THE ROLE OF FLOWERS IN REPRODUCTION

Flowering plants reproduce sexually. The sexual parts of their blossoms produce male and female sex cells. The male cells, called sperm, are in the pollen produced by the stamens. The female cells, called eggs, are in the ovules produced by the pistils. The sperm and egg cells unite in the ovary at the base of a pistil and develop into seeds. Reproduction in flowers involves two main steps: (1) pollination and (2) fertilization. Pollination is the transfer of pollen from a stamen to a pistil. Fertilization is the union of a sperm with an egg cell. Fertilization occurs in much the same way in all flowering plants. However, there are two methods of pollination: (1) cross-pollination and (2) self-pollination. Cross-pollination involves the transfer of pollen from a stamen on one plant to a pistil on another plant.

In self-pollination, pollen is transferred from a stamen of one flower to a pistil of the same flower or to a pistil of another flower on the same plant.

Cross-pollination is the method of pollination in most flowering plants. The method requires an agent to carry the pollen from flower to flower. Insects are the most common agents of cross-pollination. Many insects depend on flowers for food. Bees live on nectar and pollen. Honey bees also use nectar to make honey, which they feed on in winter. Butterflies and moths also live on nectar, and certain beetles and flies feed on both nectar and pollen. As an insect travels from flower to flower in search of food, pollen grains stick to its body. Some or all of these grains may brush off onto the stigmas of some flowers that the insect visits. One or more of these flowers may thus become cross-pollinated.

When searching for food, an insect could easily fail to visit a particular kind of flower unless the insect was attracted to it. Most flowers that depend on insects for pollination are brightly colored or heavily scented. Each kind of pollinating insect is attracted by certain colors or odors and so visits certain flowers rather than others. However, more than one kind of insect pollinates most insect-pollinated flowers. For example, moths and butterflies visit many of the same flowers. A few kinds of insects and flowers have developed highly specialized relationships with each other. These flowers are pollinated only by a particular kind of insect. For example, bumble bees are the type of insect that pollinates the red clover flower.

Pollination by bees. More kinds of flowers are pollinated by bees than by any other kind of insect. Bees cannot see the color red. Otherwise, they have a keen sense of sight. They also have a well-developed sense of smell. Bees are strongly attracted by yellow and blue blossoms, especially those with a sweet odor. Unlike people, bees can see ultraviolet light. Many flowers, particularly yellow ones, have elaborate ultraviolet markings. These markings attract bees to the flowers and even pinpoint the location of the nectaries. Many of the flowers pollinated by bees have a highly complicated structure that encourages cross-pollination and discourages self-pollination. For example, a bee can reach the nectar of a snapdragon only after brushing against the stigma. It then cannot leave the flower without touching the pollen. Furthermore, the bee cannot touch the stigma after it touched the pollen.

Pollination by butterflies and moths. Butterflies and moths are attracted to flowers that produce abundant nectar. In many such flowers, the nectaries are long and tube-shaped or are at the base of a long tube-shaped corolla. Butterflies and moths have exceptionally long, tubelike mouthparts, which enable them to reach into these structures and suck up the nectar. Butterflies, like bees, prefer flowers with sweet-smelling yellow or blue blossoms. Unlike most bees and butterflies, many moths rest during the day and search for food at night. Many of the flowers that attract moths open only at night. Most of these flowers are pale-colored or white and so are easier to see at night than dark blossoms. Many of the flowers are also heavily scented and give off their scent only at night. Flowering tobacco and various kinds of evening primroses and honeysuckles are among the plants commonly pollinated by moths. The yucca flowers of the American Southwest are pollinated only by the yucca moth. The female moth carries pollen from one yucca plant to another. She bores into the ovary of the second flower and lays her eggs inside it. She then deposits pollen from the first flower onto the stigma of the second. The moth eggs and the yucca seeds develop together. The eggs hatch into caterpillars, which feed on the seeds. But enough seeds remain uneaten to produce the next generation of yucca plants.

Pollination by beetles and flies. Beetles visit flowers in which both nectar and pollen are plentiful. They prefer white or dull-colored flowers with spicy odors, such as magnolias and wild roses. Most flies do not have mouthparts that are long enough to suck nectar from tube-shaped flowers. These flies usually visit flowers that have flat corollas, such as hawthorn blossoms and buttercups. Some flowers, such as carrion flowers and skunk cabbages, give off a foul odor that attracts flies.

Pollination by other agents. Some birds feed on nectar and so help pollinate flowers. Unlike most pollinating insects, birds have a weak sense of smell. But birds have sharp vision and see red as well as they see other colors. Most odorless red flowers are pollinated by birds. In North America, hummingbirds are the chief bird pollinators. Hummingbirds are particularly attracted to red, orange, and yellow flowers, such as columbines, fuchsias, and Indian paintbrushes. Bats and the wind are also agents of pollination. Bats pollinate certain strongly scented flowers of the tropics. The wind spreads the pollen of most plants whose flowers lack petals and sepals. These plants include oaks, ragweeds, sedges, and most wild grasses.

Self-pollination. About half of all species of plants normally pollinate themselves. Such plants include barley, oats, peas, and wheat. However, self-pollination also occurs frequently in many species that depend on cross-pollination. In such cases, pollen may simply fall onto a stigma of the same plant. Self-pollination is impossible in dioecious species because the male and female flowers are on different plants. In addition, many other plants have characteristics that discourage or prevent self-pollination. In such flowers as hibiscuses and lilies, for example, the stamens are much shorter than the pistils. Any pollen that drops from the stamens is therefore unlikely to reach a stigma of a pistil on the same plant. Many kinds of plants, such as flowering tobacco and rye, have chemicals in their cells that prevent self-pollination.