Plant Reproduction

Overview

1. Sexual reproduction in plants
2. Flower structure
3. Pollination
4. Fertilisation
5. Dispersal
6. Germination
7. Asexual reproduction in plants

Objectives

By the end of this lesson you will be able to:

1. Understand the difference between sexual and asexual reproduction in plants.
2. Understand the functions of the parts of a flower.
3. Understand how pollen is transferred from one flower to another.
4. Understand how fertilisation takes place in a flower.
5. Understand how and why seeds are dispersed.
6. Understand the conditions necessary for germination.

1. Sexual reproduction in plants
   
   Sexual reproduction takes place in nearly all organisms. Two different sexes, male and female, are involved in sexual reproduction. The male and female produce specialised sex cells called gametes. In humans the male gametes or sperm are produced in the testes. Female gametes or ova are produced in the ovaries of the female. Flowers also have specialised organs that produce gametes. The sex organs of a plant are located in the flower. The pollen contains the male gametes. The ovary contains the female gametes. In humans, fertilisation takes place when the male gamete fuses or joins with the female gamete to form a single cell called a zygote. The zygote grows into a child. Fertilisation also takes place in plants when the male gamete fuses with the female gamete to form a zygote. The zygote grows into a seed. The zygote divides into many cells, which will eventually grow into an adult organism.
   
   
2. Flower structure
   
   Male flower structure
   The sex organs are contained inside the flower. The sepals are there to protect the flower while it is in bud. The petals which are brightly coloured are there to attract insects by their colour and their scent. The stamens are the male organs and are made of two parts, the anther and the filament. Pollen grains are produced inside the anthers. The filament is there to hold up the anther.
   
   Female flower structure
   The carpel is the female organ and produces the female gametes. The carpel is composed of three parts - the stigma, the style and the ovary. The pollen lands on the stigma. The style connects the stigma to the ovary. The ovary contains the ovules. The ovule contains the female gamete. The nectary secretes sugar to attract insects. Different flowers have different sizes and numbers of sepals, petals, carpels and stamens but all of them have the same basic structure.
   
   
3. Pollination
   
   Insect pollination
   For fertilisation to take place, the male gamete inside the pollen grain must reach the female gamete inside the ovule. The transfer of pollen from the anthers to the stigma is called pollination. Insects can carry pollen. Insects such as bees are attracted to a flower by the scent and colour of the petals. The bee lands on the flower and feeds on the nectar. Nectar is a sweet sugary substance that the flower provides for the insects. Pollen from the anther is transferred to the bee's back. The bee flies to another flower of the same type, again attracted by the colour and the scent. While feeding on the second flower the stigma collects the pollen from the bee's back. Daisies, dandelions and roses are examples of plants that depend on insects for pollination. Moths and butterflies also pollinate flowers.
   
   Wind pollination
   Pollen can also be transferred from the anther to the stigma by the wind. In a wind pollinated flower the stamen releases light grains of pollen that are carried aloft by the wind. The feathery stigmas of the flowers trap the pollen grains as they float by. Only pollen from the same type of flower can grow on the stigma. Grasses are an example of plants that depend on the wind for pollination. In grass, flowers are grouped together in twos and threes forming spikelets. The flowers are small but they have the same basic structure as an insect pollinated flower. They have ovaries, stigmas, stamens and petals.
   
   Differences between wind and insect pollinated flowers
   Flowers that depend on insect pollination are different from wind pollinated flowers. Insect pollinated flowers have coloured scented petals. The stamens are located inside the flower. The stigma is also located inside the flower. They produce nectar for visiting insects. Wind pollinated flowers are smaller and have green petals. The stamens hang outside the flower and produce small light pollen grains. The feathery stigmas are also on the outside of the flower to catch the pollen as it floats by.
   
   
4. Fertilisation
   
   Fertilisation takes place after pollination. The male nucleus inside the tube fuses with the female nucleus in the ovule to form a zygote. A tube grows out of the pollen grain on the stigma and down through the style to the ovary. The male gamete is contained inside the pollen tube. As it penetrates the wall of the ovary the tube grows towards the ovule. The tube enters the ovule through a tiny hole called the microphyle. The male nucleus inside the tube fuses with the female nucleus in the ovule to form a zygote. This is fertilisation and results in the formation of a zygote.
   After fertilisation the fertilised ovule grows into a seed. As the petals and stamens wither and die the ovary wall forms the fruit. Plants produce two different types of fruit, fleshy and dry. Tomatoes, apples and pears are examples of fleshy fruits. Hazel nuts, peas and dandelion seeds are examples of dry fruits.
   
