REPRODUCTION
Reproduction is the process by which living organisms give rise to offspring of the same kind/species.
Importance of reproduction
1. Ensures the continuity of individual members of a species
2. Maintains good and desirable traits
of living things
3. It leads to perpetuation of life
on earth and increases the population of living organisms
4. It ensures that good desirable characteristics are maintained in the life of an organism.
Sexual reproduction.
Ø Sexual reproduction is a type of reproduction which involves the fusion of male and female gametes
Ø This type of reproduction involves two parents. The nucleus of the male gamete fuses with that of the female gamete
forming a zygote.
Ø The zygote undergoes
mitosis to become individual items. The fusion of male and female gametes
is called fertilization
Advantages of sexual
reproduction
Ø Creates variation between
organisms which are beneficial
Ø There is high chance of survival among living organisms
Ø Due to variations, organisms may develop
resistance to diseases
Disadvantages of sexual reproduction
Ø
Two parents must meet, male and
female which might be very difficult
Ø May lead to individual with undesired qualities. E.g. disease like hemophilia, sickle cell and anemia which
are genetically passed from the parents
Ø There is no genetic stability
Ø Time and energy
are consumed as it needs two organisms
Ø
In cases when certain organism
isolated from another it becomes difficult for sexual reproduction to take place.
Ø Gametes are lost because
there is no guarantee that the male and female
gametes would fuse to form a zygote.
Asexual reproduction.
Ø Asexual reproduction involves
only one parent.
Ø There is no fusion of male and female gametes.
Ø This type of reproduction is more common
in lower animals
such as amoeba.
Ø It usually involves
the division of cells.
Advantages of asexual reproduction
Ø No variations, hence desirable characteristics are maintained
Ø
The organisms produced by this method
grow and mature
faster
Ø Does not relay the process of pollination, which may be a hindrance to reproduction.
Ø It makes it possible for plants which do not form seed to be propagated vegetative.
Disadvantages of asexual reproduction
Ø
Because of faster rate,
it can lead to competition and overcrowding
Ø If the parent is affected by a certain disease, then it is passed to offspring because there is no variation
The difference between sexual
and asexual reproduction
|
Sexual reproduction |
Asexual reproduction |
|
Two parents are
involved |
A single parent
is involved |
|
Male and female gametes are produced during gametogenesis |
No gametes produced |
|
Diploid zygote is formed
after fertilization |
No zygote
formation |
|
Meiosis is essential for gamete formation |
Mitosis is essential for spore
formation |
|
There is variation of offsprings |
Offsprings are identical to parents |
|
It is slow in propagation |
It is rapid in favourable conditions |
|
Population number
increase slowly |
Population number
increase rapidly |
Vegetative propagation
Ø
This is a form
of asexual reproduction in plants
in which a bud
or part
of a plant grows and
develop into a new plant
vegetative reproduction
is also called
Types of vegetative propagation
Ø There are two types of vegetative reproduction which are natural and artificial vegetative propagation
Natural vegetative propagation
This involves different
parts of plants
to develop naturally
into independent plant
Types of natural vegetative propagation
This type of vegetative propagation consist of various forms
as follows
Vegetative propagation by bulbs
Ø A bulb is a modified
underground shoot, having
flesh strong leaves
Ø
Each bud grows to form a shoot which produces a new bulb at the end of the growing
season
Ø Onions are good examples in this form of propagation
Onion bulb ii). Vegetative propagation by tubers
Vegetative propagation by tubers
Ø This involves the formation of tubers which are short swollen underground storage organs formed
from a stem or root
Ø These tubers may develop into independent plants
Ø There are two types of tubers which are
Stem tubers: These are short swollen underground stems which store foods like starch. Example Irish potato
Stem tubers of Irish potato
Root tubers: These are swollen underground roots which store food like starch and they have no ability
to develop into new plants. Examples are sweet potatoes
and cassava
Root tuber of Sweet potatoes
Vegetative propagation by rhizomes
Rhizomes are horizontally growing underground stems
bearing leaves, buds and adventitious roots
This type of propagation takes place in lilies, ferns and grasses
Vegetative propagation by corms
Corns are short swollen, vertical underground stems with a flat base which has one or more buds that may
result in vegetative reproduction.
