Pollination Ecology: or, Sex in the Herb Garden

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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.

Most insect-pollinated flowers have male and female parts located close together and surrounded by colorful petals, sepals, or bracts that advertise the flower’s availability. Many–coneflowers, for example–provide a generous landing platform for some insects to light on when they arrive. The blossom may also provide an appealing refuge for the insect, perhaps a place to hide or spend the night, a warm and inviting bowl-shaped blossom that tracks the sun, or a bell-shaped bloom offering shelter from the wind or rain. The pollen of insect- and bird-pollinated flowers is larger and stickier than that of wind-pollinated flowers and often has projections that catch in the hair and feathers of visiting pollinators.

Sexual Sophistication

The choices we make, from the color of the flowers, the size and shape, to the time of year they bloom, will affect which pollinators visit our gardens.

It seems to me that too many things have to happen at the correct time for this to work, but over the past 100 ­million years or so, the plants that evolved ways to maximize their chances for ­fertilization have survived. The others became extinct.

What on earth can plants offer insects and other animals to get them to visit just when they are ready for fer­tilization? The answer is good food, ­either protein-rich pollen grains or the sweet liquid full of carbohydrates called nectar. Some insects and a few birds and mammals depend on these foods for sustenance. The animals benefit by obtaining the food that enables them to reproduce, and the plants, which must expend energy manufacturing food for these pollinators, profit by being enabled to reproduce themselves. When pollinators such as bees visit a flower, they are either hungry or are foraging for food for their offspring or others of their kind. They are rarely looking for sex, and they don’t intend to pollinate the flower, but nonetheless they ensure that there are more flowers for the next season for themselves and their descendants.

Some flowers offer rewards of food to any and all comers; botanists call them promiscuous flowers. Promiscuous flowers have a shape, color, size, smell, and food source that is appealing to a wide range of insects. They include members of the parsley family such as dill and aster family members such as chamomile and feverfew. They all welcome bees, wasps, butterflies, beetles, and flies.

Other flowers are specialized to encourage certain pollinators while excluding any visitors that might take the food without performing the sexual transfer. Members of the pea family, such as licorice and indigo, have flowers with concealed sexual parts, but bees landing on a certain spot are able to expose their pollen. Tubular flowers such as foxgloves invite pollination by bees that are willing to climb inside to seek the nectar located at the base of the flower. The stinger in the tail of a worker bee protrudes and protects her while she is feeding head down in a funnel-shaped flower. The nectar of really long, narrow tubular flowers is inaccessible to short-tongued bees. Instead, these flowers are pollinated by a butterfly or moth, whose immensely long tongue unrolls to penetrate the depths of the flower, or by a hummingbird, whose long, forked tongue forms a tube to suck up the nectar. Thymes, mints, clary sage, and other herbs of the mint family have two-lipped corollas with cunning sexual parts arranged to receive pollen from, or deposit it on, the head, back, or belly of bees that visit them in search of nectar. While flying to the next flower, honey- and bumblebees transfer much of these pollen deposits to pockets on their back legs called pollen baskets. The rest is available for transfer to the next flower visited. It has been shown that honeybees have a lot of loyalty to a flower type on foraging trips, thus maximizing the chance that pollination of the flowers of that species will occur.

Other Enticements

Plants have many ways of selecting pollinators. Flowers that open at night attract night-flying moths or bats. Other flowers time their opening to coincide with the emergence of spring- or fall-hatching insects. Early spring flowers may be thought of as getting a jump on the competition, luring hungry bees from semihibernation with fast food. Once pollinated, these early-maturing plants ripen their seed and then go dormant before the field gets crowded with big and flashy flowers.

Flower size also limits pollinators. Minute blooms appeal to minute, short-tongued insects such as flies and midges whereas larger flowers offer a good taste of nectar to larger insects, hummingbirds, or bats. Because each flower doesn’t yield enough nectar to satisfy the pollinator, it must fly from plant to plant to get its fill, effecting cross- pollination in the bargain.

Color can affect which flowers pollinators visit. Bees can see many of the colors that humans can and can also perceive ultraviolet light. They are particularly attracted to what we see as blue and yellow. Butterflies seem to go for warm and hot colors, such as the vivid orange of pleurisy root. Most beetles and flies see only in black-and-white, so they and night-flying moths seek white or pale blooms. Humming­birds are noted for being attracted to red bee balm and other red flowers with long corolla tubes, but they won’t pass up a blue salvia if no red tubular flowers are available.

Perfume in the Air

One of the most characteristic traits of herbs is their range of smells. Promiscuous flowers have generic, gentle, sugary smells that appeal to many visitors. More selective flowers tend to have more distinctive scents. The fragrance of lavender is not only attractive to humans, but coaxes bees from far and wide, broadcasting a message that food is available for the taking. Bees, butterflies, and some flies like sweet odors. Hummingbirds, however, have almost no sense of smell. Insects that eat rotting flesh or lay their eggs in carcasses or dung are fooled into visiting and pollinating foul-smelling flowers such as jack-in-the-pulpit.

Planning Encounters

February is a good time to start thinking about the sexual habits of herbs that we might plant in our gardens this spring. The choices we make, from the color of the flowers, size, and shape, to the time of year they bloom, will affect which pollinators visit our gardens and make their small contribution to the area’s ecology. Our herb gardens should be not only a place for us to collect leaves, fruits, and seeds for the table, but also a safe place for insects like bees, butterflies, and moths to gather food, too.

We may observe flowers mature during an afternoon and see pistils shift while growing wet and dewy in anticipation of receiving pollen, and if no pollinator arrives, perhaps lean toward a nearby stamen and fertilize themselves. Over the course of a few days, we can watch petals change color or fade and wither after the flower has been fertilized and ovaries swell as seeds ripen. On any sunny day, we can see bees scuttling about thyme flowers as they load up on pollen and stagger off to their hives with pollen baskets almost too heavy to carry.

There is much more sexual activity going on in your herb garden than is at first apparent. Get out and take a look. If you are looking for something to do on Valentine’s Day, find some early-blooming crocuses or houseplants and try manual cross-pollination.

Angela Overy, who has been drawing flowers since she was a child in England, now teaches botanical illustration at the Denver Botanic Garden. She is the author of Sex in Your Garden (Fulcrum, 1997).

Further reading

Barth, Friedrich G., M. A. Biederman-Thorson, trans. Insects and Flowers: The Biology of a Partnership. Princeton, New Jersey: Princeton University Press, 1991.

Capon, Brian. Botany for Gardeners: An Introduction and Guide. Portland, Oregon: Timber Press, 1990.

Proctor, Michael, Peter Yeo, and Andrew Lack. The Natural History of Pollination. Portland, Oregon: Timber Press, 1996.

These softly lit, impressionistic photographs of flowers are by Alan Pappe, PhotoDisc.

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