Pollination: The Art
and Science of Floral Sexuality
by Nancy C. Pratt and Alan M. Peter
An adult female fig wasp is on a mission, frantically searching for a place to lay its eggs. Then attracted by the intense, fruity scent of a fig tree, it finds the spot: an immature fig with a small opening at the end. Using its specially-adapted flattened head, the female wasp forces its way through the opening and into the fig, losing its wings and antennae in the struggle. The wasp then probes around for the fig's tiny flowers, into which it deposits its eggs with a long, boring ovipositor. After laying the eggs, the wasp dies inside the fig.
Over the next few weeks, the eggs develop into larvae, and then pupate into young wasps that feed on the fig's tissue. Wingless males mate with pupating females, burrow small holes through the fall of the fig, then die without ever seeing the light of day. The fresh air entering through the holes bored by the males causes a drop in carbon dioxide within the fig; this, in turn, stimulates pollen formation by the flowers. As young females move about inside the fig in preparation for leaving, they use a comb of bristles on the forelegs to fill special pollen pockets on their sides. The females then fly off through the holes to find another fig in which to lay their own eggs. In doing so, they move pollen from their "birth fig" to their "egg-laying fig." This pollen fertilizes the flowers, producing seeds surrounded by a sweet, ripening fruit that is eaten by bats, birds, and monkeys. The seeds are later excreted by the animals and thereby dispersed to germinate into new fig trees.
The fig and fig wasp story is an extreme example of the co-evolution of plants and animals, and particularly plants and their animal pollinators. Without the wasp, the fig would not be pollinated; without the fig, the wasp would have no egg-laying site or food for its larvae. And the timing of fruit maturation on the part of the fig, and of larval development and egg-laying on the part of the wasp, is incredibly precise and precisely coordinated, as it must be for this mutually beneficial system to work. Each species has become adapted through natural selection to better exploit the other, influencing the course of each other's evolutionary trajectory through their interactions. This amazing tale is just one of thousands of chapters in the fascinating story of pollination.
Pollination is the transfer of a flowering plant's male reproductive cells to a female reproductive receptacle. In a word, it is plant sex. Pollination is how plants have solved the problem of reproducing sexually and with diverse mates (which promotes genetic diversity) while they remain rooted in one place. Effective means of pollination are key to the success of angiosperms--the hundreds of thousands of plants, ranging in size from tiny herbs to tall trees, that rely on flowers for effective pollination. Flowers are highly specialized reproductive organs, adapted for the entire gamut of reproductive functions: advertising, pollination, fertilization, seed development, and dispersal of seeds. See Floral Anatomy.
Beautiful as most flowers are to us, they are strictly functional to the plant. The shapes, colors, and fragrances that we admire do not exist for our enjoyment. These characteristics serve the plant by enticing animals to visit their flowers and using them, literally, as reproductive vehicles. (While we speak of plants "enticing" animals and use similar language below, it is important to remember that this is a colorful and convenient shorthand; neither plant nor pollinator behaves purposefully or deliberately, within its lifetime or over evolutionary time, to carry out the process of pollination.)
BLOWING IN THE WIND
When it's time to mate, males and females of most animal species get up and move to find a mate or mates and then copulate or otherwise accomplish the union of male and female sex cells. On the other hand, root-bound plants depend on external forces to move their gametes. Movement through water is the most ancient method of pollination, and the only one used in the early days of plant evolution some billion years ago. Today, only a few plants use water for pollination, most notably algae and moss.
With the evolution of terrestrial life about 400 million years ago, wind replaced water as the primary means by which plants moved male sex cells. Wind pollination is common among primitive plants such as conifers. Among the flowering plants, most grasses and many tree species are wind-pollinated today. The flowers of both water- and wind-pollinated plants are usually small and inconspicuous. These plants produce copious amounts of very small, dry pollen grains that reach the stigmas of other flowers entirely by chance. Wind pollination is most effective when the individuals of a given species are growing relatively close together and is more common in temperate areas, where great stands of oaks and hickories dominate forests and grasses stretch for miles and miles across the plains. In contrast, wind pollination is rare in the tropics, where plant diversity is far greater and the distance between potential mates is often much longer.