   
5. Dispersal
   
   Reasons for dispersal
   The seeds of a plant must be dispersed or scattered as far from the parent plant as possible. Otherwise too many plants grow close together in a small area and the parents will compete with the seedlings for space, light and minerals. To prevent competition between the parent plant and its seedlings, the seedlings are scattered or dispersed away from the parent. This gives them a better chance of survival. Seeds and fruits are scattered by different methods.
   
   Mechanisms of seed dispersal
   Dandelions and thistle seeds are scattered by the wind. Animals disperse seeds by eating the fleshy fruits and passing the seeds out later. Self-dispersal is when plants use their own methods to scatter seeds. Peas are scattered by exploding pea-pods. Plants that live in water, such as the water lily, produce seeds that float and are then carried downstream by the current. When the seed lands and conditions are suitable, it will grow or germinate.
   
   
6. Germination
   
   Conditions for germination
   Many seeds are dispersed in the autumn and lie dormant in the ground during the winter. In the spring, the extra light and heat cause the seed to germinate. Seeds need warmth, oxygen and water to grow. This can be demonstrated by setting up the following experiment. Collect four test tubes and label them as A, B, C and D.
   
   
   • Test tube A contains cress seeds on moist cotton wool. These seeds have warmth, moisture and oxygen.
   • Test tube B contains cress seeds on dry cotton wool. These seeds have warmth and oxygen but no water.
   • Test tube C contains cress seeds in cold boiled water. Water that has been boiled has no oxygen. The layer of oil on the top of the water prevents oxygen in the air from dissolving. These seeds have warmth and moisture but no oxygen.
   • Test tube D contains cress seeds on moist cotton wool and is be placed in a fridge. These seeds have oxygen and water but no heat.
   
   The tubes are left for a few days and are then examined. It is observed that only the seeds in Test tube A will have germinated. Thus we may conclude that heat, water and oxygen are needed for germination.
   
   Germination of a pea
   If we soak some dried peas overnight and then place them on moist cotton wool, we can observe the stages of germination. As the pea germinates, the seed coat or testa splits and the root or radicle grows out. As the root lengthens the shoot or plumule then emerges. The root develops and the shoot pushes upwards, forming leaves. The seedling grows into a full adult plant which will produce its own flowers and seeds.
   Life cycle of a plant
   The life cycle of a plant describes the process of a seed growing into a plant, pollination and fertilisation in the plant, the dispersal of seeds and the germination of those seeds. Consider the life cycle of a sweet pea. In the spring, when conditions are right, the seeds germinate and grow. In the summer, the sweet pea produces flowers, which are pollinated and fertilised. Fruits and seeds form. In the autumn, the seeds are dispersed and the parent plant dies. The seeds lie dormant in the ground during the winter until the spring arrives, when the cycle starts over again.

7. Asexual reproduction in plants
   
   Reproduction can take place without gametes and without fertilisation. This is called asexual reproduction. When we take cuttings from a plant we are helping the plant to reproduce asexually. Cuttings when planted will grow into a new plant identical to the parent plant. Strawberries propagate themselves by runners.
   

   Summary

   This lesson has covered:
   1. In sexual reproduction the male and female produce specialised sex cells called gametes.
   2. Fertilisation takes place when the male gamete fuses or joins with the female gamete to form a single cell called a zygote.
   3. The sex organs of a plant are contained inside the flower and consist of the sepals, petals, stamens (produce the pollen with the male gametes) and carpels (which produce the ovule with the female gamete).
   4. The transfer of pollen from the anthers to the stigma is called pollination.
   5. Pollen can be transferred by wind or by insects.
   6. Insect pollinated flowers have colourful petals and nectar to attract insects. The stamens and carpels are inside the flower.
   7. Wind pollinated flowers have feathery stigmas to collect pollen and stamens that hang outside the flower.
   8. After pollination a tube grows from the pollen grain to the ovule carrying the male nucleus to the female nucleus and fertilisation takes place.
   9. The fertilised ovule grows into a seed and the ovary forms the fruit.
   10. Fruits and seeds are dispersed by different methods: wind dispersal (for dandelions), animal dispersal (for berries), self-dispersal (for pea-pods) and water dispersal (for water-lilies).
   11. The conditions needed for germination are water, heat and oxygen.
   12. The stages in the germination of a pea are firstly the testa or seed coat splits and the root or radicle emerges. Next the shoot or plumule emerges and grows above ground. Finally the root develops and leaves form on the shoot.
   13. The life cycle of a plant describes the process of a seed growing into a plant and the plant forming seeds, which are dispersed to start the cycle again.
   14. Asexual reproduction can take place without gametes and without fertilisation.