1. CLASS: X LAUREL HIGH SUBJECT: SCIENCE
How do organisms reproduce?
Chapter Notes
Reproduction is of two types: Asexual and Sexual Reproduction
Asexual Reproduction :
• Production of off springs by a single parent without the formation and fusion of gametes is called asexual
reproduction.
• It is a primitive type of reproduction in which offspring is produced by a cell or any vegetative organ of an
organism .
• In this type of reproduction off springs are genetically identical to their parents.
The different types of asexual reproduction are:
1. Binary Fission:
This is the simplest and most common method of asexual
reproduction seen in unicellular organisms. This occurs under
the favourable conditions of the environment. After the
organism grown to its full size, the parent divides into two
daughter cells which are genetically and morphologically
similar. During this process, the nucleus divides into two,
followed by the division of the cytoplasm. Ex: Amoeba,
Paramecium, Euglena etc.
1.1 BINARY FISSION IN LEISHMANIA
Leishmania is a flagellated protozoan .It is a unicellular eukaryote with
a well developed nucleus and other cell organelles. Leishmania causes
kala azar in humans. It divides by longitudinal binary fission because it
has a flagellum at one end of the cell. Due to this structure it results in
two daughter cells in the longitudinal plane.
1.2 Multiple Fission: In multiple fissions the parent nucleus undergoes
repeated divisions to form a large number of daughter nuclei. This is
followed by the division of the cytoplasm into as many parts as there
are nuclei, each part enclosing one nucleus. As a result a
number of daughter cells are formed from a single parent
cell at the same time. This process usually takes place
under unfavourable environmental conditions. The
multiple fissions occur in most algae, fungi and some
protozoans, Ex: Plasmodium (Malarial parasite)
2.REGENERATION
In this method a part of the body if the organism
if cut or broken can develop into a new
individual. Eg:-Hydra, Planaria, Star fish etc.
Regeneration is carried out by specialised cells.
These cells proliferate and make large number of
cells. From this mass of cells, different cells
undergo changes to become various cell types
and tissues. These changes take place in an
organised sequence referred to as development.
2. 3.FRAGMENTATION: In this method the body of a simple multi-cellular
organism breaks up into smaller pieces on maturation and each fragment
develops into new individuals.
Eg :- Spirogyra.
4.BUDDING
A small part of the body of parent organism grows out as a bud which then
detaches and become a new organism. For Ex:Hydra,Yeast
These organism use regenerating cells. First a small outgrowth called bud is formed on the side of its body by
division of itscells. This bud then grows gradually to form a small
hydra. Finally the tiny new hydra detaches itself from the body
of parent hydra and lives as a separate organism.
5.SPORE FORMATION
In multicellular organisms like fungus, ferns or mosses etc. the parent plant
produces hundreds of reproductive units called spores. These spores are
present inside tiny blob on a stick structures. The blobs are sporangia which
contain spores(cells) which eventually develops into new individual .the spores
are covered by thick walls that protect them until they come into contact with
another moist surface and begin to grow.
For Ex: Rhizopus, mucor(fungi),ferns, mosses(bread mould)
6.VEGETATIVE PROPAGATION: It is of two types
NATURAL ARTIFICIAL
The methods of plant multiplication with
the help of vegetative parts like
roots,stems or leaves that occur naturally
in plants.
The process of growing many plants from one plant by man made
method is called vegetative propagation.
1. ROOTS: Roots of some plants develop
adventitious buds which grow to form new
plants. Ex:Dahlia, sweet potato etc.
2. Stems:Plants are grown with the help of
stems
3. Leaves: In Bryophyllum , the leaves bear
buds in the marginal notches. As the leaf
comes in contact with the soil , the buds
sprout to form new plants.
1.Cutting:
A small part of plant which is removed by making a cut with sharp
knife is called cutting. A cut may be a piece of stem, root or leaves.
For example-The plant like rose, sugarcane, cactus are grown by
cutting. It is necessary that there are some buds on it.
2.Layering:
In this method a branch of plant is pulled towards the ground and a
part of it is covered with moist soil leaving the tip of branch
exposed above the ground. after some time new roots develop and
then it cut off from parent plant and grows as new plant. for
example- Jasmine, strawberry, hibiscus and guava etc.
3.Grafting:
It is the method in which the cut stem of two different plant one
with root and other without root and joined together in such a way
that the two stem joint and grows as a single plant. This new plant
has the characteristics of both the plants.The cut stem of plant
3. having root is called stock and the cut stem of another plant is
called scion. For example- Apple, peach, apricot, pear etc.