Most human allergies to pollen are related to wind-pollinated species of grasses, ragweed, and trees. A single ragweed plant can release a million pollen grains each day in an attempt to reproduce, and, as if to emphasize the chancy nature of wind pollination, some of those grains end up stuck to our corneas and inhaled into our throats and lungs. Our bodies recognize the pollen grains as foreign proteins, setting off an immune response--the sneezing, coughing, and watery eyes of hay fever--to oust the invaders.
ANIMAL FACTS
The most recent--and most successful--method of pollination is by animals. Insects--especially beetles, ants, flies, bees and wasps, butterflies and moths, bats, and birds--notably hummingbirds, sunbirds, sugarbirds, honeycreepers, and brush-tongued parrots--are the predominant animal pollinators. A few plants, however, have entered into pollination partnerships with such diverse creatures as mice, possums, some primates, and even snails or earthworms. Plants have evolved a variety of ways to entice these animals into doing the work of transporting pollen to other plants.
Of
course, animals don't do this work for nothing, so plants
offer rewards. Animals use flowers as sources of food for
themselves and their offspring. First of all, animal-pollinated
flowers produce nectar, a sugar-based substance that also
contains vitamins, amino acids, and other nutrients. The amount
of nectar a flower typically produces relates to the needs
of its pollinators. Hummingbirds, for instance, with their
high metabolic rates, need to earn more nectar from a flower
than a much smaller bee, to make visiting the flower worthwhile.
So hummingbird-pollinated plants generally have flowers that
produce greater amounts of nectar than bee-pollinated plants.
Second, pollen itself is a good source of protein for many
animals. Finally, a few plants reward their pollinators with
fatty oils, resin or wax.
Animal-pollinated plants have large, irregular pollen grains with lots of tiny hooks, spines, and craters on the surface. A rough texture and sticky surface ensure that the pollen will stick to a visiting animal's hair, feathers, or appendages and then stay there until the animal visits another flower. At the next flower, the pollen will be rubbed off onto a strategically placed stigma, resulting in fertilization.
DRAWING THE RIGHT CROWD
Animal pollinators use olfactory and visual cues to find flowers. A flower's scent as well as its color pattern, size, shape, and structural arrangement tell pollinators about the type and quality of the food reward it offers. Fragrant flowers such as roses and gardenias are often located by scent before they are seen by pollinators--mainly insects, which have an acute sense of smell. The fragrance gets stronger as the insect nears the nectar source, so the insect literally follows its nose until the flower is in sight. Some bees collect the scent from certain orchids in special "perfume flasks" on their hind legs and use it in their own courtship rituals. However, not all flowers smell sweet (to us). Rafflesia and devil's claw, two very large rainforest flowers, smell like rotting meat, but this aroma attracts pollinating carrion flies, which land on the flowers to lay their eggs. Flowers pollinated by birds tend to have no perfume at all because birds do not have great olfactory acuity, while those pollinated by bats tend to be very strong and musty.
Flowers use many visual tactics to attract pollinators. Most flowers are fairly conspicuous, contrasting in color, shape, and position with the rest of the plant. Many flowers sit atop a long stem or are clustered together on an inflorescence to call attention to themselves. Flowers come in widely diverse sizes, shapes, and arrangements, from a tiny, aquatic duckweed with a flower the size of a pinhead to a huge rafflesia flower that can measure up to three feet in diameter and weigh 15 pounds.
The simplest flowers, such as daisies and magnolias, display several identical petals arranged in a disc shape around a central cone. These flowers are easily pollinated by crawling beetles or flies. Other, more complex flowers, such as columbines and snap dragons, require their pollinators, usually bees or wasps, to exhibit greater dexterity or persistence to get their nectar reward. For example, the Scotch broom, a flower common throughout the United States, explodes when a large bumblebee lands on its lower petal, the keel. The five stamens and the style are curled up inside the keel, which is hooked into the wing petals on both sides. The weight of the probing bee causes the petals to come apart, creating a tiny pollen cloud that settles on the bee's back.
