Friday, March 28, 2008

The Flower


"Flower" is a general term that usually refers to four basic parts: sepals (collectively called the calyx), petals (collectively called the corolla), stamens (collectively called the androecium), and pistils (collectively called the gynoecium). These parts are attached to the tip of a specialized branch called a peduncle. The tip of the peduncle is called a receptacle. (For an illustration of the parts of a flower, see Plant Structures) The remarkable thing about all of these parts is that they are thought to be derived from leaves. Think of parts of a flower attached to a receptacle as if they were leaves attached to a twig, except that, unlike leaves, the different parts of a flower are separated by very short distances. These distances are so short, in fact, that when you look down at an open flower, the components (for example, the petals) of any one part (in this case the corolla) appear to be connected to the "twig" (the receptacle) at the same point. In a typical flower, the calyx, or outermost part of the flower, usually most closely resembles the leaves of the plant on which they occur. In fact, the calyx is frequently green and produces food from sunlight via photosynthesis, just as the leaves do. The calyx protects the developing flower bud in many plants. The corolla, the frequently colored part of the flower, is located just inside the calyx. Showy corollas can attract pollinators such as insects or birds. Wind-pollinated plants frequently have minute corollas or lack corollas entirely. The androecium is just inside the corolla. The individual stamens, the male reproductive organs that make up the androecium, are composed of an anther (usually a swollen structure) and a filament (a long stalk supporting the anther). With magnification, you can see that the anther is actually made up of sacs. It is in these sacs that pollen grains are produced. Pollen grains are capable of producing sperm, the male sex cells. In the center of the flower (actually at the top of the receptacle) is the gynoecium. Each individual pistil, or female reproductive organ, of the gynoecium typically looks like a flower vase and has three recognizable parts: a stigma, which will receive pollen grains, at the top of the vase; a style, the "neck-like" portion of the vase, which supports and typically elevates the stigma; and an ovary, the swollen part at the bottom. Eggs, the female sex cells, are produced in ovules, which are inside the ovary. It is important to note that the above is a description of a "typical" flower. In many species, some flower parts may be fused with others, extremely reduced, or completely absent. One example of a flower "missing" parts is the Anemone, or windflower, which lacks petals; its sepals have taken on a petal-like appearance (the sepals are colored, not green as in the typical flower). "Perfect" or "imperfect" and "complete" or "incomplete" are terms botanists use to describe the various types of variation in flowers. Perfect flowers possess male (stamens) and female (pistil) parts, while imperfect flowers lack one of these parts. Complete flowers have sepals, petals, stamens, and pistils, while incomplete flowers lack one of these parts. The typical flower described above is both perfect and complete, while anemones are perfect and incomplete. Flowers can be perfect and complete, perfect and incomplete, or imperfect and incomplete, but never imperfect and complete.

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