Plant Reproduction – Sexual Reproduction
In sexual reproduction, sperm carried in the
pollen form the male part of a flower fuses
with the egg in the female part of the flower.
SR gives the plant species the means to
change with a changing environment. Every
time SR occurs there is a recombining of
genetic material.
Plants receiving genes that enable them to
adapt to the environment are more likely to
survive to pass onto their offspring.
Species evolve in order to survive.
Birds, insects, bats and other animals and
sometimes wind help in the pollination
process, which is the transfer of pollen from
the male to the female part of a plant.
When the pollen of a plant pollinates a flower
the same plant, it is called self-pollination.
Cross-pollination is when pollen from one
plant pollinates a flower on a different plant.
Once the pollen lands on the stigma, it grows
a thin pollen tube down the style to the
ovary. The cell within the grain of pollen
divides to form two sperm nuclei. These
travel down the pollen tube to the embryo sac
that holds the egg.
Flowering plants have double fertilization.
One occurs when one sperm fuses with the
egg. The sperm carries genetic material from
the male part of the flower and the eggs
contain genetic material from the female part.
When the sperm and the egg combine, a cell
called the zygote develops.
The other fertilization has the second sperm
nuclei fusing with 2 nuclei in the embryo sac,
it develops into the endosperm.
The ovule of the flower becomes the seed.
The offspring of genetically different parent
is said to be a hybrid.
The advantage to hybridizing is the best
traits of each parent may be expressed in the
offspring.
Genetic information is stored in every cell of a
plant or animal in long molecular chains made
of DNA (Deoxyribonucleic Acid)
Genes, segments of DNA, code for life
processes and the appearance of a plant.
Genes are arranged in a set of chromosomes.
Normal cells contain a double set of
chromosomes and are called diploid (2n).
Reproductive cells have a single set of
chromosomes and are haploid (1n).
When fertilization occurs, the sperm and the
egg both contribute to a single set of
chromosomes.
After fertilization, the ovary wall enlarges
and forms the fruit. Seeds are formed in the
fruit.
The major parts of the seed include:
cotyledon, seed coat, endosperm and the
embryo.
Seeds of a flowering plant have a seed coat
that surrounds the embryo and endosperm
and it prevents them from drying and
physical injury. It also plays a role in
determining when conditions for germination
are right.
Food in the form of starch and protein is
stored in the seed for the embryo.
Seeds are designed to wait for favorable
conditions to begin growth, this may take
years.
One mechanism seeds have to help ensure
survival is called stratification; this is where a
seed must go through a period of cold
temperatures before it will germinate.
(Example: tulips)
Another mechanism is scarification; this is
breaking down of the seed coat. For this to
happen, the seed must pass through the acid
stomach of an animal or lay in the soil where
microorganisms can eat away the seed coat.
(Example: geranium)
Environmental factors play key roles in seed
germination. These include: water, oxygen,
temperature and light.
Water stimulates chemical reactions within
the seed.
Oxygen is important because starches stored
in the seed are converted to energy through
cellular respiration.
Most plant seeds germinate at 60 to 80
degrees F.
Seed quality refers to two factors, viability
and vigor.
Viability is the ability of a seed to germinate
under optimal conditions.
Vigor is the ability of a seed to germinate
under different conditions but still produce
healthy seedlings.