Butterflies and moths, with their long, hollow proboscises, can sip nectar from flowers with more elongated floral tubes than insects with shorter tongues can. Favorite butterfly plants include sweetly fragranced flowers with tubular blooms such as lantana and honeysuckle. An orchid found in Madagascar has the longest floral tube known. Charles Darwin, who based much of his evolutionary theory on observations of orchid pollination, predicted that this large, ivory-colored flower must be pollinated by a hawk moth with a proboscis long enough to reach the nectar at the bottom of the tube. Forty years later he was proved right. A hawkmoth with a 12-inch-long proboscis was found on Madagascar and named Forma predicta, or "predicted form."
The highly specialized tubular flowers, such as fuschias and trumpet vine, exclude insect pollinators because their nectar is hidden deep in the bottom of the tube; these flowers also have no "landing pad" so it is difficult for flying insects to alight on a flower. Instead, hummingbirds, which can hover in front of a flower and probe it with their long beaks, commonly pollinate these flowers. The smallest bird, the bee hummingbird, is slightly larger than a bumblebee. This tiny bird can fly at speeds of 40 miles an hour and beats its wings 100 times a second as it hovers in front of a flower. The bird's reward for such an energy-expensive effort is a large amount of nutrient-rich nectar.
Larger birds, such as sunbirds and honeyeaters, tend to pollinate larger, brush-like flowers, such as protea and eucalyptus, or sturdy flowers with rigid perching bracts, such as bird of paradise plants. Some South African proteas are pollinated by mice, and the flowers of these plants grow very close to the ground. The time of day when flowers are open can be important in attracting their pollinators. Night-opening flowers, such as many cactus species, are usually white or cream colored and attract nocturnal pollinators such as bats and moths.
IN THE EYE OF THE BEHOLDER
To us, the colors of flowers are a big part of their attraction. [See Looking at Flowers] Flower colors also attract insects, which have color vision but see color differently than we do. Colors correspond to specific wavelengths of light. We can see wavelengths that range from red to violet, with shades of orange, yellow, green, and blue in between. Special cells in the eye's retina, called cones, respond to certain wavelengths of light and signal the brain to "see" specific colors. Humans have between five and ten million cones. These cones are sensitive to either red, green, or blue light. All the other colors we see are the result of reflected light stimulating different combinations of these cones. When an object reflects all the wavelengths in the spectrum, we see it as pure white; when it absorbs all the wavelengths we see black.
Insects see a spectrum of colors that is shifted toward the shorter wavelengths of light. Their three kinds of cone cells respond to green, blue, and ultraviolet light. Ultraviolet light, like infrared at the other end of the spectrum, is invisible to us. Many insect-pollinated flowers display patterns that are visible only in the ultraviolet range. Flowers that look uniformly white, yellow, or blue to us often show striking patterns--like bull's eyes and road maps--that seem to guide insects to their nectar or pollen reward. (In other flowers, these patterns are visible to us.) Most insects do not see red, so a red flower would blend into the background for an insect. Red flowers are thus likely to be pollinated by birds, which have color vision more like our own.
Some flowers change their colors to take advantage of their pollinators' visual systems. Yellow lantana flowers attract butterfly pollinators; then, after pollination, the flowers turn red, which is invisible to the butterflies. This shift ensures that pollen is transferred only to those flowers awaiting pollination and, further, signals to the butterflies that a flower has been fertilized and is no longer producing nectar. Scarlet gilia shifts from dark red blooms in early summer to white blooms in late summer. Hummingbirds pollinate the red flowers, which then change to white, attracting nocturnal hawkmoths to do the job.
Probably the best known animal pollinators are the honeybees. These insects are highly adapted for pollination and use both nectar and pollen from flowers. Honeybees live in large colonies, often referred to as "superorganisms," in which each individual --queen, worker, and drone--performs specific tasks that are critical to the colony's survival. The bodies of worker bees are fully equipped for collecting pollen. Bristly hairs that cover their bodies and legs pick up the dusty pollen, which the bees then transfer to their middle legs with their forelegs. Then they use a comb-like structure on the back to comb out and pack the pollen into a pollen basket. A stiff bristle holds the pollen basket during the flight back to the hive. The honeybee's Z-shaped, extendable proboscis is used to collect nectar from flowers. Both nectar and pollen are brought back to the hive for honey production and for storage in the comb; the stored pollen feeds developing larvae. In the course of this process, honey bees "lose" some of the pollen they collect from one flower when they visit another one. Honeybees thus pollinate many kinds of flowers. People also use honeybees to pollinate many crop plants. See People in the Partnership.
