A flower is a part of a plant that forms pollen or seeds or both. Only seed-bearing plants have true flowers. Other plants like the algae, fungi, and mosses have none. And, only those parts of a plant which have to do with the forming of seeds are parts of the flower.
The bright colors, sweet smell, and nectar are necessary to attract insects. The insects are needed to help the flower make seeds that will grow into a new plant.
A person, who thinks of a flower simply as something brightly colored which grows on a plant, may be led far astray. The brilliant scarlet poinsettia blossoms which one sees at Christmas are not true flowers, but simply brilliantly colored leaves which surround the small and dull flowers at the tip of the cluster. Similarly, the large creamy “petals” of the lovely dogwood “blossoms” that bloom in spring are not petals at all but simply brightly colored bracts surrounding the small inconspicuous flowers. In the callas that are used so much at Easter, the great white sheath is not a single flower but a bract or leaf which surrounds and protects a club-shaped mass on which many small flowers are tightly crowded together.
Thus, many of the bright, showy parts of plants which are commonly called “flowers” are not true flowers in the strict sense. On the other hand, there are many true flowers which are seldom noticed at all, or if they are, not considered as flowers. For example, the bearded tufts at the tips of grasses, the heads of cattails, the unripe ears of corn and the small tassels that somewhat resemble certain caterpillars and hang from birches and alders early in spring, are all clusters” of real flowers.
The flower is a branch with very special kinds of leaves. The outermost ones are called sepals. They are usually green; sometimes they are separate one from another, sometimes they are not separate and form a cup. When the flower buds are small, they serve as a protective covering. All together, they are known as the calyx. Next inside the sepals are the petals, known all together as the corolla. The petals may be separate from one another, as in the buttercup, or they may be partially united, as in the tomato, or form a broad flaring trumpet, as in the petunia. Petals are the brightest parts of most flowers. The most important parts, however, are not the brightly colored petals, but the pistils and stamens. Many flowers contain both—the pistil or pistils in the center, surrounded by the stamens. Pistils are often greenish in color. In an enlarged part at the lower end of the pistil (the ovary) are the ovules. In the sweet pea the ovary is long and hairy; in it are ten or more ovules arranged in two rows. Each ovule may grow into a seed. The most important part of an ovule is a tiny egg cell, which can be seen only under a microscope. From this egg cell a new plant may grow.
Some plants have more stamens than others. Each stamen is made of a pollen sac (anther) at the end of a stalk (filament). Before they are ripe they are smooth. Later they open by slits or tiny holes and release the pollen, which they contain as a fine dust, usually yellow. These pollen grains are just as important as the ovules in the production of seed. They produce the sperm cells which unite with the egg cells in the ovules. After a sperm cell has united with an egg cell, the product is called an embryo. From this embryo the new plant will grow. But before an embryo can be formed, the pollen grains must be transferred from the anthers to the pistil, and eventually to the egg. This transfer of pollen is called pollination.