Session 11 Reading
SEXUAL REPRODUCTION IN PLANTS
INTRODUCTION
Pollination, transfer of pollen grains from the male structure of a
plant to the female structure of a plant. The pollen grains contain male
sex cells, or sperm, and 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. 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, is intricately sculpted
with designs that are unique for each plant species.
HOW POLLINATION WORKS
POLLEN
A pollen grain contains a sperm cell that
fertilizes an egg. If fertilization is successful, a
seed is produced. The pollen grains of each
species display unique sculpting of the pollen
wall, and fossilized pollen serves to identify
ancient species. The pollen grains shown here
are about 1000 times their actual size.
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.
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
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this time the scales gradually become brown and stiff, the cones typically associated with conifers. When the
seeds are mature, the scales separate and the mature seeds are dispersed by the wind or animals.
ROSE HIPS
When rose hips become ripe, they change in color from green to
red. Attracted by the red color, both birds and other mammals
eat the rose hips as a part of their diet. The individual seeds of
the rose hip have a tough outer skin that allows them to pass
through the digestive tract of an animal undigested, ensuring
successful wide-ranging dispersal.
Dorling Kindersley
In plants with flowers, such as roses, maple trees, and corn, pollen is produced within the male part
of the plant, called the stamen. The female sex cells, or eggs, are produced within the female part of the
plant, the carpel. With the help of wind, water, insects, birds, or small mammals, pollen is transferred from
the stamen to the stigma, a sticky surface on the carpel. Pollination is 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
carpel, and into the ovary, located at the base of the carpel. 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.
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
many species are capable of both.
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BRAZILIAN BIRTHWORT
The Brazilian birthwort uses insects as
pollinators. The putrid odor of this species—
like that of rotting flesh—especially attracts
flies, which enter the plant and become
trapped overnight. While they are trapped,
they become completely dusted with pollen.
They escape the following day as the plant
withers and are attracted to other Brazilian
birthworts, which they then inadvertently
pollinate as they enter and again become
trapped.
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. 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.
In self-pollination, pollen is transferred from the stamen to the carpel 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.
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.
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BURDOCK PLANT
The lesser burdock plant
has a fruit that is encased
in a burr covered with
hooks. These hooks
enable the burr to easily
attach to the fur of
passing animals, which
ensures wide-ranging
dispersal of the seeds.
Most plants, however, do not live close enough to one another to rely on the wind. These plants
employ pollinators—bees, butterflies, and other insects, as well as birds, bats, mice, and even fish—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. 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 in clumps 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.
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.
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