The common example is yam plant
Figure: Propagation by corms in yams
Vegetative propagation by suckers
Ø Suckers are short branches arising from the main stem just below the ground level
Bananas, sisal and pine apples reproduce vegetatively by suckers
Figure: Propagation by suckers in banana plant
Artificial vegetative propagation
•
This is the type of propagation
which is done by human being to serve his/her
own interest of getting food
•
This method also involves
different forms as follows
Vegetative propagation by cuttings
•
A cutting is a piece of a plant body which
can be used to produce
a new plant
•
Plants such as lemons, sweet potatoes, sugar cane and cassava are propagated by cutting
Figure: Propagation by cuttings in sugar cane
Advantages of vegetative propagation in plants
Ø Vegetative propagation is a more rapid, easier and a less expensive
method of multiplying plants
Ø It also helps us to introduce plants in new areas where seed germination fails to produce plants due to
change in the soil and environmental conditions.
Ø Plants like Bermuda
grass or doob grass which produce only a small quantity of seeds are mostly propagated vegetatively.
Ø Vegetative propagation is the only known method of multiplication in plants like banana, seedless grapes and oranges, rose and jasmine that have lost their capacity to produce
seeds through sexual
reproduction.
Ø Grafting permits the physical and physiological joining of separate individuals for the best economic
advantage i.e. good qualities of two varieties
can be combined in one composite plant.
Reproduction in flowering plants
Flowering plants or
angiosperms reproduce sexually, Flowering plants have specialized structures called
the flowers, which is used for the reproduction process.
HIBISCUS FLOWER
A flower comprises of the following parts
PEDICEL
It is a flower stalk attached to the plant. It is where
flower develops. If it is branched
so as to bear many flowers each branch is called radical.
RECEPTACLE
It is the top
of the flower stalk ‘pedicel
to which other parts
are attached.
CALYX/SEPAL
It
is the outermost ring of floral leaves.
Are usually great and protect
the inner floral
structure when the flower is not open. Some species of plants have flowers with rings of sepals. The
outermost ring is called the epicalyx
COROLLA
This is a ring of petals on a flower.
In some plants the petals
are brightly colored. They may fuse to form
corolla tube.
STAMEN
This is the male reproductive organs.
It consists of filament on top with a head of filament
called anther The anther contains pollen
grains inside. In the hibiscus flower there are many
stamens and filaments which join to
forma stamina tube, which is connected
to the receptacle.
CARPEL
This is the female reproductive organ, it consists of three parts called ovary, style and stigma -
Ovary contains eggs/ stigma.
- Style is a tube connected
to the ovary.
Dissection of a
Flower
Identify the relevant flower by assessing the floral
parts and location of the following
in the whole flower:
Ø
sepals forming the calyx;
Ø
receptacle is the swollen
part below the calyx;
Ø
flower stalk (pedicel);
Ø
petals that form a corolla;
Ø stamens with anthers and filaments; and pistil with stigma, style and ovary.
(a)
Dissection to half flower
(i)
Count the number of
sepals, petals and stigma.
(ii)
Cut the flower longitudinally
into half and separate it into two halves.
(iii)
Use the hand lens to observe
the sepals, styles, petals, stamen,
stigma, ovary and the ovules.
Diagram of the half dissected
flower
(b)
Dissection to display carpel
(i)
Use forceps to cut and
make a small opening between
the two sepals, then use hand to peel the calyx
completely from the flower.
(ii)
Detach the petals by pulling each one downwards towards the receptacle until the entire
corolla is removed
from the flower.
(iii)
Use forceps to cut and make a
small opening at the base of the staminal
tube, then expose the ovary with your finger nails by pilling off the staminal tube of the flower around the ovary.
(iv)
Use needle to cut at the base of the staminal
tube, then extend
the incision to the base of the stigma. Care must be taken not to damage the ovary and the style under it.
(v)
Repeat the cuttings as much as
possible, and then pull apart the
strands to completely remove the staminal tube from the flower.
(vi)
Look at the flower you will see
stigma, style and ovary collectively known as the carpel.
(vii)
Observe the ovary, stigma, and
style of your flower with a magnifying glass.
(c)
Dissection to display stamen
(i)
Use forceps to cut and make
a small opening
between the two sepals, then use hand to peel the calyx completely from the flower.
(ii)
Detach the petals by pulling each one downwards
towards the receptacle until the entire
corolla is removed
from the flower.
(iii)Detach the filament and use the hand
lens to observe anther and filament.
