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Plants Reproduction

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Plants Reproduction

Sexual reproduction is not just limited to humans and other animals; plants also undergo sexual reproduction. Just like sexual reproduction in humans, gametes (sex cells) from both the male and female fuse to form potential offspring. Let’s take a look at this in some more detail.

What is the structure of a flower?

The flower is the reproductive organ of angiosperms (flowering plants); most flowers possess both male and female reproductive organs (monoecious) while some only possess one type (dioecious).

[INSERT FLOWER STRUCTURE IMAGE HERE]

A typical flower consists of four whorls (parts): the calyx, corolla, androecium and gynoecium.

  • The sepals, collectively known as the calyx, make up the outermost whorl of the flower.

  • The second outermost whorl is made up of the petals, collectively known as the corolla. Flowers with petals in multiples of three are known as monocots, while those with petals in multiples of four or five are known as dicots.

  • The androecium is the third whorl of the flower which contains the male reproductive structures, including stamens and anthers.

  • The gynoecium makes up the innermost whorl – this contains the female reproductive system, including the stigma, style, and ovary, collectively known as the carpel.

Male gametes are produced in the anthers of the flower and can be found in grains of pollen, while female gametes are produced in the ovary of the flower and are stored in the ovules.

How does the ovule develop?

  1. An ovule begins as a group of cells called the nucellus. Within the nucellus resides the embryo sac mother cell, which divides to form four haploid cells. Remember that a haploid cell contains just one copy of chromosomes - in the case, just the female copies.

  2. Three of these cells die; the remaining young embryo sac cell grows larger.

  3. The nucleus of this cell divides multiple times to form an 8-haploid nuclei.

  4. This cluster of nuclei is rearranged to form a megagametophyte, where two nuclei move to the centre to become the polar nuclei. Three cells become antipodal cells (provide nutrients to endosperm), two become synergids (guide pollen tube growth) while the egg cell is formed between the synergids.

Further development and differentiation (specialisation) form an inner and outer integument that surrounds and protects the nucellus. A small opening at the tip of the ovule remains, called the micropyle.

[INSERT OVULE DEVELOPMENT IMAGE HERE]

What are the steps of sexual reproduction in flowering plants?

1. Pollination

As we learned in the previous sections, the male gametes are made in the pollen grain and the female gametes are made in the embryo sac. Both are non-motile but are required to be brought together to be fertilised. This is achieved through pollination, which is the transfer of pollen grains (that have been shed by the anther) to the stigma.

Pollination can either occur through cross-pollination or self-pollination. Cross-pollination occurs when the pollen from the anther of one plant is transferred to the stigma of a different plant. Transfer can be via wind or insects, such as bees. Self-pollination occurs when the pollen from the anther on a plant is transferred to the stigma on the same plant.

There are advantages and disadvantages of each pollination method:

Self-pollination

Cross-pollination

Advantage

Self-pollination is more reliable since the distance for the pollen to travel is relatively short which results in more successful fertilisation, compared to a plant that is not very close.

Cross-pollination results in greater genetic variation as there is breeding between individuals – this is genetically favourable.

Disadvantage

Since both gametes come from the same parent, it will result in inbreeding. Thus, any disadvantageous recessive characteristics in the parent will be greatly amplified, potentially resulting in defects in the plant.

Cross-pollination is relatively unreliable as the distance between plants is variable and can be large, resulting in less successful fertilisation events.

Angiosperms have evolved strategies to favour cross-pollination:

  • Some plants are dioecious – which means that they either possess a stamen or carpel i.e. either male or female.

  • In monoecious plants, the timing of the development of female and male reproductive systems is different, which reduces the likelihood of self-pollination. As discussed above, self-pollination can result in inbreeding.

  • Protandry: the development of stamens or pollen release occurs before the maturation of carpels or the ability of carpels to receive pollen

  • Protogyny: the stigma and carpels mature before the stamens or pollen release.

  • If self-pollination does occur, some reproductive structures may not grow or function well, such as the pollen tube. This prevents self-fertilisation.

2. Double fertilisation

Pollen grains contain two cells: the pollen tube cell and the generative cell. These two structures are involved in initiating the process of double fertilisation:

  1. When the pollen grain is released from the anther and lands on a stigma, a pollen tube grows from the pollen tube cell. The growth of this tube is controlled by the tube nucleus located at the tip of the tube.

  2. The pollen tube grows downwards in response to chemicals secreted by the synergids in the ovary (chemotropism). Chemotropism is the growth of a plant or plant part in response to chemical stimuli.

  3. As the pollen tube extends, the generative nucleus undergoes division by mitosis, producing two male haploid gametes (sperm cells).

  4. Eventually, the pollen tube enters the ovule through the micropyle before penetrating the embryo sac wall.

  5. The tip of the pollen tube bursts, resulting in one of the gametes fertilising the egg cell – a diploid zygote is produced in the process.

  6. The other gamete fuses with the two polar nuclei of the ovule, producing the triploid primary endosperm nucleus.

These two fertilisation events are known as double fertilisation. The newly fertilised ovule is known as the seed, and the ovarian tissue envelopes the seed, becoming the fruit.

[INSERT DOUBLE FERTILISATION IMAGE HERE]

3. Embryonic development

Embryonic development begins soon after double fertilisation.

The newly formed zygote divides numerous times to form an embryo, before differentiating (specialising) to become a plumule (embryonic shoot), radicle (embryonic root), and cotyledons (seed leaves). It is attached to the embryo sac wall by a supporting structure, called the suspensor.

