Reproduction ——————— Sexual —>is a process involving the fusion of two gametes(sex cells) to form a zygote and the production of offspring that are genetically
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Asexual —> is the process resulting in the production of genetically identical offspring from one parent
Asexual reproduction
- It does not involve sex (or the production of gametes or a zygote)
- eg, bacteria reproduce by cell fission where they simply divided into two and each daughter cell becomes an independent bacterium
- In unicellular organisms, the organism just divides into 2 whereas in complex organisms or multicellular organisms a part of the body grows and develops into a separate individual
Asexual reproduction in Fungi
Fungi have sexual and asexual methods of reproduction
In asexual methods they form single celled, haploid spores that are dispersed into the air. Here, upon achieving suitable conditions, the grow new hyphae which develops into a mycelium
Asexual reproduction in flowering plants
Asexual methods for plant reproductions knows a s vegetative propagation
Stolons and rhizomes
- Eg of stolons , strawberry and buttercup
- These plants have very short shoots and most of their shoot is below the ground
- These stems o these short shoots are called rootstocks
- The rootstock can give rise to leaves and flowers
- Once the main shoot grows, the lateral buds produce long shoots that grow horizontally over the ground.
- These shoots are called stolons and have only small leaves at their nodes and very long internodes
- At each node isa bud that is capable of producing shoots as well as roots.
- Thus a complete plant may develop at the node
- The plant may be nourished initially by it’s parent by food send through the stolon
- This changes however and the plant is able to fend for itself once the stolon dries up and withers.
- In may plats, horizontal shoots arise from lateral buds near the stem base and grow under the ground.
- Such underground horizontal stems are called rhizomes
- At the nods of the rhizomes are buds which may develop to produce shoots above the ground.
- The shoots become independent plants when the connecting rhizome dies
- Eg of rhizomes – cough crass, bracken
Bulbs and corms
- Eg of bulbs, daffodil and snowdrop
- They have very short shoots
- Their stem is only a few mm long and they they have leaves which encircle the stem that are thick and fleshy with stored food
- the food stored in the leaves is used up by a rapidly growing bud in spring and this produces a flowering stalk and a number of small leaves
- During the growing season, food made is sent to the leaf bases and stored
- These leaf bases swell and and form a new bulb ready for growth in the following year
- Vegetative reproduction occurs when some of the food is sent to a lateral bud as well as the leaf bases. The lateral bud grows inside the parent bulb and is all produce a new, independent plant in the following year.
- corms have a life cycle similar to that of bulbs, except that instead of the leaf bases, the stem swells up with stored food
- Vegetative reproduction takes place when a lateral bud on the short, fat stem grows into an independent plant
Potatoes are stem tubers. lateral buds as the base of the potato shoot produce underground shoots(rhizomes)
These rhizomes sweet u with stored stretch and form tubers. Since the tubers are stems, they have buds
If left in the ground, the tubers ca produce shoots, using food stored. Thus, potato can propagate vegetatively
Artificial Propagation
This is method to exploit vegetative reproduction in order to produce fresh stocks of plants
It does not occur naturally
There are 2 methods involved – cuttings and tissue culture
Cuttings
- This a method of producing new individuals from certain plants by inserting a cut end of a hoot into water or moist earth
- Roots grow the base of the stem and the shoot continues to grow and produce leaves.
- The cut end of the stem may be treated with rooting hormones to promote root growth
- Evaporation from the shoot may also be reduced by covering it with a polythene or glass jar
- Eg of plants produced by cuttings – carnations, germaniums and chrysanthemum
Tissue culture
- One a cell has become part of a tissue, it loss its ability to divide
- However the nucleus of the cell holds all the instructions for making a complete plant
- In lab conditions, a single cell is cultivated and induced to divide and grow not an independent plant
- One method is to take small pieces of the plant tissue from a root or stem and treat it with enzymes to separate it into individual checks
- The cells are provided with plant hormones to induce ceel division
- This eventually leads to the formation of roots, stems and leaves
- Another method is place a small piece of tissue on a nutrient jelly. This causes the cells in the tissue to start dividing to produce many cells, forming a shapeless mass called a callus.
- This callus is then provided with the appropriate hormones to cater for good growth of the plant.
