Structure Of Flowers

Regardless of form, all flowers share certain basic features. A typical flower develops several different parts, each with its own function (Fig. 8.5). Each flower, which begins as an embryonic primordium that develops into a bud (discussed in Chapter 6), occurs as a specialized branch at the tip of a stalk called a peduncle, which may in some instances have branch-lets of smaller stalks called pedicels. A peduncle or pedicel swells at its tip into a small pad known as a receptacle. The other parts of the flower, some of which are in whorls, are attached to the receptacle (a whorl consists of three or more plant parts—such as leaves—encircling another plant part— such as a twig—at the same point on an axis).

The outermost whorl typically consists of three to five small, usually green, somewhat leaflike sepals. The sepals of a flower, which are collectively referred to as the calyx, may, in some flowers, be fused together. In many species, the calyx protects the flower while it is in the bud.

The next whorl of flower parts consists of three to many petals; the petals collectively are known as the corolla. Showy corollas attract pollinators, such as bees, moths, or birds. Bees, which can see in ultraviolet light, detect in some corollas special markings invisible to humans. The petals are distinct separate units in peach flowers, but in some flowers (e.g., petunias), the petals are fused together into a single, flared, trumpetlike sheet of tissue (Fig. 8.6). The corolla may not be showy or conspicuous in many tree and weed species and is often missing altogether or highly modified in wind-pollinated plants such as

Petunias Attract Bees
Figure 8.2 A Rafflesia flower. These flowers may be as much as 1 meter (3 feet 3 inches) in diameter. (Courtesy Charles H. Lamoureux)

grasses. The calyx and the corolla together are referred to as the perianth. Bracts are specialized leaves that may be as colorful as petals and can attract pollinators the way petals do; they are discussed in Chapter 7.

Several to many stamens are attached to the receptacle around the base of the often greenish pistil in the center of the flower. Each stamen consists of a semirigid but otherwise usually slender filament with a sac called an anther at the top. The development and dissemination of pollen grains in anthers is described in Chapter 23. In most flowers, the pollen is released through lengthwise slits that develop on the anthers, but in members of the Heath Family (Ericaceae) and those of a few other groups, the pollen is released through anther pores.

The pistil, which often is shaped like a tiny vase that is closed at the top, consists of three regions that merge with one another. At the top is the stigma, which is connected by a slender, stalklike style to the swollen base called the ovary. The stigma may be little more than a point, a slight swelling, or may consist of up to several divergent arms or branches. The ovary later develops into a fruit.

There is evidence that ovaries first developed when the margins of leaves bearing ovules rolled inward. Such ovule-bearing leaves were called carpels. In some instances, two or more carpels eventually fused together, and many ovaries are now compound, consisting of two to several united carpels. The number of carpels present is often reflected by the number of lobes or divisions of the stigma. Each segment of a mature ovary, such as a tomato or orange, for example, represents a carpel. Carpels are discussed further in Chapter 23. Some texts refer to pistils as carpels, but since a pistil can consist of one to several carpels, such references are unfortunate and unnecessarily confusing.

The ovary is said to be superior if the calyx and corolla are attached to the receptacle at the base of the ovary, as in pea and primrose flowers. In other instances, the ovary becomes inferior when the receptacle grows up around it so that the calyx and corolla appear to be attached at the top, as in cactus and carrot flowers. A cavity containing one or more egg-shaped ovules lies within the ovary; ovules are attached to the wall of the cavity by means of short stalks. An ovule, the development of which takes place after fertilization has occurred, eventually becomes a seed. Details of fertilization and ovule development are given in Chapter 23.

Peach flowers are produced singly, each on its own peduncle, but many other flowers such as lilac, grape, and bridal wreath are produced in inflorescences. Inflorescences are groups of several to hundreds of flowers that may all

Chapter 8

Chapter 8

Ovule Shaped Plant

Figure 8.4 Spanish moss (Tillandsia). Spanish moss is a non-parasitic flowering plant that goes through its life cycle suspended from other plants or objects such as wires. This plant should not be confused with lichens, which consists of an alga and a fungus (see Chapter 18), and which also may hang suspended from other objects.

Figure 8.3 Male catkins of an alder, (Alnus). Each catkin consists of numerous tiny, inconspicuous, wind-pollinated flowers that have no petals.

Figure 8.4 Spanish moss (Tillandsia). Spanish moss is a non-parasitic flowering plant that goes through its life cycle suspended from other plants or objects such as wires. This plant should not be confused with lichens, which consists of an alga and a fungus (see Chapter 18), and which also may hang suspended from other objects.

open at the same time, or they may follow an orderly progression to maturation (Fig. 8.7). The single peduncle of an inflorescence has many little stalks called pedicels attached to it—one pedicel for each flower.

A discussion of primitive and specialized flowers, along with their evolutionary development and ecological adaptations, is given in Chapter 23.

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