Pollination

I

 

INTRODUCTION

Pollination, transfer of pollen grains from the male structure of a plant to the female structure of a plant. The pollen grains contain cells that will develop into male sex cells, or sperm. The female structure of a plant contains the female sex cells, or eggs. Pollination prepares the plant for fertilization, the union of the male and female sex cells. Virtually all grains, fruits, vegetables, wildflowers, and trees must be pollinated and fertilized to produce seed or fruit, and pollination is vital for the production of critically important agricultural crops, including corn, wheat, rice, apples, oranges, tomatoes, and squash.

Pollen grains are microscopic in size, ranging in diameter from less than 0.01mm (about 0.0000004 in) to a little over 0.5 mm (about 0.00002 in). Millions of pollen grains waft along in the clouds of pollen seen in the spring, often causing the sneezing and watery eyes associated with pollen allergies. The outer covering of pollen grains, called the pollen wall, may be intricately sculpted with designs that in some instances can be used to distinguish between plant species. A chemical in the wall called sporopollenin makes the wall resistant to decay.

Although the single cell inside the wall is viable, or living, for only a few weeks, the distinctive patterns of the pollen wall can remain intact for thousands or millions of years, enabling scientists to identify the plant species that produced the pollen. Scientists track long-term climate changes by studying layers of pollen deposited in lake beds. In a dry climate, for example, desert species such as tanglehead grass and vine mesquite grass thrive, and their pollen drifts over lakes, settling in a layer at the bottom. If a climate change brings increased moisture, desert species are gradually replaced by forest species such as pines and spruce, whose pollen forms a layer on top of the grass pollen. Scientists take samples of mud from the lake bottom and analyze the pollen in the mud to identify plant species. Comparing the identified species with their known climate requirements, scientists can trace climate shifts over the millennia.

II

 

HOW POLLINATION WORKS

Most plants have specialized reproductive structures—cones or flowers—where the gametes, or sex cells, are produced. Cones are the reproductive structures of spruce, pine, fir, cycads, and certain other gymnosperms and are of two types: male and female. On conifers such as fir, spruce, and pine trees, the male cones are produced in the spring. The cones form in clusters of 10 to 50 on the tips of the lower branches. Each cone typically measures 1 to 4 cm (0.4 to 1.5 in) and consists of numerous soft, green, spirally attached scales shaped like a bud. Thousands of pollen grains are produced on the lower surface of each scale, and are released to the wind when they mature in late spring. The male cones dry out and shrivel up after their pollen is shed. The female cones typically develop on the upper branches of the same tree that produces the male cones. They form as individual cones or in groups of two or three. A female cone is two to five times longer than the male cone, and starts out with green, spirally attached scales. The scales open the first spring to take in the drifting pollen. After pollination, the scales close for one to two years to protect the developing seed. During this time the scales gradually become brown and stiff, the cones typically associated with conifers. When the seeds are mature, the scales of certain species separate and the mature seeds are dispersed by the wind. In other species, small animals such as gray jays, chipmunks, or squirrels break the scales apart before swallowing some of the enclosed seeds. They cache, or hide, other seeds in a variety of locations, which results in effective seed dispersal-and eventually germination-since the animals do not always return for the stored seeds.

Pollination occurs in cone-bearing plants when the wind blows pollen from the male to the female cone. Some pollen grains are trapped by the pollen drop, a sticky substance produced by the ovule, the egg-containing structure that becomes the seed. As the pollen drop dries, it draws a pollen grain through a tiny hole into the ovule, and the events leading to fertilization begin. The pollen grain germinates and produces a short tube, a pollen tube, which grows through the tissues of the ovule and contacts the egg. A sperm cell moves through the tube to the egg where it unites with it in fertilization. The fertilized egg develops into an embryonic plant, and at the same time, tissues in the ovule undergo complex changes. The inner tissues become food for the embryo, and the outer wall of the ovule hardens into a seedcoat. The ovule thus becomes a seed—a tough structure containing an embryonic plant and its food supply. The seed remains tucked in the closed cone scale until it matures and the cone scales open. Each scale of a cone bears two seeds on its upper surface.

In plants with flowers, such as roses, maple trees, and corn, pollen is produced within the male parts of the plant, called the stamens, and the female sex cells, or eggs, are produced within the female part of the plant, the pistil. With the help of wind, water, insects, birds, or small mammals, pollen is transferred from the stamens to the stigma, a sticky surface on the pistil. Pollination may be followed by fertilization. The pollen on the stigma germinates to produce a long pollen tube, which grows down through the style, or neck of the pistil, and into the ovary, located at the base of the pistil. Depending on the species, one, several, or many ovules are embedded deep within the ovary. Each ovule contains one egg.

Fertilization occurs when a sperm cell carried by the pollen tube unites with the egg. As the fertilized egg begins to develop into an embryonic plant, it produces a variety of hormones to stimulate the outer wall of the ovule to harden into a seedcoat, and tissues of the ovary enlarge into a fruit. The fruit may be a fleshy fruit, such as an apple, orange, tomato, or squash, or a dry fruit, such as an almond, walnut, wheat grain, or rice grain. Unlike conifer seeds, which lie exposed on the cone scales, the seeds of flowering plants are contained within a ripened ovary, a fleshy or dry fruit.

III

 

POLLINATION METHODS

In order for pollination to be successful, pollen must be transferred between plants of the same species—for example, a rose flower must always receive rose pollen and a pine tree must always receive pine pollen. Plants typically rely on one of two methods of pollination: cross-pollination or self-pollination, but some species are capable of both.