STRANGE BEDFELLOWS
Interesting adaptations between plants and their pollinators abound. The Central American pelican flower gets its name from the fishy odor it produces when it is ready to be pollinated. The large yellow flower is U-shaped, with one end of the U closed off by a transparent panel of cells called a "window pane." Flies are attracted by the smell and fall into the bottom of the flower, where they are trapped by downward-pointing hairs. The flies keep moving toward the light they see through the window pane and, in the process, pass over the flower's anthers and stigmas. Eventually, the trapping hairs dry and shrivel, allowing the flies to escape and later become trapped by another pelican flower.
American yucca plants and yucca moths form another unique partnership. At night, a female moth flutters from fragrant white flower to fragrant white flower collecting yucca pollen in a small lump under its head. It then lands on a flower, bores a hole into the ovary, and lays its eggs among the ovules. Next, it packs the lump of pollen into the same flower's stigma, ensuring its pollination. This act is critical because the moth's larvae will feed on some of the seeds before growing large enough to gnaw their way out of the ovary. The uneaten seeds will then produce new yucca plants, full of flowers for moths to lay eggs in. Interestingly, yuccas introduced to other parts of the world do not produce seeds because they lack their moth partners.
Vanilla, an orchid native to parts of Central and South America, depended on particular co-occurring bees for pollination. When 18th-century Europeans tried to cultivate the prized spice elsewhere, the plants thrived but failed to produce vanilla beans, the plant's fruit, without the proper bees. Painstaking hand pollination was and is the only solution, accounting in part for the high price this fragrant spice commands. Hand pollination does increase the plant's productivity, however, so all commercial vanilla is now hand-pollinated. Particularly in isolated parts of the world, such as Australia and Madagascar, species of possums, lemurs, shrews, rats, and mice participate in the pollination. The Australian honey possum, for instance, pollinates banksia and eucalyptus flowers. The honey possum's adaptations for pollination include a pointed snout, an extremely long tongue, grasping feet, and a prehensile tail that allows it to hang from the branches of trees as it searches for flowers.
CHEATS AND ROBBERS
Some plants and pollinators "cheat" at the game of pollination by reaping the benefits of the partnership without the costs. The fly orchid deceives its pollinator, a digger wasp, by looking and smelling like a female wasp. In search of a mate, a male wasp will unwittingly land on the orchid and "pseudocopulate" with it, even depositing sperm in the process. The wasp becomes laden with pollen and when it moves on to copulate with another deceptive orchid, pollen is transferred at no nectar cost to the orchid.
Many flowers look and smell like rotting flesh, attracting a variety of flies, beetles, and midges looking for a meal or a place to lay eggs. One such is devil's claw, a plant in the milkweed family. The flower's stench attracts blowflies, which lay their eggs on the blossoms and pollinate them as they go from flower to flower. But the maggots that hatch on the flowers die because, unlike real carrion, the flowers offer no food for them. Conversely, some animals, such as certain species of bumblebees, are "nectar robbers," boring holes in the base of a flower to reach the nectar pockets without entering the top of the flower to collect pollen. Flower-piercers, small Neotropical birds, specialize in stealing nectar and have beaks adapted to gripping and poking holes into nectaries. Interestingly, some bumblebees take advantage of the holes pierced by these birds to get nectar they can neither reach nor steal on their own.
Understanding the basic processes of pollination reveals the intricate purpose that underlies the beauty of flowers and opens our eyes to the amazing natural events going on all around us. Next time you walk through a colorful garden, a meadow of wild flowers, a towering forest, or even down a weedy city street, take a moment to look for the butterflies, the birds, and the bees moving from flower to flower, all partners in the sex of live plants.
Nancy C. Pratt was a curatorial intern and Alan M. Peters is an assistant curator at the National Zoo. Alan, with Nancy's assistance, coordinated the creation of the National Zoo's new Pollinarium.
(ZooGoer 24(4) 1995. Copyright 1995 Friends of the National Zoo. All rights reserved.)
