UNIT 16: A SEXUAL REPRODUCTION IN PLANTS
UNIT 16: ASEXUAL REPRODUCTION IN PLANTS
Key Unit Competence
Account for various methods of asexual reproduction as means of increasing crop
yield.
Learning Objectives
By the end of this unit, I should be able to:– Describe the various methods of asexual reproduction: fragmentation,Introductory activity
budding, and spore formation.
– Demonstrate asexual reproduction mechanisms in lower organisms.
– Discuss the advantages and disadvantages of asexual reproduction.
– Describe the characteristics of vegetative reproductive parts in a flowering
plant.
– Apply principles of artificial propagation in growing varieties of plants that
are economically important.
– Apply the knowledge acquired to produce economically important plants.– Appreciate the use of artificial propagation in increasing crop yields.
The kingdom Plantae comprises about 260,000 known species including flowering
and non-flowering plants. All plants have a general organization which includes
vegetative and reproductive organs. Plants reproduce through different ways:
Use the books and other source of information to1. Write on how lower organisms such unicellular plant and another like16.1. Asexual reproduction
cassava, sugar cane and apple reproduce.
2. Describe the techniques used by people to grow Irish potatoes, cassava
and bananas.
3. Describe each of the following methods of asexual reproduction:fragmentation, budding and spore formation.
Activity 16.1
Using textbooks or internet and search on asexual reproduction.
Asexual Reproduction
This is a type of reproduction without production of gametes. It usually results in the
production of identical offspring; the genetic variation arises as a result of random
mutation among the individuals.
Self-assessment 16.1
Write an essay on asexual reproduction.
16.2. Types of asexual reproduction
Activity 16.2.1
Discuss on asexual reproduction in lower organisms and write a report as an outof-class activity.
There are five common modes of asexual reproduction: fission, budding, vegetative
reproduction, spore formation and fragmentation.
16.2.1. Fission
An important form of fission is binary fission. In binary fission, the parent organism
is replaced by two daughter organisms, because it divides in two, i. e: Unicellular
Fungi. Another type of fission is multiple fission that occurs in many algae. The
nucleus of the parent cell divides several times by mitosis, producing several nuclei.
The cytoplasm then separates, creating multiple daughter cells.
16.2.2. Budding
Some cells split via budding resulting in a ‘mother’ and ‘daughter’ cell. The offspringorganism is smaller than the parent. Budding is also known on a multicellular level.
16.2.3. Vegetative reproduction
Vegetative reproduction is a type of asexual reproduction found in plants where
new individuals are formed without the production of seeds or spores. Examples
of vegetative reproduction include the formation of miniaturized plants called
plantlets on specialized leaves (for example in kalanchoe) and some produce new
plants out of rhizomes or stolon (for example in strawberry). Other plants reproduce
by forming bulbs or tubers (for example tulip bulbs and dahlia tubers). Some
plants produce adventitious shoots and suckers that form along their lateral roots.
Vegetative reproduction may form a clonal colony where all the individuals are
clones.
16.2.4. Spore formation
Many multicellular organisms form spores during their life cycle in a process called
sporogenesis. On the other hand, plants and many algae undergo sporic meiosis
where meiosis leads to the formation of haploid spores rather than gametes. These
spores grow into multicellular individuals called gametophytes. Fungi and some
algae can also utilize true asexual spore formation, which involves mitosis giving
rise to reproductive cells called mitospores that develop into a new organism after
dispersal.
Activity16.2.2
Demonstration of asexual reproduction by fragmentation in algae
Requirements:
Glass beakers of 500ml, Scalpel, Forceps, Pins, Spatula, Weighing balance, Labels,
Artificial, fertilizers, Clear river water and Spirogyra (algae).
Procedure1. Label five beakers of the same size as A, B, C, D and E. Pour water in eachDraw a table as this shown here and record your observation.
beaker. Weigh several measures of artificial fertilizers of 1 g each.
2. Transfer 1g of fertilizer to beaker A, then 2g to beaker B, 3g to beaker C,
4g to beaker D and 5g of fertilizers to beaker E. Note the concentration of
fertilizers is increasing from A- E.
3. Using forceps pick spirogyra and put it on a tile. Add several drops of
water to avoid drying. Tease off a piece of spirogyra using a pin. Cut that
piece into 5 fragments of the same length and transfer each piece into the
beaker.
