Plant Reproduction

Asexual reproduction involves a single parent producing offspring without any contribution from another offspring. This type of reproduction gives many offspring that are very similar to each other and to the parent because they have identical genes Genes are the coded instructions, found inside the cell nucleus, that determine how the organism will grow and what features it will have. So if offspring have identical genes, they will have identical features and will be clones of each other and the parent that produced them. 

E.g. dandelions reproduce like this. 

The term asexual means without the use of gametes or sex cells (without sex) and so no fertilisation is involved. It is sometimes called vegetative reproduction and involves the separation of parts of the parent plant, which then grow into new plants.

Examples are:

• Bulbs such as garlic where each clove will grow into plant

• Underground runners as seen in dune grass and strawberries which produce new shoots and roots

• Tubers as in potatoes are enlargements of the end of a rhizome (underground stem) that specialise in food storage

• Cuttings can be taken from stems, leaves or roots; they are usually dipped in a hormone that promotes roots to grow from the cut end.

The new plants grown by these types of reproduction are identical to the parent plant. Not only will they look the same but also they will be susceptible to the same diseases as the parent.

Sexual reproduction involves combining genes from two parents. There are two types of sex cells (also known as gametes), that carry the genetic information required for reproduction. The smaller type of cells are called the male gametes (sperm), and they need to be moved to where the larger female gametes (eggs) are located. The male gamete fuses (joins) with the female gamete in an event called fertilisation. This brings about a new combination of genetic material and thus variation in offspring. 

Sexual reproduction is more difficult than asexual reproduction because of the problem of ensuring that male gametes are safely transferred. But it does produce offspring htat are different from each other and from their parents. This diversity can be important in the survival of a species when conditions change. Some of the varying individuals will be better adapted to the new conditions and thrive, eventually producing offspring. 

Flowering plants carry out sexual reproduction using flowers, but many of these plants can also reproduce asexually (e.g. by producing tubers or bulbs).

Flowers are the reproductive parts of plants. Most flowers contain both male and female organs. The passionfruit flower shown has both male and female organs. 

A few species, like the kiwifruit plant, have male and female flowers on separate plants (they are said to be dioecious). So you need both a male and a female plant!

The basic function of each part of the flower is outlined here:

• Petals - attract pollinator

• Sepals - protect flower bud

• Anther (male) - produce pollen grains which hold male gamete

• Fiilament (male) - holds up anther

(ANTHER + FILAMENT = STAMEN)

• Stigma (female) - catches the pollen either by being sticky or feathery

• Style (female) - tube that joins stigma to ovary, holds stigma up to receive pollen

(STIGMA + STYLE = CARPEL)

• Ovary and ovule (female) - the ovule holds the female gamete.

Male gametes are found inside the tiny pollen grains. The pollen grains need to be moved from the anther of one plant to the stigma of another plant of the same species. This helps ensure that the offspring plants are all different from each other and the two parent plants. Transferring pollen grains to a flower on another plant of the same species is called cross-pollination. Some flowers will pollinate themselves (self-pollination) if cross-pollination fails. This will produce seeds that can grow into new plants, but they will all be identical to the parent and each other. To prevent self-pollination, some plants make a chemical that stops pollen from the same flower developing on the stigma. Others have the stamen and carpel maturing at different times within the same flower.

Cross-pollination (i.e. the pollen goes to a stigma on a flower of another plant of same species) produces a greater variety in offspring which helps in colonisation of new environments, improved competition with other plants and production of better quality seeds.

Once a pollen grain lands on the stigma (top of the female organ), the pollen grain germinates. This means that the pollen grain absorbs water and nutrients to begin growing  a pollen tube down the style until it reaches a female gamete (egg) inside the ovary. The egg is enclosed in the ovule.

Two male gametes (sperm) travel down the pollen tube and one will enter the egg. Its nucleus will fuse with the egg's nucleus to form a new cell called the zygote. This event is called fertilisation, and it is the start of a new plant with different genes from either parent plant. The zygote will grow by cell division and develop into an embryo plant.

The other male gamete that went down the pollen tube fuses with a cell in the embryo sac and develops into the endosperm that will provide nourishment for the developing embryo.