Shades of Green: Earth's Forests -- Flowers & Pollen

Many plants rely on insects or other animals for flower pollination. Flowers are brightly colored and conspicuous in order to attract pollinators, and therefore plants that rely on wind instead of animals to disperse their pollen usually have much smaller, less elaborate blooms.

Flowers from both temperate and tropical regions show an amazing variety of colors, forms, and sizes, all designed to be as attractive as possible to pollinating insects. Photos by Maya Walters.

Flowers that are pollinated by insects are usually brightly colored and odorous, two characteristics that advertise their presence to pollinators. The insects are not interested in the act of carrying pollen from plant to plant, but they are attracted by the nectar inside the flowers. In the process of feeding on the nectar, the insects pick up sticky pollen grains from the flower, and also deposit pollen grains from other flowers. This pollination service is so important to the plants that they spend great amounts of energy producing quantities of the sweet nectar, which is of no use to them, and serves only to attract the insects.

The most common pollinators are the bees. Some bees live in colonies and are particularly efficient pollinators, because when one bee finds a source of food, she can communicate its location to the rest of the bees in the hive. Other bees, such as bumble bees, are not colonial. Wasps are also important pollinators, as well as butterflies, moths, and flies. Even some beetles are important carriers of pollen. Pollination is one of the most successful forms of interdependence between two groups of organisms as different as insects and plants. The success and diversity of flowering plants is due in part to the insects, which are much more efficient pollinators than the wind. The enormous variety of flower shapes and colors is a result of each species competing with others for the attention of pollinators.

A bee in a white crocus flower. Photo by Maya Walters.

While most insects will feed from many kinds of flowers, there are certain specialists in both groups. Some insects have adapted to feed efficiently from just one species of flower, and some plants have evolved flower structures that encourage pollination only by certain kinds of insects. For example, open, upright flowers are favored by butterflies, while low, drooping flowers are pollinated by bees. The nectar at the base of a long, trumpet-shaped flower is more accessible to a butterfly than to short-tongued bee. Some short-tongued bees have a trick for reaching this nectar, however: they cut small holes at the base of the flower, so they can drink the nectar directly instead of reaching through the flower. But this is of no advantage to the plant, because nectar is consumed with no movement of pollen. Red clover is pollinated mostly by large bumble bees, while the smaller white clover flowers attract honey bees, whose tongues are too short to reach the nectar within red clover flowers.

The petals on most flowers are patterned, which helps insects find their way to the center of the flower where the pollen and nectar are located. The petals of other flowers appear to be a plain color, which would offer no help to a confused bee. But patterns are there, they are just invisible to humans. In addition to seeing regular colors, bees can also detect ultraviolet wave lengths. These uniformly colored petals have patterns that are only visible in ultraviolet light, and where we see a plain colored flower, bees see a variety of colors. For example, certain sunflowers that look completely yellow to us have ultraviolet patterns visible to bees.

On this page: Flowers are brightly colored to attract insects, but the colors that insects see are different from what we see. Photos by Maya Walters.

Most tree species are pollinated by the wind, and their inconspicuous flowers are located high in the canopy. Often, they don't even look like flowers since they are long, hanging structures called "catkins". Plants that make use of wind-pollination have separate male and female flowers. This ensures that pollen will be moved between more different flowers, resulting in more genetic diversity in the seeds.

Tiger lilies are common flowers in open areas and gaps in temperate forests, where sunlight is not blocked by the trees. Photo by Maya Walters

Open habitats and tree tops high in the forest canopy are the best places for wind-pollinated flowers, because the wind is does not blow strongly enough in lower, more sheltered locations for pollen distribution. Tree blossoms often open before their leaves, to ensure their catkins have maximum exposure to wind.

In addition to the seed cones which are easy to find, coniferous trees also produce much smaller pollen cones. Photo by Maya Walters.

All coniferous trees have two kinds of cones: seed cones and pollen cones. The seed cones are much more common to see, and provide the seeds that are such an important food source to squirrels and birds. The pollen cones are much smaller, and dry up in the spring as soon as they release their pollen. All these cones release so much pollen that it can often be seen as a yellow haze blowing in the air, or a floating layer on the surface of ponds. Millions of microscopic pollen grains make up these clouds of pollen, and it is possible to determine what species of plant any pollen grain came from. Since each plant species produces its own distinct pollen, and pollen grains form "microfossils" which persist for millions of years, pollen is important to the study of prehistoric vegetation. By studying ancient pollen samples, it is possible to discover the locations of ancient forest cover and what plants were most abundant in different places and different times. This information about prehistoric vegetation can then help to reveal how regional climates have varied over time.

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