4. Stand the beakers in a place where they can receive adequate sunlight for
the seven days. On the next day, start to examine the fragments in each
beaker every day and record any observable changes such the increase in
size of the spirogyra.Table16.1: Rate of growth
16.2.5. Fragmentation
Fragmentation is a form of asexual reproduction where a new organism grows
from a fragment of the parent. The fragment develops into a mature and fully
grown individual. Fragmentation is seen in many organisms such as fungiplants
and gemma in liverworts. Most lichens, which are a symbiotic union of a fungus
and photosynthetic algae, reproduce through fragmentation to ensure that new
individuals contain both symbionts. These fragments can take the form of soredia,
dust-like particles consisting of fungal hyphen wrapped around photobiont cells.
Self-assessment 16.2
1. Write a short essay on asexual reproduction.
2. What is fragmentation? give one example of plant which reproduces by
fragmentation
16.3. Advantages and disadvantages of asexual reproduction
16.3.1. Advantages of asexual reproduction.
Activity 16.3
Discuss on asexual reproduction in lower organisms and higher plants, and
outlining advantages and disadvantages.
Since asexual reproduction does not involve the process of gamete formation. It
can be completed much more quickly, allowing the individual to spread its genetic
material in a shorter period of time. It is also an advantage for small populations.
as there may not be an adequate number of plants to pollinate one another to
reproduce. If the parent plants are successful, it is a good idea to make clones of the
plant that can be successful as well.
16.3.2. Disadvantages of asexual reproduction.
Asexual reproduction does not have genetic diversity. With sexual reproduction,
mixing a gene pool can produce diversity. The organism becomes less adapted to
certain environmental changes when there is less variation produced in offspring.
The entire communities of the genetically identical organisms might not survive
such changes. Asexual reproduction usually leads to struggle for existence as well as
overcrowding. The main distinction of asexual reproduction from sexual reproduction
is that there is no need for two parents as well as special cells to reproduce. It means
that special the mechanisms of combining sex cells for fertilization is no longer
required. This form of reproduction process is simply mitosis that copy a parent
organism.
Self-assessment 16.3
Explain the advantages and disadvantages of asexual reproduction.
16.4. Vegetative and artificial propagation in flowering plants
Activity 16.4
Demonstration of asexual reproduction in plants by cuttings
Requirements
Growth medium or moist soil, sweet potatoes vines, elephant grass, sugarcane or
cassava stems, secateurs/sharp knife and rooting hormone.
Procedure1. Collect clean and healthy stems from cassava, sugarcane or potato plants.Draw and record what you will observe after 13 days on the development of roots
2. Using a secateurs/sharp knife, cut the stem of either cassava, sugarcane or
sweet potato stems into fragments of suitable sizes.
3. Place them in either suitable medium of growth with rooting hormone if
available or plant them in moist soil in the school garden.
4. Leave the set up for about 13 days, and then observe the development of
roots and leaves at nodes.
and leaves at nodes.
Artificial vegetative propagation is the deliberate production of new plants from
parts of old plants. This can be done by following three methods: Cutting, layering,
and grafting.
a. Cutting
This is a simple procedure in which part of the plant called a ‘cutting is removed
from the parent plant by cutting and placed in a suitable medium for grow. In this
method, one-year-old stem of root is cut from a distance of 20 to 30 cm and buried
in the moist soil in natural position. After sometime, roots develop from this cutting
and it grows into a new plant. This method is commonly used in rose and sugar cane.
Care is taken that nodes which were lower in parent plant (morphologically) are put
in the soil, while the morphologically higher nodes are kept up. Adventitious rootsare given off at the lower nodes.
b. Layering
This method of vegetative propagation is used in those plants with soft branches
such as jasmine plant. In this method, a branch of the plant which is near to the
ground is pulled towards the ground and covered with moist soil leaving the tip
of this branch above the ground. After sometime, roots develop from that part of
the branch which was buried in the soil. This branch is then cut of along with the
roots from the parent plant and develops into a new plant. This method of asexual
reproduction is also used in the production of plants such as Bougainvillea, guava,strawberries, lemon, China rose etc.
c. Grafting
In this method of vegetative propagation, the stems of two different plants are
joined together so as to produce a new plant containing the characters of both
plants. The plant of which the root system is taken is called ‘stock’, while the other
plant of which the shoot is selected is known as ‘scion’ or ‘graft’. These two stems i.e.
the stock and the scion are fitted together by making slanting cuts into them and
bound tightly with a piece of cloth and then covering it with a polythene sheet.
