Pollination Ecology: or, Sex in the Herb Garden

The Birds and the Bees Do More Than You Think

| February/March 1998

We share our passion for flowering plants with many insects and a few birds, but while we are drawn to their beauty and sensuous fragrance, the birds and the bees are after a free meal. None of us gives much thought to the fact that flowers contain a plant’s sexual organs.

Unlike most animals, that can move about and select a mate for themselves, plants must depend on wind, water, insects, or other animals. Either by chance or enticement, the male genes of one plant are transferred to the female parts of another for cross-fertilization and eventual seed production to occur. In many kinds of flowering plants, these parts may be on separate plants or on separate flowers of a given plant. Over eons, plants have developed fascinating adaptations, including flowers with alluring fragrances, colors, shapes, and sizes, to maximize their chances of reproductive success. This variety of adaptations makes our herb gardens colorful and intriguing.

How Plants Do It

Plants have male and female organs as we do, but their sexual practices are even more varied than those of mammals. A typical flower (shown above) consists of a stalk topped by a ring of sepals (calyx) surrounding a ring of petals (corolla). Inside the corolla are the stamens, each of which has a filament topped with a sac of pollen called an anther. These are the male organs. The anthers disperse millions of two-celled pollen grains, each of which has the potential to give rise to a sperm that can fertilize an egg (female sex cell) of the same kind of plant. As with mammals, there are many more pollen grains than there are eggs to be fertilized.

The female organ of a typical flower, located at the top of the stalk inside the ring of stamens, is called a pistil. It consists of a sticky stigma, an elongated style, and an ovary housing ovules (undeveloped seeds) which contain eggs waiting to be fertilized. At just the right time, the pistil may lengthen and exude sticky liquids or expose a hairy surface to trap more pollen grains. When a pollen grain lands on a compatible stigma, it can germinate, after which one of its cells forms a pollen tube that grows down the style into the ovary, where it enters an ovule through a microscopic opening. The second cell divides to form two sperm, which descend through the pollen tube to the ovule. One can imagine the ovule calling out, “Get over here, I’m ready.” One sperm unites with the egg; the ­resulting zygote will grow into the embryo of a new plant.

The flowers of some plants, such as many trees and most grasses, are wind-pollinated. They don’t need pretty petals or flamboyant bracts to attract a pollinator. Instead, their sex organs hang out in the wind. Many wind-pollinated flowers have the sexes in separate flowers, or, if in the same flower, the male and female parts mature at different times. Because there’s only a minute chance of a pollen grain’s landing in the right place at the right time, wind-pollinated plants produce huge quantities of pollen. The pollen grains of wind-pollinated plants are usually small and light.

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