The triploid primary endosperm nucleus undergoes mitotic division to form an endosperm, which provides a regular supply of nutrients and plant hormones to the embryo. The nucellus is crushed as the embryo sac expands, transferring all of its nutrients to the embryo and endosperm for development. The testa (seed coat) is formed from the integuments enveloping the embryo sac. This seed coat is very tough to ensure that the seed is well protected.

The ovary becomes the fruit as the ovary wall forms the pericarp (fruit wall). The fruit protects the seed and aids in seed dispersal. The sweet and colourful nature of fruits encourages animals to eat the fruit and scatter the seeds.

4. Germination

Through imbibition (water uptake), the seed takes up water through the micropyle. The uptake of water is crucial in the first steps of germination. Water allows the hydrolysis of biomolecules to form reactants used in cellular respiration, as well as activating a number of enzymes used in cellular reactions. The embryonic shoot, root and cotyledons respire and grow. Soon, the young shoot pushes its first leaves out of the seed into sunlight, allowing photosynthesis to take over.

[INSERT GERMINATION IMAGE HERE]

What are the advantages of sexual reproduction over asexual reproduction?

Asexual reproduction in plants is a form of reproduction that does not require the fusion of male and female gametes. It produces plants that are genetically identical to the parent plant. Plants may reproduce asexually from a fragment or cutting of the plant. Refer to the Reproduction article to learn more about asexual reproduction in plants.

Advantages

Disadvantages

Sexual reproduction results in greater genetic variation in populations. This variation allows the species to adapt to new environments – providing a survival advantage over asexually-reproducing plants.

If an individual is isolated from other plants it may not be able to reproduce through sexual reproduction, whereas an isolated plant reproducing asexually would be able to.

A disease outbreak would be less likely to affect all individuals in a sexually-reproducing population due to the genetic variability, but it would likely affect most individuals in asexually-reproducing populations

Sexual reproduction is less time and energy-efficient when compared to asexual reproduction.

Plants Reproduction - Key takeaways

  • Most flowering plants are monoecious; they possess both male and female reproductive systems. These reproductive systems are found in the flower of the plant.

  • Male gametes are produced in the anthers and female gametes are produced in the ovaries

  • Ovules develop from a mass of cells called the nucellus through a process of differentiation.

  • Cross-pollination is preferred over self-pollination due to genetic advantages, and many methods have been evolved by angiosperms to favour cross-pollination.

  • Double fertilisation occurs when one male gamete fertilises an egg forming a diploid zygote, while another male gamete fuses with two polar nuclei.

Frequently Asked Questions about Plants Reproduction

Sexual reproduction in flowering plants is the production of a new seedling through the fusion of male and female gametes. It produces offspring that are variable and genetically diverse among populations.

The ovule is an organ in flowering plants that produces a seed upon fertilisation. It contains a mass of cells, called the nucellus, and is surrounded by integuments. A small opening at the tip of the ovule is known as the micropyle, which allows the pollen tube to enter during fertilisation.

Through imbibition (water uptake), the seed takes up water through the micropyle. The embryonic shoot, root and cotyledons respire and grow. Soon, the young shoot pushes its first leaves out of the seed into the sunlight, allowing photosynthesis to take over.

Sexual reproduction results in greater genetic diversity in populations. This variation allows the species to adapt to new environments – providing a survival advantage over asexually-reproducing plants, A disease would be less likely to affect all individuals in a population due to the genetic variability, however it would likely affect most individuals in asexually-reproducing plants.

Cross-pollination results in greater genetic variation as there is breeding between individuals – this is genetically favourable. 

Final Plants Reproduction Quiz

Question

What term best describes the female reproductive system of a flower?

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Answer

Gynoecium

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Question

What is the role of synergids during fertilisation?

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Answer

Promote the growth and extension of the pollen tube by secreting chemicals

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Question

What is the ‘testa’?

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Answer

Seed coating

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Question

What is the ‘pericarp’?

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Answer

Fruit wall

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Question

What is the difference between monoecious and dioecious flowers?

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Answer

Monoecious flowers possess both male and female reproductive organs while dioecious flowers only possess one type; either male or female.

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Question

Another term for ‘flowering plant’ is angiosperm. True or false?

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Answer

True.

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Question

The outermost whorl of a flower contains the androecium. True or false?

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Answer

False, the sepals, collectively known as the calyx, make up the outermost whorl of the flower

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Question

The gynoecium contains the male reproductive system, while the androecium contains the female reproductive system. True or false?

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Answer

False, androecium contains the male reproductive system, while the gynoecium contains the female reproductive system.

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Question

Fill in the blanks. The androecium is the _____ whorl of the flower which contains the male reproductive structures, including _____ and anthers. 

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Answer

Third, stamens.

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Question

Fill in the blanks. The gynoecium makes up the _________ whorl – this contains the female reproductive system, including the stigma, style, and ovary, collectively known as the ______.

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Answer

Innermost, carpel.

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Question

What is the role of the endosperm during embryo development?

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Answer

The endosperm provides a regular supply of nutrients and plant hormones to the embryo.

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Question

Why is water uptake so crucial in the first stages of germination?

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Answer

Water allows the hydrolysis of biomolecules to form reactants used in cellular respiration, as well as activating a number of enzymes used in cellular reactions. The embryonic shoot, root and cotyledons respire and grow.

Show question

Question

Fill in the blanks. During ovule development, the inner and outer integuments form, which surround and protect the ______. A small opening at the tip of the ovule remains, called the _______.

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Answer

Nucellus, micropyle.

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Question

Why may self-pollination be considered a disadvantage for some plants?

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Answer

Since both gametes come from the same parent, it will result in inbreeding. Thus, any disadvantageous recessive characteristics in the parent will be greatly amplified, potentially resulting in defects in the plant.

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