Flower
Virtually everywhere on earth flowers blossom on plants and trees. They are essential for reproduction of vegetation, since they develop the seeds for further growth, and flowering plants are the nutrient source for almost all species of animals. Humans consume grains, fruits and vegetables, and the animals many people use for food, such as cattle, hogs and sheep, also live on flowering plants.
Any plant that produces some sort of flower, even a small colorless one, is a flowering plant. Thus, grasses, roses, apple trees, lilacs and oaks are all flowering plants. At least 200,000 kinds of flowers have been classified, from the microscopic water blossom to the three-foot-wide tropical plant called the Giant Rafflesia. A blossom's shape may be long and thin, such as grass, or flowers that resemble stars, balloons or even insects. Flowering plants also may have pleasant fragrances to attract the birds and insects that help to fertilize the plant, or they may have unpleasant odors to repel enemies. These and other variations have emerged over millions of years, as plants adapt to their environment.
Despite various shapes, sizes, and smells, each flowering plant includes some or all of the following parts: roots, stems, leaves, and flowers. The roots are underground and serve as anchors or support structures, and absorb water and dissolved minerals which nourish the plant. Stems may be partially underground, such as bulbs of onions or tulips or tubers of potatoes, but most of the stems grow upright above the earth. They support the rest of the plant and convey the food and water up and down the plant. Leaves spread out and absorb sunlight, which acts on their green chlorophyll to produce sugar and starch, which feed the plant. Leaves may be simple or compound. Simple leaves contain a single undivided blade, while compound leaves are divided into leaflets.
Flowers are the reproductive parts of the plant. In the center of the flower is the pistil, which produces the seeds. At the flower's base is the ovary, where the ovules that will grow into seeds after fertilization are stored. Above the ovary is the style, a stalk that grows to the right height for the plant to receive pollen, or the fertilizing substance. Five arms at the style's top form the stigma, which open when the plant is ready to be fertilized.
The pollen is produced in the stamens, which encircle the pistil. A thin stalk called the filament holds up the anther, the small sac that comprises the top part of the stamen. Inside the anther is the yellowish, powdery pollen. The size and shape of these parts vary according to whether the plants pollinate via insects or the air. Around the stamens are the protective petals, which collectively are called the corolla. Their shape and color attract and provide a landing base for insects, and also protect the reproductive parts they surround. Holding the blossom are leaves called sepals (collectively called the calyx). Flowers that have all of these parts are termed perfect or complete, while those missing one or more parts are called incomplete or imperfect flowers. Either type may grow singly or in clusters.
The flowering plants of the world have been classified into approximately 300 families according to these flower parts. Thus, even though certain plants may live in different climates and soils and have vastly differing shapes, they may be part of the same family because of how their reproductive organs look and operate. The lily family, for example, is found around the world, and includes plants that are pollinated by insects. They contain three sepals and petals which closely resemble one another, six stamens, one pistil, long sheathlike leaves with parallel veins, and fruit that contains many seeds within one capsule. Examples of the lily family include the tulip, hyacinth, trillium, lily-of-the-valley, onion, garlic, asparagus, crocus, yucca and aloe, and all the lily plants. The pea family includes trees such as the locust and rosewood; vines like the wisteria and bean; herbs that include clover, lupine, licorice and alfalfa; and the protein-rich seed plants of the peas, beans, lentils, peanuts, and soybeans.
Flower families are further divided into two large groups that have evolved separately for millions of years: monocots (having one sheath or casing) and dicots (having two casings). Monocots, or moncotyledons, have only one seed leaf, petals and sepals which resemble one another, simple leaves with parallel veins without teeth or serrated edges, and singular stems that are all or partially underground. Because they are mostly low plants, tree forms of monocots are rare. Examples of moncots include lilies, grasses, and orchids. Dicots, or dicotyledons, are a large and varied group that range from a violet to an oak tree. They all have two seed leaves, which means they send down roots and send up a stem, their petals are distinct from the sepals, the leaves are simple or compound and have a wide variety of shapes, and there are multiple branches from one or more stems. The leaves' veins also have a radiating network of veins, versus the parallel veins of a monocot. While almost all flowering plants in either group need soil to survive, tropical plants called epiphytes, or air plants, live on the trunks and branches of trees. They get their food from the air and dirt and decaying leaves that collect around their open roots.
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