Introduction
The various modes of plant reproduction include sexual reproduction (also known as sexual reproduction) in plants, asexual reproduction and asexual reproduction. Sexual reproduction produces offspring that are genetically identical to each other while asexual reproduction produces offspring that are genetically distinct from each other.
The reproduction of organisms by asexual reproduction is common in nature, but when it occurs in humans, it is termed asexual reproduction. Asexual reproduction can be either clonal (also known as clonal reproduction) or parthenogenic (also known as parthenogenic reproduction).
In general, sexual reproduction is rare in plants, and asexual reproduction (through parthenogenesis, asexual reproduction, apomixis, and allogamy) is much more common. Clonal reproduction includes both vegetative propagation and propagation by seeds, whereas asexual reproduction may be either clonal (by the division of a single individual) or apomictic (by the fusion of two genetically distinct gametes). In apomictic reproduction, the resulting offspring do not contain genetic information from either parent. Sexually, plants reproduce by the fusion of gametes.
Plant Reproduction
Asexual reproduction is usually termed seed reproduction because plants can often reproduce asexually by means of seeds. Seeds of many plants contain genetic information for the production of a plant of the same species. Seeds are more successful at surviving harsh environments because they contain nutrients and energy that are ready to be used for a new plant when the seed falls to the ground. Seeds are the only means of reproduction for most flowering plants, the only exception being the seedless flowering plant the sunflower.
In plants, the act of reproduction is called a zygote. The zygote can be produced in a number of ways, including fertilisation of two cells, double fertilisation of two sperm and egg cells, and parthenogenesis. In seed plants, the two parental species usually have very different reproductive mechanisms and must first come together to fuse their nuclei and then produce an embryo. Sexual reproduction in flowering plants involves either dioecious or monoecious species. In dioecious species, male and female flowers grow on separate plants. Monoecious plants produce both male and female flowers on the same plant.
Dioecious Plants
In dioecious species, the two parental species usually have very different reproductive mechanisms and must first come together to fuse their nuclei and then produce an embryo. Male plants usually contain two sets of chromosomes, one from each parent. Each male plant usually has one or two genetically distinct pollen and one or two genetically distinct sperm.
Male plants produce pollen, which then swims through the air to another male plant and, if the pollen is successful, it will grow into an embryo containing its own paternal set of chromosomes. The pollen must be successful in finding a female partner because if no successful sperm are found the plant will not produce seeds.
If pollen fertilises a female plant but is unsuccessful, the plant will have the same number of chromosomes as the male plant, but a different set of DNA. The offspring will be a dizygotic (two sets of chromosomes) monoecious plant. This is a very rare type of reproduction and is often genetically unstable, leading to poor progeny and variable traits. This type of reproduction is also found in fungi.
Dioecious plants are rare in nature. In dioecious plants, the male and female plants are separate, meaning that if the two are not pollinated by another plant or by wind, the plant will produce only females or males. Dioecious plants have males and females on separate plants and are not monoecious.
In dioecious plants, the male flower has a pollinator while the female flower lacks one. For example, the apple, pear, strawberry, fig, cherry, grape, and rose are all dioecious, because males only grow on these plants. The apple and pear are pollinated by moths, wind, or flying insects, while the strawberry is pollinated by bees and hummingbirds. Female strawberry plants are not visited by male fruit flies, so only female plants reproduce. Because of this, dioecious plants are difficult to breed and are hard to find.
Monoecious Plants
In monoecious species, the two parental species usually have very different reproductive mechanisms and must first come together to fuse their nuclei and then produce an embryo. In monoecious species, the males and females are separate plants and produce seeds independently. Monoecious plants are usually found only in plants in the family of Caryophyllaceae, which includes the common ice plants.
In the plant, male and female reproductive organs are both on the same plant. Male and female flowers grow on separate plants and are pollinated by a type of fly called the flower fly. Monoecious plants are dioecious because male and female flowers grow on different plants. There is a small, inconspicuous structure on the male flower called the stamen that produces pollen. In order for the plant to reproduce, the male plant needs pollen from the female plant to fertilise the female.
To be successful, the male plant must then get close to the female plant and then open its corolla, a type of flower, which exposes its pollen-carrying stamen. The fly, usually attracted by nectar, finds the male and female plants and opens the corolla. The fly deposits its sperm on the pollen and then lays eggs, effectively fertilising both male and female plants. Monoecious plants are often pollinated by wind or bees.
Reproduction
The production of new offspring of plants is called plant reproduction. Reproduction ensures continuity of species even after several generations.
Modes of Reproduction : asexual and sexual.
Differences Between Asexual and Sexual Reproduction
Asexual Reproduction
Many plants reproduce asexually in which a part of the parent plant is involved to produce a new plant.
Modes of Asexual Reproduction
Vegetative propagation
Budding
Fragmentation
Spore formation
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.
Modifications in Stem Which can Reproduce Vegetatively are:
Bulb - It is a 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.
Examples:
i. Cutting - Rose, Chrysanthemum
ii. Layering - Jasmine, bougainvillaea
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 favourable conditions to form new plants. E.g bread mould, mushroom.
Advantages of Asexual Reproduction:
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.
Sexual Reproduction
When male and female gametes fuse together, it is known as sexual reproduction.
