Flower

Every flower is a terminal branch consisting of a modified stem, the floral axis, or receptacle. The floral axis bears one to four types of specialized appendages, or modified leaves, usually arranged in whorls in the more advanced flowers and spirally arranged in the more primitive ones. In a typical flower the outermost whorl, the calyx, consists of a number of sepals that protect the flower bud before it blooms. The next whorl on the floral receptacle, the corolla, is composed of a number of petals, often bearing nectar-producing glands that aid in attracting pollinators (see Pollination). The next whorl, the androecium, consists of a number of stamens that produce in anthers the pollen necessary for reproduction. Two whorls of stamens may be present. The next, or innermost, whorl of the flower, called the gynoecium, consists of several carpels frequently fused to form a pistil. Each carpel contains at least one placenta to which are attached ovules, or immature seeds. The calyx and corolla are collectively known as the perianth.

Flowering plants are divided into two major classes. They are Dicotyledons and Monocotyledons. In the Dicotyledons, floral organs in multiples of five or four predominate and in the Monocotyledons, multiples of three are usual.

Most angiosperm species bear flowers that deviate from the norm described above. Flowers that bear sepals, petals, stamens, and carpels are termed complete; a flower lacking any of these whorls is called incomplete. If the parts involved in reproduction—the stamens or pistils—are lacking, the flower is said to be imperfect. It is a perfect flower if both pistils and stamens are present. If only pistils are present the flower is said to be pistillate; with stamens only, staminate. Typical flowers are bisexual. When staminate and pistillate flowers occur on one plant, it is said to be monoecious; when they occur on different plants, dioecious.

In many flowers, sepals and petals are uniform in size and arranged in a star-shaped, or radially symmetrical, form. Bilaterally symmetrical flowers, however, have petals that differ in size or shape. The five petals of the Sweet Pea, for example, include a large, showy banner, or standard petal; two smaller, wing-like petals at the side of the flower; and between them the keel, two petals that encase the pistils and stamens. These petals are united along their edges.

Floral parts vary in their relative positioning. In a hypogynous flower the sepals form the lowest whorl, followed by successively higher whorls of petals, stamens, and pistils. In a perigynous flower, there is a floral cup that surrounds the gynoecium, with the other floral parts attached to the rim of the cup. In some cases the floral cup is the result of the fusion of the basal portions of the other floral parts; in other cases it is the result of an upward extension of the receptacle. In an epigynous flower, the floral cup is fused to the gynoecium, and the other floral parts are on top of the ovary, as in apple flowers.

Flowers that have numerous spirally arranged parts separately attached to their floral axes appeared earlier in the evolution of angiosperms. Flowers that vary from this condition are more derived. Thus, whorling, reduction of parts, fusion of parts, loss of parts, and bilateral symmetry indicate modification. The flower in possession of all or any one of these characteristics is more derived. If only one characteristic is present, the flower is considered derived for that characteristic alone. Buttercups and magnolias are among the oldest plants on Earth in terms of resemblance to fossil ancestry, while it is believed that the earliest flowering plants were most closely related genetically to Amborella, a small, cream-coloured flower from New Caledonia; Nymphaea, or water lilies; and Austrobaileya, a plant native to Australia. Snapdragons, mints, composite flowers, and orchids are among the most advanced—that is, more recently evolved.

The composite is a special case. The flower of the composite (a Daisy, for example) is not a flower at all but a mass of many small flowers or florets called a head, or capitulum. The petals of the daisy are not single petals; they are small, bilaterally symmetrical florets that are inserted at the rim of the head. The centre of the daisy is composed of complete, perfect, radially symmetrical florets—from a few to many—with five fused petals shaped to form a tube. See also Inflorescence.

Two types of pigment are responsible for the coloration of flowers: fat-soluble pigments contained in chromoplasts and water-soluble pigments contained within the vacuoles of the epidermal cells of the petals. Most blues and purples seen in flowers are due to vacuolar pigments known as anthocyanins. These change colour depending on the degree of acidity or basicity, as well as on the type of anthocyanin pigment present—if the vacuolar solution is basic, they are blue; if neutral, purple or violet; and if acid, red. Reds may also be due to the presence of chromoplast pigments. Yellows are often due to the presence of flavones, as in the primrose. A white appearance in a petal is caused by numerous minute air pockets between its cells.

The fragrance of flowers is caused by minute quantities of volatile oils formed by the alteration of essential oils in petals. Natural perfumes are made from such flowers as hyacinth, heliotrope, mimosa, jasmine, orange, Rose, and Violet, among others. The various fragrances attract pollinators to the flower. Some flowers give off putrid odours, again to attract their pollinators, usually the housefly or a fly relative. Such flowers are called carrion flowers and usually smell like rotting flesh. Carrion flowers are not limited to any one family or order among the Flowering Plants.