Unit A Reproduction

Figure 5 Male and female cones are
found on the same fir tree. The larger
female cone contains the developing seeds.
Separate Sexes
Many complex animals and some plants have separate female and male
individuals. The females produce eggs, and the males produce sperm.
Not all plants have male and female reproductive structures in the same
flower. Conifers, such as pine and fir trees, have separate male and female
cones on the same plant (Figure 5). Some species have separate male and
female flowers on the same plant. Other species, such as poplar trees, have
separate male and female flowers on separate plants.
In animals that have separate sexes, there are two methods of fertilization:
external fertilization and internal fertilization.
If you would like to learn
more about a salmon’s
life cycle, go to
www.science.nelson.com
GO
External Fertilization
In many aquatic animals, such as salmon, the female releases her eggs into the
water and the male then releases his sperm over the eggs. Since the sperm and the
eggs unite outside the body, this method is called external fertilization (Figure 6).
Hundreds or even thousands of eggs and sperm are produced to ensure that
enough will unite and grow to be adults. Both the gametes and the developing
young are vulnerable to environmental conditions and predators. GO
Did You
KNOW?
Slim Chance of Survival
Female salmon lay between 2500 and
7500 eggs in a nest. Because of the
many hazards they face, only four or
five fertilized eggs from a nest will
survive long enough to reproduce.
www.science.nelson.com
Figure 7 Thousands of coral animals
each release an egg to be fertilized.
GO
(a)
Figure 6 After a female sockeye salmon (top)
lays eggs in a nest she made in the gravel, the
male (bottom) fertilizes the eggs (a). The eggs are
fertilized externally when the male releases sperm
(seen here as a milky substance) over the eggs (b).
5 mm
A spectacular reproductive event occurs in coral reefs. Coral reefs are
made of millions of tiny animals, called corals. Each animal builds a hard
shell. These shells fuse together to make up a reef. Reproduction occurs when
the males and the females both release their eggs and sperm into the ocean at
the same time. Fertilization is by chance, as millions of tiny cells fill the water
and are carried by the movement of the water (Figure 7).
Internal Fertilization
Some aquatic animals, such as sharks, and most terrestrial (land) animals,
require the sperm from the male to be deposited inside the body of the
female. These animals have specialized reproductive structures to accomplish
this. (The reproductive structures of humans are discussed in Chapter 4.)
Since the union of the sperm and the egg occurs inside the female, this
method is called internal fertilization.
(b)
86 Unit A Reproduction
NEL