Reproduction in plants

Plants reproduce sexually when a pollen grain from the male part of the flower fuses with the ovule from the female part of a flower. Plants will rely on insects or the wind to transfer their pollen to different plants. Some plants can reproduce asexually by growing runners. Gardeners may clone plants to produce a large number of genetically identical plants with desirable characteristics.

Sexual vs asexual reproduction

Sexual reproduction involves the fusion of two gametes (sex cells) from two parents. Gametes are created by meiosis which produces cells with half the genetic information (haploid). When two haploid gametes join during fertilisation, an organism is produced which is genetically distinct from either of its parents.

Asexual reproduction occurs when cells from one parent divide by mitosis to produce a whole new organism. Since there is no mixing of genetic information, the offspring is genetically identical (a clone) of its parent. Organisms which can reproduce asexually include yeast, bacteria and fungi. Even some plants can produce clones of themselves, in a process called vegetative propagation. The disadvantage to asexual reproduction is that there is no variation in the offspring, making them equally susceptible to harmful environmental pressures such as disease or drought. However, it is a much quicker method than sexual reproduction as the organism doesn’t need to waste time finding a mate.

Sexual reproduction in plants

Sexual reproduction in plants requires the male gamete (the pollen) from one flower coming into contact with the female gamete (the ovule) of another flower. Since plants are unable to move physically, they rely on insects or the wind to do this job for them.

Insect-pollinated flowers

These flowers have bright petals and sweet-smelling nectar to attract insects to the reproductive parts of the plant. When the insects land on the flower to collect its nectar, pollen grains will brush onto the insect from the anther of the flower. When the insect moves onto another flower, the pollen grains are transferred to the stigma, which has a sticky surface to catch the pollen grains.

Wind-pollinated flowers

Plants that use the wind for pollination tend to look a bit more boring (think of grasses and wheat) as they have no need to attract insects. Instead each anther is attached to a long stamen, which swing about in the breeze. A gust of wind easily carries the pollen grains away, which are small and light, to (hopefully) land on the stigma of another flower. These stigmas are long and feathery, giving them a large surface area for catching pollen.

Fertilisation

When a pollen grain lands on a stigma, a pollen tube grows down the length of the style through the action of digestive enzymes ‘eating away’ at it. This forms a tunnel through the style which allows the pollen to pass through to reach the ovary. The pollen tube penetrates the ovule (egg) at a small opening called the micropyle, allowing the nucleus of the pollen to fuse with the nucleus of the ovule during fertilisation. After fertilisation:

• the ovules becomes the seeds

• the ovary becomes the fruit

Seeds have a huge food store which provide energy for the seedling until it is able to photosynthesise to produce its own food.

Asexual reproduction in plants

Instead of producing a new plant through fertilisation of a pollen grain and ovule, some plants can reproduce simply by growing more cells by mitosis to create genetically identical offspring or clones. Some plants, such as the strawberry plant do this by forming ‘runners’ which are off-shoots or side branches which young plants grow from.

Plants can also reproduce asexually by artificial methods, such as taking cuttings. This is a simple and easy way to clone a plant and is a common practice among gardeners. It involves removing a branch from the parent plant and dipping the end in rooting powder. The rooting powder contains plant hormones which stimulate fresh roots to develop. The cutting is kept in moist and warm conditions and planted in compost once roots appear.

The other method of cloning plants is by micropropagation (also known as tissue culture). This method is a little more complicated but has the advantage of producing large numbers of genetically identical plants at any time of year. Using a scalpel, small pieces of plant tissue (explants) are removed from the parent plant. The explants are grown in a petri dish containing sterile agar jelly with plant hormones and nutrients. The explants grow into small plants (plantlets) which can be transferred to compost.

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