Most plants are designed for cross-pollination, in which pollen is transferred between different plants of the same species. Cross-pollination ensures that beneficial genes are transmitted relatively rapidly to succeeding generations. If a beneficial gene occurs in just one plant, that plant’s pollen or eggs can produce seeds that develop into numerous offspring carrying the beneficial gene. The offspring, through cross-pollination, transmit the gene to even more plants in the next generation. Cross-pollination introduces genetic diversity into the population at a rate that enables the species to cope with a changing environment. New genes ensure that at least some individuals can endure new diseases, climate changes, or new predators, enabling the species as a whole to survive and reproduce.

Plant species that use cross-pollination have special features that enhance this method. For instance, some plants have pollen grains that are lightweight and dry so that they are easily swept up by the wind and carried for long distances to other plants. Other plants have pollen and eggs that mature at different times, preventing the possibility of self-pollination.

In self-pollination, pollen is transferred from the stamens to the pistil within one flower. The resulting seeds and the plants they produce inherit the genetic information of only one parent, and the new plants are genetically identical to the parent. The advantage of self-pollination is the assurance of seed production when no pollinators, such as bees or birds, are present. It also sets the stage for rapid propagation—weeds typically self-pollinate, and they can produce an entire population from a single plant. The primary disadvantage of self-pollination is that it results in genetic uniformity of the population, which makes the population vulnerable to extinction by, for example, a single devastating disease to which all the genetically identical plants are equally susceptible. Another disadvantage is that beneficial genes do not spread as rapidly as in cross-pollination, because one plant with a beneficial gene can transmit it only to its own offspring and not to other plants. Self-pollination evolved later than cross-pollination, and may have developed as a survival mechanism in harsh environments where pollinators were scarce.

IV

 

POLLEN TRANSFER

Unlike animals, plants are literally rooted to the spot, and so cannot move to combine sex cells from different plants; for this reason, species have evolved effective strategies for accomplishing cross-pollination. Some plants simply allow their pollen to be carried on the wind, as is the case with wheat, rice, corn, and other grasses, and pines, firs, cedars, and other conifers. This method works well if the individual plants are growing close together. To ensure success, huge amounts of pollen must be produced, most of which never reaches another plant.

Most plants, however, do not rely on the wind. These plants employ pollinators—bees, butterflies, and other insects, as well as birds, bats, and mice—to transport pollen between sometimes widely scattered plants. While this strategy enables plants to expend less energy making large amounts of pollen, they must still use energy to produce incentives for their pollinators. For instance, birds and insects may be attracted to a plant by its tasty food in the form of nectar, a sugary, energy-rich fluid that bees eat and also use for making honey. Bees and other pollinators may be attracted by a plant’s pollen, a nutritious food that is high in protein and provides almost every known vitamin, about 25 trace minerals, and 22 amino acids. As a pollinator enters a flower or probes it for nectar, typically located deep in the flower, or grazes on the pollen itself, the sticky pollen attaches to parts of its body. When the pollinator visits the next flower in search of more nectar or pollen, it brushes against the stigma and pollen grains rub off onto the stigma. In this way, pollinators inadvertently transfer pollen from flower to flower.

Some flowers supply wax that bees use for construction material in their hives. In the Amazonian rain forest, the males of certain bee species travel long distances to visit orchid flowers, from which they collect oil used to make a powerful chemical, called a pheromone, used to attract female bees for mating. The bees carry pollen between flowers as they collect the oils from the orchids.

Flowers are designed to attract pollinators, and the unique shape, color, and even scent of a flower appeals to specific pollinators. Birds see the color red particularly well and are prone to pollinating red flowers. The long red floral tubes of certain flowers are designed to attract hummingbirds but discourage small insects that might take the nectar without transferring pollen. Flowers that are pollinated by bats are usually large, light in color, heavily scented, and open at night, when bats are most active. Many of the brighter pink, orange, and yellow flowers are marked by patterns on the petals that can be seen only with ultraviolet light. These patterns act as maps to the nectar glands typically located at the base of the flower. Bees are able to see ultraviolet light and use the colored patterns to find nectar efficiently.

These interactions between plants and animals are mutualistic, since both species benefit from the interaction. Undoubtedly plants have evolved flower structures that successfully attract specific pollinators. And in some cases the pollinators may have adapted their behaviors to take advantage of the resources offered by specific kinds of flowers.

V

 

CURRENT TOPICS

Scientists control pollination by transferring pollen by hand from stamens to stigmas. Using these artificial pollination techniques, scientists study how traits are inherited in plants, and they also breed plants with selected traits—roses with larger blooms, for example, or apple trees that bear more fruit. Scientists also use artificial pollination to investigate temperature and moisture requirements for pollination in different species, the biochemistry of pollen germination, and other details of the pollination process.

Some farmers are concerned about the decline in numbers of pollinating insects, especially honey bees. In recent years many fruit growers have found their trees have little or no fruit, thought to be the result of too few honey bee pollinators. Wild populations of honey bees are nearly extinct in some areas of the northern United States and southern Canada. Domestic honey bees—those kept in hives by beekeepers—have declined by as much as 80 percent since the late 1980s. The decline of wild and domestic honey bees is due largely to mite infestations in their hives—the mites eat the young, developing bees. Bees and other insect pollinators are also seriously harmed by chemical toxins in their environment. These toxins, such as the insecticides Diazinon and Malathion, either kill the pollinator directly or harm them by damaging the environment in which they live.