower is one that has both androecium and gynoecium and is thus bisexual.
Structural Variations
Many plant species produce flowers that deviate from the idealized format. Certain lilies, for example, do not have sepals and petals that are clearly distinguishable from each other. In magnolias and some water lilies, each flower produces perianth parts pollination.
Flowers with a well-developed corolla or a calyxmade up of petal-like sepals are attractive to animals and insects, which function to pollinate them. A petalous and naked flowers are wind-pollinated; they do not need to waste their energy making showy flower parts.
Some incomplete flowers lack either the androecium or the gynoecium. These imperfect flowers are unisexual and fall into two categories: staminate flowers are male flowers, having only stamens and no pistils; pistillate flowers are female, having only pistils and no stamens. Forced into cross-pollination, imperfect flowers benefit the plant by preventing some of the harm inherent in self-pollination.
In the idealized flower, the parts are free down to the receptacle. Many flowers, however, exhibit connation, in which similar parts are fused above the receptacle. The petals of the morning glory are fused to form a funnel-shaped corolla.
Carnations have connate sepals, forming a calyx tube. Many plants have pistils composed of individual fused segments, called carpels, while others, such as the mallow, have connate stamens, forming a stamen tube.
Other flowers show adnation,which involves the fusion of different parts.The stamens of phlox flowers are fused to the petals. The sepals, petals, and stamens of roses are all fused, forming a cupshaped structure called a hypanthium.
The presence of a hypanthium can be best observed in plum and cherry blossoms, whose individual sepals, petals, and stamens are attached to the rim of the hypanthium.
Finally, many flowers have a hypanthium that is fused to the wall of the ovary. The result is that the sepals, petals, and stamens emerge from the top of the ovary, a good example being the apple blossom. Flowers of the latter category are said to have an inferior ovary, whereas the others have superior ovaries.
Flowers’ corollas also vary: In flowers with a regular corolla, such as buttercups, lilies, and roses, all the petals are equal in size and shape, giving the flower a star shape.
In flowers with irregular corollas, such as the snapdragon, pea, and orchid, one or more of the petals are unequal. Some irregular flowers, such as the violet, touch-me-not, and columbine, have a rounded, cone-shaped, or pointed extension of the corolla called a spur, which serves to store nectar.
Although not technically floral structures, color, shape, and inflorescences (the loose or dense clusters in which flowers appear on a plant) are other ways in which flowers differ, important because they allow certain pollinators to enter but exclude others.
Bowl-shaped flowers are visited by a variety of insects, such as beetles and bees. Irregular flowers are typically pollinated by honeybees and bumblebees, and in some cases the insects fit the flower like a key fits a lock. Flowers with long spurs are pollinated by long-tongued insects such as moths.
Color, determined by special molecules called pigments that occur within the cells of the plant, attracts different pollinators as well: red flowers are pollinated by birds, specifically hummingbirds and butterflies. White flowers are often open at night and are visited by moths. One group of plants have brown or maroon flowers and an odor of rotting flesh.
These “carrion flowers” are pollinated by an array of insects, particularly beetles and flies. Interestingly, the way that humans perceive color is often different from the way that other animals perceive color. For example, xanthophylls reflect not only yellow but also a deep violet that bees can perceive but that humans cannot.
