Sexual reproduction in Angiosperms

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1 Sexual reproduction in Angiosperms Name: ANGIOSPERMS Angiosperms are plants that have their seeds enclosed in an ovule inside their flower. About 80% of the plants we see and know are angiosperms. The other type of plant is a gymnosperm and instead of having their seeds in flowers, their seeds are exposed, such as the cones of a pine tree. Gymnosperms have no flowers. Angiosperms have flowers that have male parts and female parts. This allows them to be able to reproduce sexually, as we will learn about later. The Outer Parts of an Angiosperm Flower There is a huge variety of angiosperms with very different looking flowers but all flowers have these structures: The peduncle is the stalk or stem that attaches the flower to the plant. The receptacle is the top part of the stalk where all the flower parts attach to on the peduncle. The sepals are the small green leaves that protect a new flower bud. The petals are the colourful parts that attract pollinators such and bees, birds and butterflies. DID YOU KNOW? A gamete or sex cell is a special cell like an egg or sperm cell (reproductive cells). Every cell in your body has DNA (deoxyribonucleic acid) that contains your genetic code. Gametes also contain DNA but only half a set of DNA. When a male gamete and a female gamete join or fuse together, the half of the female DNA joins with the half of the male DNA to make a new set of DNA which will become the DNA of the baby.

2 The Male Parts of the Angiosperm Flower The male structures include the stamen (anther and filament) and the pollen grains which contain the male sex cells. The stamen is made of the anther which rests on top of the filament. The anther makes pollen. Each pollen grain contains two male sex cells or male gametes. The Female Parts of the Angiosperm Flower The female structures include the pistil (stigma, style and ovary) and the ovule which contains the female sex cells. The pistil is made of the stigma which rests on top of the style. The style is attached to the ovary. Inside the ovary there is an ovule or ovules, depending on the plant, and inside the ovule are the female sex cells or female gametes. ACTIVITY Fill in the missing labels in the diagram below. Colour the male parts blue and the female parts pink. ANGIOSPERM FLOWERS Cross section of typical flower stamen pistil

3 POLLINATION AND FERTILISATION In order for a seed to form, a male gamete must fuse, or join, with a female gamete. This is the process of FERTILISATION. In order for fertilisation to take place, the male gamete has to get to the female gamete in the female part of the flower. This is the process of POLLINATION. 1. Pollination Stamens are the male part of a flower and produce pollen in sacclike structures called anthers. The anther (of the male part of the flower) produces pollen. The pollen grains each contain two male gametes and rest on top of the anther. The pollen not only needs to move away from the anther, but it needs to be transferred to the stigma (female part) of another flower. The plant needs help to do this. Here are some ways that plants receive help in transferring their pollen. a) Zinnia and butterfly b) Hibiscus and hummingbird c) Saguaro cactus and bat POLLINATION BY ANIMALS These animals are called pollinators. They could be insects (like bees and butterflies) or birds (like sunbirds). The plant is adapted to attract the pollinators to its flowers. Plants use their colourful petals, strong aroma (smell) and sweet nectar to attract insects and birds. Pollinators come to the flower to drink its sweet nectar. As they sit on the inside of the flower (insects) or put their face into the flower (birds), pollen grains brush off the anthers onto them. The pollinator moves from one flower to another. As they reach a new flower some of the pollen from previous flowers rubs off onto the stigma and the pollinator picks up new pollen.

4 POLLINATION BY WIND AND WATER Plants that use wind and water don t need the adaptations that will attract pollinators (big colourful flowers with a sweet smell and tasty nectar) but they have other adaptations that help them become pollinated more easily. Plants using water and wind to help them become pollinated need the following adaptations: They have many anthers with lots of pollen on them, which hang out of the flowers so that pollen can be swept away easily. Each plant has lots of flowers so the chances of pollination are greater more flowers mean more chance of pollination. Pollen grains are not sticky like in other flowers, rather they are smooth and dry, light and small so they can be carried easily by the wind and water. Stigmas are much bigger and look feathery which make it easier for them to catch the pollen grains. Plants using the water to help them with pollination must live in the water (they are aquatic plants). The pollen is released and floats on top of the water. The stigmas of another plant must be close to the surface of the water so that they can catch the pollen as it floats by. Why is pollination important for us? Most of our food comes from plants or from animals that eat plants. Plants need to reproduce so that there can be enough to supply the demand of providing food. Every piece of fruit or vegetable consumed, including wheat, comes from plants. These are only available to us because they have been able to reproduce. Reproduction can happen if pollination happens. We need to preserve our pollinators so that we can preserve our food crops. 2. Fertilisation Pollination is the transfer of pollen from one plant to another. What happens next is what makes it possible for a plant to produce seeds. Once the pollen grain has landed on the stigma, it releases chemicals that make a pollen tube from the top of the stigma to the ovary. Then the male gametes travel from inside the pollen grain down the tube and into the ovary. Inside the ovary, the pollen tube goes into the ovule which contains the embryo sac which has the female gametes in it.

5 One of the male gametes fuses with one of the female gametes (the egg) inside the embryo sac and this forms the zygote. This zygote will grow into the seed of the plant. Remember that the male gamete contains half of the DNA of the male plant and the female gamete contains half of the DNA of the female plant. When they fuse, the DNA combines so that the zygote has a new set of DNA. The other male gamete fuses with the two other female gametes inside the embryo sac and this forms the endosperm. The endosperm becomes the food that will keep the growing seed alive. This process is called double fertilisation because fertilisation happens twice in the plant. Once fertilisation has happened, no other male gametes can enter the ovule or embryo sac. After this, the ovary begins to swell and enlarge to form a fruit. Inside the ovule in the ovary, the zygote grows to form a seed. pollen grain stigma style ovary two sperm nuclei pollen tube tube nucleus One sperm fertilises the central cell (3n) Double Fertilisation One sperm fertilises the egg cell (2n) central cell (2n) ovule egg nucleus ACTIVITY Draw a flow diagram to show each step in the process of fertilisation of a plant.

6 SEED DISPERSAL There are many different types of fruit and each have different types of seeds in them. The reason a plant makes seeds and fruit is so that these can be spread (dispersed) to new places so the seed can germinate and grow into a new plant. Plants use a variety of ways to spread or disperse their seeds. Gravity Fruit falls off the tree or plant and into the ground where is can start to germinate or grow. The fruit may also roll further away from the tree if it is on a hill. Explosive force Some plants have seeds that burst open when it is ready and the seeds shoot out of the pod and away from the parent plant. Some plants need to be burned to activate this force. Wind Seeds dispersed by the wind need to be lightweight and have parts adapted to being carried by the wind (eg. Feathery-parts or wings). Water Plants that live in or near the water often use the water to disperse their seeds. Seeds float in the water to a different place where they land on the ground and germinate. Animals - externally Seeds with sticky or spiky parts stick to an animal s hair and are carried to a new area where they, drop off and grow in a new place. Animals - internally Animals eat the fruit and carry the seeds to a new place. Seeds have a tough outer covering that prevents them from being digested. When they pass out of the animal, they grow in a new place.

7 Answer sheet ACTIVITY Fill in the missing labels in the diagram below. Colour in the male parts in blue and the female parts in pink. stamen ANGIOSPERM FLOWERS Cross section of typical flower anther filament pistil stamen petal (corolla) sepal (calyx) receptacle peduncle stigma style ovary (with seeds) pistil