Advantages of vegetative propagation:
1. The new plant produced by artificial vegetative propagation will be exactly like parent plant.
2. The fruit trees grown from cutting or grafting start to bear fruits much earlier.
3. Many plants can be grown from just one parent plant.
4. We can also get seedless plant by artificial propagation.
Disadvantages of vegetative propagation:
1. Overcrowding: It causes overcrowding and hence severe competition.
2. Diseases: Diseases from the parent spread to all off springs.
3. Absence of variations: Due to absence of variations, asexually reproduced organisms are unable to adapt
themselves to changes in the environment.
4. Characters: Undesirable characters cannot be eliminated. Desired characters cannot be introduced.
Tissue culture (Micro propagation):
The production of new plants from a small piece of plant tissue removed from a growing tip of plant in a suitable
growth medium (culture or jelly like synthetic medium) is called tissue culture.
Procedure:
• The tissue is placed on a culture medium which contain nutrients and plant hormones, which make the cells
in the plant tissue to divide rapidly o form callus.
• This callus is transfer to another medium containing plant hormone which stimulates the callus to develop
roots.
• Then it is put on another medium containing another plant hormone which stimulates the development of
shoot.
• The plantlets are then transplanted into soil where they can grow to form mature plant. This technique is
used to the production of ornamental plants like orchids, carnation etc.
Advantages:
1. It is a very fast technique.
2. New plants produced by tissue culture are disease free.
3. Very little space is needed.
4. By this method plants can be grown around the year irrespective of weather or seasons.
Advantages of Asexual Reproduction:
1. A large number of individuals are produced within a short period from a single parent.
2. The offspring’s are genetically identical to the parent.
3. It occurs only through simple mitotic division.
4. It helps in dispersal of young ones to far off places.
5. It also helps the animal to tide over unfavourable environmental conditions.
Disadvantages of Asexual Reproduction:
1. Continuous binary fission for several generations makes the daughter individuals genetically weak and requires
rejuvenation.
2. Animals produced by asexual reproduction are generally less adaptable to changing environmental conditions.
3. Since the genetic constitution of the daughter individuals is similar to the parent there is no genetic variation in
the off springs and hence does not contribute to speciation.
4. SEXUAL REPRODUCTION IN FLOWERING PLANTS
The reproductive parts of angiosperms are located in the flower.
There are two types of floral organs:
a) Accessory and non-essential floral organs: Sepals and petals are called accessory organs. They have no direct
function in sexual reproduction.
b) Essential floral organs: Stamens and carpels are essential floral organs. Stamen is male reproductive part and
carpel is female reproductive part of the plant.
TYPES OF FLOWERS:
CARPEL
1. The female part of the flower is called carpel. Each carpel has a swollen base like structure called ovary, a
stalk like structure style and a terminal receptor part called stigma.
2. Stigma is sticky, hairy or feathery to pick up pollen grains.
3. Ovary contains one or many ovules.
4. An ovule consists of an embryo sac.
5. The embryo sac has actually three celled egg apparatus (one female gamete and two synergids), a central
cell with two polar nuclei and three antipodal cells.
STAMEN:
1. The male part consists of a stalk like structure called filament and a terminal knob like structure called the
anther.
2. There are four pollen sacs in an anther.
3. They produce yellowish coloured powder of haploid cells called pollen grains through meiotic
divisions.(Reductional)
4. Each pollen grain has a tube cell with tube nucleus and two male gametes.
POLLINATION
AGENTS OF POLLINATION
a) Wind: Anemophily
b) Water: Hydrophily
c) Insects: Entomophily
d) Birds: Ornithophily
e) Bats: Chiropterophily
5. FERTILISATION
1. After falling over stigma, the pollen grain absorbs water and nutrients. It produces a tube called pollen
tube.
2. Pollen tube reaches through style and reaches the ovary.
3. Pollen tube contains a tube nucleus and two male gametes.
4. The pollen tube reaches the embryo sac and one male gamete fertilises with the egg cell to form a
zygote and the other male gamete fertilises with the central cells ( two polar nuclei) to produce
endosperm .This is known as triple fusion. This type of fertilisation is known as double fertilisation.
5. Double fertilisation is fusion of two male gametes brought by a pollen tube with two different structures
of the same embryo sac , one forming embryo and the other forming endosperm.
6. The zygote develops into embryo and endosperm provides nourishment for the growing embryo.
POST FERTILISATION EVENTS
1. After fertilisation, the zygote divides several times to form an embryo within the ovule.
2. The ovule develops a tough coat and is gradually converted into a seed.
3. The ovary grows rapidly and ripens to form a fruit.
4. Meanwhile, the petals, sepals, stamens, style and stigma may shrivel and fall off.
STRUTURE OF A SEED
COTYLEDONS(SEED LEAVES)
Cotyledons are referred to as seed leaves because they
act as the leaves of the seed during the process of
germination. This in the sense that, as the leaves of
plants provide food for the plants, so does the
cotyledons provide food for the germinating plant. This
is possible because the cotyledons store food necessary
used during this process.
