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Flowers, which generally are the showiest part of a plant, have sexual reproduction as their sole function. Their beauty and fragrance have evolved not to please humans but to ensure continuance of the species. Fragrance and color attract pollinators (insects or birds) that play an important role in the reproductive process.

Flowers are important for plant classification. The system of plant nomenclature we use today was developed by Carl von Linné (Linnaeus) and is based on flowers and/or reproductive parts of plants. One reason his system is successful is because flowers are the plant part least influenced by environmental changes. Thus, a knowledge of flowers and their parts is essential for anyone interested in plant identification.

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  As a plant's reproductive part, a flower contains a stamen (The male flower part; consists of an anther and a supporting filament) and/or pistil (The female flower part; consists of a stigma, style, and ovary), plus accessory parts such as sepals, petals, and nectar glands (Figure 19).

The stamen is the male reproductive organ. It consists of a pollen sac (anther) and a long supporting filament. This filament holds the anther in position, making the pollen available for dispersement by wind, insects, or birds.

The pistil is a plant's female part. It generally is shaped like a bowling pin and is located in the flower's center. It consists of a stigma, style, and ovary. The stigma is located at the top and is connected by the style to the ovary. The ovary contains eggs, which reside in ovules. If an egg is fertilized, the ovule develops into a seed.

Sepals are small, green, leaflike structures located at the base of a flower. They protect the flower bud. Collectively, the sepals are called a calyx.

Petals generally are the highly colored portions of a flower. Like nectar glands, petals may contain perfume. Collectively, the petals are called a corolla. The number of petals on a flower often is used to help identify plant families and genera. Flowers of dicots typically have four or five sepals and/or petals, or multiples thereof. In monocots, these floral parts typically come in threes or multiples of three.

If a flower has a stamen, pistil, petals, and sepals, it is called a complete flower (Figure 19). Roses are an example. If one of these parts is missing, the flower is called incomplete.

The stamen and pistil are the essential parts of a flower and are involved in seed production. If a flower contains both functional stamens and pistils, it is called a perfect flower, even if it does not contain petals and sepals. If either stamens or pistils are lacking, the flower is called imperfect. Pistillate (female) flowers possess a functional pistil or pistils, but lack stamens. (Figure 20) Staminate (male) flowers contain stamens, but no pistils.

Plants with imperfect flowers are further classified as monoecious or dioecious.

Monoecious plants have separate male and female flowers on the same plant (e.g., corn and pecan). Some monoecious plants bear only male flowers at the beginning of the growing season, but later develop both sexes (e.g., cucumbers and squash).

Dioecious species have separate male and female plants. Examples include holly, ginkgo, and pistachio. In order to set fruit, male and female plants must be planted close enough together for pollination to occur. In some instances (e.g., holly), the fruit is desirable. In the case of ginkgo, however, the fruit generally is not desirable due to its putrid smell when ripe. Kiwis are complicated because they may have one plant with bisexual flowers and another plant with only male flowers. The plant world doesn't always have absolutes!

İnfloresans tipleri

Some plants bear only one flower per stem, which is called a solitary flower. Other plants produce an inflorescence--a cluster of flowers. Each flower in an inflorescence is called a floret.

Most inflorescences belong to one of two groups: racemes and cymes. In the racemose group, the florets start blooming from the bottom of the stem and progress toward the top. In a cyme, the top floret opens first and blooms progress downward along the peduncle. Detailed discussions of flower types are found in many botany textbooks.

Tohumlar nasıl oluşur

Pollination is the transfer of pollen from an anther to a stigma, either by wind or by pollinators. Species pollinated by insects, animals, or birds often have brightly colored or patterned flowers that contain fragrance or nectar. While searching for nectar, pollinators transfer pollen from flower to flower, either on the same plant or on different plants. Plants evolved this ingenious mechanism in order to ensure their species' survival. Wind-pollinated flowers often lack showy floral parts and nectar because they don't need to attract pollinators.

A chemical in the stigma stimulates pollen to grow a long tube down the style to the ovules inside the ovary. When pollen reaches the ovules, it releases sperm, and fertilization typically occurs. Fertilization is the union of a male sperm nucleus from a pollen grain with a female egg. If fertilization is successful, the ovule develops into a seed. It is important to remember that pollination is no guarantee that fertilization will occur.

Cross-fertilization combines genetic material from two parent plants. The resulting seed has a broader genetic base, which may enable the population to survive under a wider range of environmental conditions. Cross-pollinated plants usually are more successful than self-pollinated plants. Consequently, more plants reproduce by cross-pollination than by self-pollination.