Reproductive Parts of a Plant
Flowers are the parts which bear organs of reproduction. There are four whorls in a flower; sepals, petals, stamens and pistils.
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 a 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 produce 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, goldmohur
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.
Fertilisation
After pollination, fusion of male and female gametes takes place inside the ovary. This is called fertilisation.
The resultant product is zygote.
Steps of Fertilisation
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 a zygote. The second male gamete fuses with the nucleus in the central cell and forms the endosperm. This phenomenon is called double fertilisation.
Zygote develops into an embryo.
The ovule develops into a seed and the ovary becomes the fruit.
Post Fertilisation Changes
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.
FAQs on Different Modes of Plant Reproduction
1. What are the two main modes of reproduction in plants?
Plants reproduce through two primary modes: asexual reproduction and sexual reproduction. Asexual reproduction involves a single parent producing offspring that are genetically identical to it. Sexual reproduction involves the fusion of male and female gametes, typically from two parents, resulting in offspring with genetic variation.
2. What is asexual reproduction in plants and what are its common types?
Asexual reproduction is a method where a new plant is generated from a part of a single parent plant, without the involvement of gametes. The offspring are exact copies of the parent. The main types include:
- Vegetative Propagation: New plants grow from vegetative parts like roots, stems, or leaves.
- Budding: A small outgrowth, or bud, forms on the parent organism (like yeast) and detaches to become a new individual.
- Fragmentation: The parent body breaks into two or more fragments, and each fragment develops into a new plant. Example: Spirogyra.
- Spore Formation: The parent plant produces tiny spores which can grow into new plants under favourable conditions. Example: Ferns and fungi like bread mould.
3. What are the key stages involved in the sexual reproduction of flowering plants?
Sexual reproduction in flowering plants follows a sequence of key stages:
- Pollination: The transfer of pollen grains from the anther to the stigma of a flower.
- Fertilisation: After pollination, a male gamete from the pollen grain fuses with the female gamete (egg) inside the ovule. This fusion is called fertilisation and results in a zygote.
- Seed and Fruit Formation: The fertilised ovule develops into a seed, which contains the embryo. The ovary surrounding the ovule develops into a fruit, which protects the seed.
- Seed Dispersal and Germination: The seeds are scattered away from the parent plant by agents like wind, water, or animals. Under suitable conditions, the seed germinates to grow into a new plant.
4. What is the difference between a unisexual and a bisexual flower?
The main difference lies in the reproductive organs they possess. A unisexual flower contains only one type of reproductive organ—either the male part (stamen) or the female part (pistil). Examples include flowers of papaya and cucumber. A bisexual flower, on the other hand, contains both the male (stamen) and female (pistil) reproductive organs in the same flower. Examples include flowers of rose, mustard, and hibiscus.
5. How does pollination occur and what are its different types?
Pollination is the essential process of transferring pollen grains from the male anther to the female stigma. There are two main types:
- Self-Pollination: Occurs when pollen from the anther lands on the stigma of the same flower or another flower on the same plant.
- Cross-Pollination: Occurs when pollen from the anther of one flower is transferred to the stigma of a flower on a different plant of the same species. This transfer is carried out by agents like wind, water, insects, and birds.
6. Why is sexual reproduction considered more advantageous for a species than asexual reproduction?
Sexual reproduction is generally more advantageous because it creates genetic variation among the offspring. The mixing of genes from two different parents results in new combinations of traits. This diversity helps a species to adapt to changing environmental conditions, resist diseases, and evolve over time. In contrast, asexual reproduction produces genetically identical offspring, which may all be vulnerable to the same diseases or environmental changes.
7. How does the structure of a flower facilitate the process of pollination?
A flower's structure is highly adapted for pollination. For example, insect-pollinated flowers often have brightly coloured petals and produce sweet nectar to attract insects. Their stigmas are often sticky to catch pollen from an insect's body. In contrast, wind-pollinated flowers are typically small, lack bright colours, and produce large amounts of light, dry pollen that can be easily carried by the wind. Their stigmas are often large and feathery to effectively trap airborne pollen.
8. What is the significance of 'double fertilisation' in angiosperms?
Double fertilisation is a unique process in flowering plants (angiosperms) and holds great significance. It involves two separate fusion events in the ovule:
- One male gamete fuses with the egg cell to form the zygote, which develops into the embryo.
- The second male gamete fuses with the two polar nuclei in the central cell to form the endosperm.
9. What is the difference between vegetative propagation and spore formation?
While both are types of asexual reproduction, they differ fundamentally. Vegetative propagation involves the growth of a new plant from a multicellular vegetative part of the parent, such as a stem, root, or leaf (e.g., growing a potato from a tuber). Spore formation involves the production of single-celled reproductive units called spores, which are typically produced in large numbers and can survive harsh conditions before germinating into a new organism (e.g., bread mould growing from spores).
10. Can a plant use both sexual and asexual modes of reproduction? Explain with an example.
Yes, many plants are capable of reproducing both sexually and asexually. This allows them to benefit from the advantages of both methods. For example, a strawberry plant can reproduce asexually by sending out runners (stems) that grow into new plants, which is a form of vegetative propagation. At the same time, it also produces flowers that can be pollinated to produce seeds (sexual reproduction), allowing for genetic diversity.