The diameter of the stock and scion chosen for grafting should be equal. Scion gets
the mineral and water from the soil through the stock and develops branches and
produce fruits. This method of propagation is used in mango, apple, banana, pear,grape, pineapple and peach.
Vegetative reproductive parts in flowering plants
The reproductive part of the plant is a flower. The union of male and female
gametes to form a zygote is called fertilization. The transfer of pollen grains from the
anther to the stigma of the same flower or the different flower is called pollination.
Plants reproduce asexually in a variety of ways. The vegetative reproductive parts
in flowering plant are stem, branches, and leaves and they have the following
characteristics:
Characteristics of Stem:
Stem develops from the plumule of embryo. It is generally the ascending part of
the plant axis. It bears a terminal bud for growth in length and is differentiated into
nodes and internodes. The stem nodes possess dissimilar appendages called leaves.
The young stem is green and capable of performing photosynthesis. In the mature
state, stem bears flowers and fruits. Leaves and stem branches develop exogenously.
Stem exposes leaves, flowers and fruits to their most suitable position in the aerial
environment for optimum function. Stems usually show positively phototropic,negatively geotropic and negatively hydrotropic.
Characteristics of Leaf– It is dissimilar lateral flattened outgrowth of the stem,
– The leaf is exogenous in origin
– It is borne on the stem in the region of a node,
– An axillary bud is often present in the axil of the leaf.
– Leaf has limited growth. An apical bud or a regular growing point is absent,– The leaf base may possess two lateral outgrowths called stipules,Characteristics of branches
– A leaf is differentiated into three parts: leaf base, petiole and lamina.
– The lamina possesses prominent vascular strands called veins,
– It is green and specialized to perform photosynthesis,
– Leaf bears abundant stomata for exchange of gases and it is the major seat oftranspiration.
A branch is a woody structural member connected to but not part of the central
trunk of a tree. Large branches are known as boughs and small branches are known
as twigs. Due to a broad range of species of trees, branches and twigs can be found
in many different shapes and sizes.
Self-assessment 16.41. Discuss on the methods of artificial vegetative propagation.16. 5. Application of artificial propagation in production of
2. Cassava produces flowers, fruits and seeds. Why people prefer to grow
cassava by cutting rather than using seed?
3. Describe the characteristics of vegetative reproductive parts in a floweringplant
improved varieties of plants
Activity 16.5
Use textbooks and other sources of information to discuss on application ofartificial propagation in production of improved varieties of plants.
Artificial vegetative propagation is usually used in agriculture for the propagation
(or reproduction) of those plants which produce either very few seeds or do not
produce viable seeds. Such plants include: Banana, Pineapple, Orange, Grape, Rose,
etc.
Reasons for preferring vegetative rather than sexual means of reproduction vary,
but commonly include greater ease and speed of propagation of certain plants.
Another major attraction is that the resulting plants are clones of the parent plants.
The methods of vegetative reproduction range from rooting cuttings to grafting
and artificial propagation by laboratory tissue culture. In horticulture, a “cutting” is
a piece that has been cut off from a mother plant and then caused to grow into a
whole plant. A popular use of grafting is to produce fruit trees, sometimes with more
than one variety of the same fruit species growing from the same stem. Rootstocksfor fruit trees are either seedlings or propagated by layering.
Self-assessment 16.5
Explain the application of artificial propagation in production of improved
varieties of plants.
End of unit assessment16
A. Multiple choice questions: choose the best answers.
1. In vegetative propagation, cuttings are mainly taken froma. Leaves of parent plant2. Artificial methods of vegetative propagation includes
b. Roots or stems of parent plant
c. Shoots of parent plant
d. Buds of parent planta. Cloning3. Example of plant in which vegetative propagation is occurred by leaves is
b. Grafting
c. Cuttings
d. Both b and c
calleda. Cannabis4. Which of the following is NOT an advantage of asexual reproduction?
b. Chrysanthemum
c. Bryophyllum
d. Brassicaa. Rapid reproduction.B.Questions with short and long answers
b. High genetic diversity.
c. No need for a mate.
d. Low resource investment in offspring.1. Name the plants which are grown by grafting method.
2. What do you understand by grafting?
3. Show how vegetative propagation takes place in potatoes?
4. Explain the method by which the sugarcane and rose are propagated.
5. Give the names of the different methods of artificial vegetative reproduction.
6. Explain the term vegetative reproduction and give one example of plantwhich reproduces by using this type of asexual reproduction.