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BIOLOGY FORM THREE TOPIC 6: REPRODUCTION

Concept of Reproduction
Reproduction
is the ability of living organisms to form new individuals of the same
species from those already in existence. Here, the new organisms replace
those that have died and so life continues. It can also be defined as
the process whereby organisms produce new individuals of the same
species. It is one of the important features of living things.
The Merits and Demerits of Sexual and Asexual Reproduction
Explain the merits and demerits of sexual and asexual reproduction
Sexual Reproduction
This
is a type of reproduction in which new organism is produced when a male
gamete fuses with a female gamete. Sexual reproduction involves the
fusion of two gametes. The process of fusion of gametes is called
fertilization.
These
two gametes differ in form and function and each is produced from a
different organ. In animals the gametes producing organs are called
gonads. These include the ovaries and testes. In flowering plant
structures concerned with the production of gametes are the ovaries and
anthers. The testes and anthers produce the male gametes while ovaries
produce the female gametes.
Merits of Sexual Reproduction
  1. It ensures genetic stability
  2. It ensures perpetuation of life
  3. It brings variation
  4. Leads to the interaction among organisms
Demerits of Sexual Reproduction
  1. Offspring have a great chance of inheriting diseases from the parent
  2. The reproduction takes long time
  3. It needs energy
  4. The sexual reproduction produces few numbers of offspring
  5. It depends on presences of two parents
  6. It leads to great chance of spreading diseases
  7. It takes a long time until offspring are produced
Asexual Reproduction
This
is the type of reproduction whereby production of offspring is from
single organism without the production of gametes. There is no fusion of
gametes.
Examples
of organisms who produce asexually are amoeba and bacteria. Asexual
reproduction involves only one individual organism. That means no change
of genetic material is passed from a parent to an offspring. The
offspring are also identical to their parents.
In this kind of reproduction, in same organisms, body part such as roots, stems or leaves may become reproductive body organs.
Depending on the type of organisms asexual reproduction may be of different forms or ways such as:
  • Fission
  • Sporulation/Spore formation
  • Budding
  • Fragmentation
  • Vegetative propagation
  • Binnary Fission (Splitting)
  • Suckers
  • Bulbils
Binary Fission (Splitting)
This is an asexual reproduction in which an organism divides into two equal parts, which are identical to each other.
Each part then grows to attain the original size of the parent cell and hence become a separate and independent organism.
Fragmentation
This
is a form of asexual reproduction in which organisms (parent) breaks
into two or more parts. Fragments grow and develop into a new organism
with identical features as the parent. Example worms such as Nematodes
and flat worms.
Sporulation/Spore formation
Sporulation
is asexual reproduction by the use of spores. The spore develops from
single cell as a result of mitosis, forming a structure known as
sporangium. When the sporangium is truly developed, the wall bursts to
release the spores which when placed in suitable area they germinate
into new organisms.
Other organisms, which reproduce by sporulation, are ferns and mosses.
Budding
Budding
is a type of asexual reproduction in which a new organism arises as an
outgrowth (bud) of the older organism (parent). The bud later separates
from the parent and grows to become an independent organism to attain
the size of the parent. Examples: yeast and hydra
Some
flowering plants reproduce through the formation of structure called
buds. Roots may form such buds, leaves or underground stems and such
buds sprout to form new independent plants.
Vegetative propagation
Vegetative
propagation is a form of asexual reproduction found in plants in which a
bud grows and develops into a new plant. The detached plant, root, stem
or leaves at some stages grows and develops into an independent plant.
Artificial vegetables propagation:
This is vegetative propagation, which occurs through man’s
manipulation. Man can learn from plants’ natural vegetative propagation
and can intervene and make propagation of plants artificially.
Natural vegetative propagation: Vegetative propagation involves different parts of plants as discussed below. Such as:
Bulb
In this type each bud grows to form a shoot, which produces a new bulb at the end of the growing season.
Bulb is a modified underground shoot having fresh strong leaves
Tubers
These
are short swollen underground storage organs formed from a stem or a
root. New tubers are made at the end of the growing season but do not
arise from old tubers.
Stem tubers
These
are short swollen underground stems, which store food, such as starch.
Normally, yam plants form a number of tubers each of which can rise to a
new plant. Such new plant continues to live after the death of the
parent.
Root tubers
These
are swollen adventitious underground roots. Roots tubers such as sweet
potatoes and cassava store their food in root tubers and do not bear
leaves or bud.
Rhizomes
These
are horizontally growing underground stems, bearing leaves, buds and
adventitious roots. Examples are lilies, ferns and grass.
Stolons
These
are slender stems, creeping horizontally as they grow along the ground
surface. Examples: strawberries, black currant and oxalis.
Suckers
These
are short horizontal branches, arising from the main stem or just below
ground level. Suckers contain food reserves. Examples: Bananas, sisal
and pineapples
Tap root
A taproot is the main root that arises from a radical. Tap roots may become swollen and act as storage organs.
Tillers
These
are collection of shoots. Grass plants consist of a number of tillers.
Each tiller has a number of leaves, which arise from the stem of nodes
at the base of the leaves.
Leaves
Some
plants such as cactus propagate vegetative using leaves. When the
leaves fall off from a plant they develop adventitious roots and buds,
which later may grow and develop into mature plants.
Merits of Asexual Reproduction
  1. Asexual reproduction results into an individual with the same genetic constitution as their parent.
  2. Its offspring matures faster than sexually reproduced organisms
  3. It does not depend on processes of pollination, seed or fruit dispersal
Demerits of Asexual Reproduction
  1. Asexually reproducing organisms are at a great risk to perish or get destructed when environmental conditions are unfavorable
  2. The
    parents may pass undesirable characteristics to the offspring since
    only one individual organism is involved in asexual reproduction
  3. Competition for resources such as food and shelter may occur due to large number of organisms being produced
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