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).

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.

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:

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.

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.

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.