Reproduction - The production of new offsprings of plants is called plant reproduction. Reproduction ensures continuity of species even after several generations.
Modes of Reproduction : asexual and sexual.
Asexual Reproduction - Many plants reproduce asexually in which a part of the parent plant is involved to produce a new plant.
1. Vegetative Propagation - In higher plants vegetative parts of plants like roots, stems and leaves may give rise to new plants. This method of propagation is called vegetative propagation.
Natural Methods of Vegetative Propagation - Plant parts that can propagate naturally are:
Roots - In some plants, the roots are modified into tubers. In tubers roots form adventitious buds that have a tendency to grow into new plants. E.g sweet potato.
Stems - New plants may develop from nodes of stems which bear buds.
Bulb - It is swollen underground modification of the stem. E.g onion, garlic
Tuber - Underground stem has ‘eyes’ which contain buds that develop into new shoots. E.g potato
Rhizome - These are also underground stems that have nodes. E.g ginger
Runner - Some plants have horizontal stems that grow parallel to the ground. The nodes on the stem form buds which grow into new shoots. E.g strawberry
Leaf - In some plants adventitious buds are formed along leaf margins which fall on the ground and develop into new plants. E.g Bryophyllum, Begonia
Artificial Methods of Vegetative Propagation - Often gardeners use these methods to produce new and unique plants.
i. Cutting - Rose, Chrysanthemum
ii. Layering - Jasmine, bougainvillea
iii. Grafting - Mango, rose, citrus plants
iv. Tissue Culture/ Micropropagation - A part of the plant tissue is grown under laboratory conditions. Cells of tissue grow into an undifferentiated mass of cells called callus. Soon this callus differentiates into different parts of the plant. This method is used to produce disease free, rare and endangered species of plants. E.g orchids.
2. Budding - Organisms like yeast grow small outgrowths called buds on their bodies. The nucleus divides and one of the nuclei moves into the bud. Later the bud detaches from the parent cell and grows into an individual yeast cell.
3. Fragmentation - Some plants simply break into fragments, each capable of growing into a new individual plant. E.g Spirogyra
4. Spore formation - Plants like fungi reproduce by producing spores which germinate under favorable conditions to form new plants. E.g bread mould, mushroom.
The new plants produced are exact copies of parent plants; as there is no variation. Such plants are able to survive better in stable conditions.
Large numbers of plants can be produced in a short time.
There is no need for pollination and seed dispersal.
Some varieties of plants grow best with vegetative propagation such as seedless varieties of grapes, banana and pineapple.
When male and female gametes fuse together, it is known as sexual reproduction.
Flowers are the parts which bear organs of reproduction. There are four whorls in a flower; sepals, petals, stamens and pistil.
Calyx/ Sepals - These are green leaf like structures that protect the flower during the bud stage.
Corolla/ Petals - These are colourful parts of flowers. They attract insects for pollination.
Androecium - It forms the third whorl in flowers.
Male reproductive organ is called stamen. It produces male gametes.
A stamen has two parts; a long thin stalk called filament and a bilobed structure at the tip of filament and anther.
Inside the anther pollen grains are produced which contain male gametes.
Gynoecium - It forms the fourth whorl in flowers.
Female reproductive organ is called a pistil. It produces female gametes.
Pistil consists of ovary, style and stigma.
Ovary is a swollen basal part which contains ovules that produces female gametes.
Style is the tube like part that connects stigma to the ovary. It helps in passage of pollen grain to the ovary.
Stigma is the sticky receptacle that receives pollen grains during pollination.
Types of flowers - Depending on whether the stamens and pistil are present on the same flower or different flowers, flowers are of two types.
Bisexual Flowers - When both stamen and pistil are present on the same flower, it is called bisexual.
Examples: rose, mustard, Hibiscus, gulmohar
Unisexual Flowers - When both male and female parts are present on different flowers, they are called unisexual.
Examples: papaya, maize, cucumber
Pollination - It is the process of movement of pollen grains from anther to stigma. There are two types of pollination;
Self-Pollination - This refers to the pollen grains from anthers falling atop the stigma of the same flower.
Cross Pollination - This refers to the pollen grains from the anthers of one flower falling atop the stigma of yet another flower on the same or another plant.
Agents of Pollination - Transfer of pollen grains can be facilitated with the help of wind, water, insects, birds, reptiles and animals.
Wind Pollination - In some plants pollen grains are very light and dry. They get carried away by winds and are deposited on sticky stigmas. Such plants produce pollen grains in large numbers. The flowers are inconspicuous and lack nectar. Examples: maize, wheat, rice
Water Pollination - In aquatic plants pollination is carried out by water. Pollen grains fall on water and are transported to other flowers by water currents. Examples: hydrilla, coconut
Insect Pollination - Most flowers are brightly coloured and produce sweet smelling nectar that attract insects such as butterflies and bees. When an insect sits on a flower, the sticky pollen grains stick on their wings and legs. These pollen grains are carried from one flower to another where they fall on the stigmas. Examples: orchids, sunflower, pea
Birds like hummingbirds and sun birds also help in pollination.
Reptiles like garden lizards bring about pollination in some plants.
Mammals like squirrels are also agents of pollination.
After pollination fusion of male and female gametes take place inside the ovary. This is called fertilization.
The resultant product is zygote.
After landing on the stigma a pollen tube grows out from one end of pollen grain.
It moves down the style and enters the ovule through an opening called micropyle.
It carries two male gametes and deposits them into the embryo sac of the ovule.
One male gamete fuses with the egg and forms zygote. The second male gamete fuses with the nucleus in the central cell and forms the endosperm. This phenomenon is called double fertilization.
Zygote develops into an embryo.
The ovule develops into a seed and the ovary becomes the fruit.
Zygote develops into an embryo.
Ovary develops into fruit.
Seeds are formed inside the fruit.
Sepals and petals fall off.
Seed contains the embryo.
The seeds get dispersed by wind, water and insects. Under favourable conditions they germinate to produce new plants.