GROWTH
Seed germination
Seed germination is the process by which plant grows from the seed.
Parts of the seed
The seed consists of the following parts
a.
An embryo: this part consists of a radicle that develops into a shoot after germination
b.
A seed coat(testa):it is a protective layer surrounding a seed
c.
Cotyledons: this part grows to form the first embryonic leaves after germination. It also stores and provides food for young plants.
d.
Micropyle: This is a pore through which
water enters the seed during germination
e.
Hilum: This is a scar that shows the
seed’s point of attachment to the ovary wall.
Changes which occur during seed germination
During germination, physical
and chemical changes occur in a seed.
The physical changes include
Ø Swelling of the seed after absorption of water through the micropyle.
Ø The water absorbed
by the seed softens the seed coat and activates enzymes in the cotyledons.
Ø This process is followed by the bursting of the seed coat and the emergence of the radicle
and plumule.
Ø The radicle is the one that forms a primary root and the plumule gives rise to a shoot.
Ø The radicle continues to elongate and give rise to many side roots.
Conditions necessary for seed germination
For a seed to germinate, it requires certain conditions.
These conditions include
the following:
Water
Germination occurs when a seed is supplied with water. A
seed absorbs water through the
micropyle. Water plays an important role
during seed germination. When a seed absorbs water, the embryo cells enlarge due to pressure. Such action leads to the rapture of the seed coat. Water softens
the seed coat, hence increases seed
permeability. Water also activates enzymes necessary for seed germination and growth.
Oxygen gas
Oxygen gas is necessary for aerobic respiration to generate energy
that is required for the growth of the embryo of a seed.
Temperature
Seeds need the optimum temperature to germinate. The
seed of each plant species has its optimum
temperature for germination. Temperature is essential for the activation of enzymatic reactions
during respiration.
Enzymes and hormones
The viability of seeds depends on the presence of the
relevant enzymes and hormones in the required
amounts. These help to break seed dormancy and catalyze all
metabolic activities associated with the process
of germination
Types of seed germination
There are two types of seed germination
i.
Epigeal germination
ii. Hypogeal germination
i.
Epigeal germination
This is the type
of germination in which the cotyledons of the seed are pushed above the ground.
This type of germination is seen in many dicotyledonous
plants, such as pea, beans, sunflower, castor, and bean.
ii. Hypogeal germination
Hypogeal germination is a type of germination
in which the cotyledons remain
underground. It occurs in plants such as maize, pigeon peas, wheat, etc.