- Tissue cultures are an advantage as large number of plants can be produced from a small amount of tissue.
- These plants also have the advantage that of being free from fungal or bacterial infections.
- The plants produced in this way are called clones of each other as they have been produced by the same parent
Asexual reproduction in animals
Eg – hydra
- A group of cells on the column of the hydra start dividing and form bulge
- The bulge develops tentacles
- The daughter hydra pulls itself off the parent
- The daughter becomes an independent animal
Advantages and disadvantages of asexual reproduction
Advantages Disadvantages
No mate is needed There is little variation created so adaptation to any environment is unlikely.
No gametes are needed The offspring is susceptible to diseases that parents might have
All the good characteristics are passed on from the parent to the offspring lack of dispersal can lead to competition for resources like water light and nutrients
When there is no dispersal, the offspring will grow
in the same favourable environment as the parent
The plants tore large amounts of food that allow rapid growth under suitable conditions
Sexual reproduction
Fertilisation is the fusion of gametes
Sexual reproduction involves the production of sex cells or gametes. The male and female gametes fuse together to form a zygote which eventually grows into a new individual
In flowering plants, the male gametes are found in pollen grains and the female gametes, called egg cells, in the ovules
In animals, male gametes are sperm and female gametes are eggs.
in both plants and animals, the male gamete is microscopic and mobile whereas the female gametes are larger than the male gametes and ar immobile
Pollination in plants and mating in ms animals causes the male and female to come close together
Chromosome numbers
Diploid number – somatic cell – 46 chromosomes
Haploid number – gametes – 3 chromosomes
The haploid chromosomes in the gametes fuse together to give a diploid number of chromosomes
Advantages Disadvantages
There is variation in the offspring, making adaptation feasible Two parents are usually needed
New varieties can be created(with disease resistance for example) The growth of a new plant from a seed is slow
Seeds are produced in plants, allowing dispersal which educes the risk of competition for resources
Sexual reproduction in plants
Male organs – Stamens
Female organs – Carpels
After fertilisation, the carpel modifies itself and some part of it becomes the fruit and contains the seeds.
In the flowers of most plants there are both stamens and carpels. Side they contain both male and female parts, they are known to be hermaphrodite or bisexual.
The process that brings the male and female gametes close in plants is known as pollination
Flower structure
Petals
- Brightly coloured and scented sometimes to attract insects for pollination
- Usually arranged in a circle or cylinder
- Have about 4 to 10 petals
- Flowers of grasses and many tress don’t have flowers but have small, leaf-like structures that enclose the reproductive organs
Sepals
- Present outside petals in the form of a ring
- They are often green
- Much smaller than the petal
- Protects the flower in its bud stage
Stamens
- Male reproductive organs
- Consists of a stalk called the filament along with an anther on the end
- Plants may have many or a few stamens
- Each anther consists of 4 pollen sacs in which pollen grains are produced by cell division
- When the pollen grains are ripe, the pollen sac bursts and release the pollen
Pollen
- Insect pollinated flowers produce smaller amounts pf pollen grain which are often round and sticky, or covered in tiny spikes to attach to the bodies of insects
- Wind pollinated flowers tend to produce larger amounts of pollen grains which are smooth and light so it may easily carried away by the wind; large amounts sea needed as they have a great chance of being lost
Carpels
- Female reproductive organs
- Plants may have many carpels or just one
- Consists of an ovary, bearing a style and stigma
- Inside the ovary are one or more ovules
- After fertilisation, the ovary becomes the fruit ad he ovules become the seeds
- The style and stigma project out from the top of the ovary
- The stigma has a stick surface and pollen grains stick to it during pollination
- The style need not be long
Receptacle
- All of the above flower structures are attached to the expanded end of a flower stalk. This is called the receptacle
- In some cases, after fertilisation , the receptacle becomes fleshy and edible
Pollination
Pollination is the transfer of pollen grains from the anther to the stigma
Insect pollination
- the plants are adapted to be able to become insect pollinated
- They have bright petals and a nice scent which attract a variety of insects
- Some flowers also produce nectar
- Some of them are adapted to be pollinated by one one type of insect(honeysuckle ad foxglove)
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