pollen between plants. This is called
cross-pollination. Other plants have
both male and female parts. They can
pollinate themselves. This is called selfpollination.
Pollen travels in many ways.Wind,
water, and animals can carry it. Many
plants have fragrant or colorful flowers
to attract birds or insects (especially
bees). These animals come to drink a
sweet liquid called nectar. While an animal
drinks the nectar, pollen from the
flower sticks to its body. The animal
Polk was 49
years old
when he
became president.
He was
the youngest
U.S. president
up to that
time.
A plant can pollinate itself if it has both stamens
and pistils.
116 Pollen BRITANNICA STUDENT ENCYCLOPEDIA
carries the pollen to other flowers when
it moves on for more food.
Hay Fever
Some people have an allergic reaction
when they breathe in pollen. They may
sneeze, get a stuffy nose, or have itchy
eyes. This condition is known as hay
fever. People with hay fever suffer most
during seasons when there is a lot of
pollen in the air.
#More to explore
Allergy • Conifer • Flower • Plant • Seed
Pollution
Pollution happens when the environment
is contaminated, or dirtied, by
waste, chemicals, and other harmful
substances. Pollution is a problem all
over the world. But it is especially bad in
large cities with a lot of industries and
automobiles. There are three main forms
of pollution: air, water, and land.
Air Pollution
Wildfires, volcanoes, and industrial
chemicals cause some air pollution. But
most air pollution comes from burning
fossil fuels. These include coal, oil, and
natural gas. Factories, electrical plants,
and automobiles burn these fuels for
power. The burning of fossil fuels may
release solid particles, such as ash and
soot, into the air. It also may release
harmful gases. This type of pollution
may be seen in the form of smog over
Most plants depend on a carrier, such as a bee, to bring pollen to them from another plant.
The smokestacks of chemical plants send
pollution into the air.
BRITANNICA STUDENT ENCYCLOPEDIA Pollution 117
big cities. The word smog is a combination
of “smoke” and “fog.” Smog-filled
air is hazy, or hard to see through.
Air pollution may cause such diseases as
cancer and asthma. Air pollution also
leads to acid rain, or polluted rain that
can harm living things. In addition, air
pollution may be a cause of global
warming, which is a steady rise in
Earth’s average temperature. Finally, air
pollution damages a part of the
atmosphere called the ozone layer. The
ozone layer is important because it
protects Earth against harmful rays
from the sun.
Water Pollution
Some causes of water pollution are easy
to see. People dump garbage and sewage
into creeks, rivers, ponds, lakes, and
oceans. Factories or cities sometimes
release oils, poisonous chemicals, and
other wastes into water.
Other causes of water pollution are not
so direct. The use of chemicals in farming
is one example. Farmers use some
chemicals as fertilizers, or substances
added to soil to help crops grow. They
use other chemicals as pesticides or herbicides.
These are substances that kill
pests or weeds. When these chemicals
seep into the ground, they may make
the groundwater unfit to drink. Chemicals
also may drain into rivers and lakes,
where they can harm fish and other
forms of life.
Land Pollution
Littering, or tossing garbage on the
ground, is a form of land pollution. Litter
is unpleasant to look at. It also can
destroy the habitats, or homes, of plants
and animals.
The buildup of dangerous chemicals in
the ground is another form of land pollution.
The chemicals may come from
farms or factories. These chemicals can
spread to plants and animals. They may
even harm people who eat the contaminated
plants and animals.
Controlling Pollution
Many governments, environmental
groups, and ordinary people are working
to control pollution. Governments have
passed laws to keep people from releasing
dangerous chemicals into the environment.
Some companies and people
are trying to use fewer fossil fuels.
Instead they are getting power from the
sun, wind, water, and other energy
sources that produce less pollution.
Many communities use recycling to
reduce pollution. Recycling is the process
of making new products out of used
Workers clean a seacoast that is coated
with oil spilled by a tanker ship.
118 Pollution BRITANNICA STUDENT ENCYCLOPEDIA
writer who helped him to describe his
travels in a book. Polo was released from
prison in 1299. He returned to Venice,
where he died on January 8, 1324.
#More to explore
China • Mongol Empire • Venice
Pomo
The Pomo are Native Americans of
northern California. There are about 72
separate tribes grouped together as
Pomo.
Pomo along the coast of the Pacific
Ocean traditionally lived in cone-shaped
homes made from the wood and bark of
redwood trees. The Pomo farther inland
lived in larger rectangular houses built
from poles, brush, and grass. The Pomo
fished and hunted deer, birds, and small
animals. They also gathered and ate
many types of wild plants.
Spanish explorers may have visited the
Pomo in 1542. In 1811 or 1812 Russian
fur traders founded Fort Ross in Pomo
