Different Modes of Plant Reproduction

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.