IGCSE Double Award Extended Coordinated Science Biology 8.2 - Sexual Reproduction in Plants Sexual Reproduction in Plants In a plant, the flower is the sexual organ, and it has both male and females parts. For a plant to sexually reproduce, a pollen from one flower must be delivered to another flower. This is called pollination - transfer of pollen grains from the male part of the plant to the female part of the plant. There are two main methods - insect-pollination and wind-pollination - And the structure of flowers for each method is slightly different You need to know the structure of a flower in a plant. Insect pollinated flower - Some plants have pollens that are carried by insects. - The flowers have special features that help to attract insects. - Petal : have attractive colours and scents so that pollinators are attracted to the flower - Sepal : protective leaf that protects the flower when it is in the bud - Petiole : joins the flower to the stem - Nectary : produces and contains nectar, a sweet liquid, to attract insects and birds - Stamen : the male sex organ of a flower, divided into the anther and filament - Anther : contains pollen sacs that open to release pollen - Filament : stalk that at holds the anther - Carpel : the female sex organ of a flower, divided into the stigma, style, and ovary - Stigma : collects and receive pollens - Style : tube that connects the ovary and the stigma - Ovary : contains ovules ( egg cells), after fertilisation, the ovary becomes the fruit.
Wind-pollinated flowers Have the same structures of the male and female sex organs, but some unnecessary parts are removed - Nectar - Because the flower does not need to attract any insects, it is a waste of resources - Colourful Petals - Again, because the flower does not need to attract any insects They also have some special features which the insect pollinated flowers don t have - Long, extended anthers (hangs outside the flower) - So that they can be blown in the wind easily - Large and feathery stigmas (hangs outside the flower) - So that they can catch more pollen in the wind - So now we can say pollination is - transfer of pollen grains from the anther of the plant to the s tigma You need to know characteristics of a pollen. A pollen is the male sex cell ( gamete ) which contains a nucleus which contains half of the DNA of the organism. The anther produces huge amounts of microscopic pollens, to increase the chances of it reaching the stigma. The pollens for an insect-pollinated plant and a wind-pollinated are different - Insect-pollinated pollens are spiky and sticky so that they can easily stick onto insects - Wind-pollinated pollens are round and light so they are easily blown by the wind
Fertilisation After pollination, (when the pollen arrives at the stigma), - the pollen (male gamete ) grows a pollen tube down the stigma, down the style, into the ovary. - The nucleus of the pollen travels down the tube and meets the ovule (female gamete ) - Gamete is a sex cell that contains only half the genetic information - The nucleus of the pollen and the ovule fuse, giving a cell with cull genetic information. This is called fertilisation: - The fusion of nuclei of the male gamete (pollen) and the female gamete (ovule) The fertilised ovule then becomes a seed, and the ovary becomes the fruit. They are ready to be distributed, and the seed will grow a new plant, by germination. The Seed You need to know structure of a seed. Testa - the hard protective cover on the outside of the seed Cotyledon - food store until the plant grows leaves for photosynthesis Micropyle - this is where the pollen tube connected with the ovule. Plumule - grows upwards into a shoot (upper part) of the plant Radicle - grows downwards into a root (lower part) of the plant (the plumule/radicle movement is because of geotropism - Unit B7.3) You need to know that germination of a seed is: - is the process of seeds developing into new plants. Few factors are necessary for a seed to successfully germinate - Remember it as WOW - W ater - the water activates certain enzymes in the seed that initiate seed growth, - However, too much water will inhibit germination - O xygen - the seed needs to respire for energy, using the food store and oxygen. - W armth - each species of plant seeds have a optimum temperature of germination
Seed Dispersal The seeds will have to be dispersed somewhere away from the parent plant - So that when the seed grows into a new plant, it does not compete for resources with big plants - So that the plant can colonise new areas and spread. There are various methods for a plant to make their seeds disperse away Wind dispersal - The seeds are designed so that they can be carried away by wind. - Some seeds like dandelions, have a light, parachute like seed - When wind blows, it takes these seeds away. - Some seeds like maple seeds have a helicopter like seed. - As they drop, they are suspended in air to travel further. Animal dispersal - There are few methods via animals. - Some seeds have spikes or hooks. - They stick to animal fur and are carried away with animals - Some seeds are meant to be eaten by the animal - Their fruits are colourful and attractive so the animals eat it. - The seeds are designed so that they will not be digested - The animal later releases the seeds in their faeces. Water dispersal - The plant simply drops the seeds in water - Seeds follow flow of water to be deposited elsewhere. - Like coconuts (yes - coconuts are seeds) Explosion dispersal - The seed pod explodes open, launching the seeds far away. The syllabus says you should be able to, (SO check if you can): - Identify and draw, using a hand lens if necessary, the sepals, petals, stamens, anthers, carpels, ovaries and stigmas of one locally available, named, insect-pollinated, dicotyledonous flower, and examine the pollen grains under a light microscope or in photomicrographs. - Use a hand lens to identify and describe the anthers and stigmas of one locally available, named, wind-pollinated flower. - State the functions of the sepals, petals, anthers, stigmas and ovaries. - Candidates should expect to apply their understanding of the flowers they have studied to unfamiliar flowers. - Define pollination as the transfer of pollen grains from the male part of the plant (anther or stamen) to the female part of the plant (stigma). - Name the agents of pollination. - Compare the different structural adaptations of insect-pollinated and wind-pollinated flowers. - Investigate and describe the structure of a non-endospermic seed in terms of the embryo (radicle, plumule and cotyledons) and testa, protected by the fruit. - Investigate and state the environmental conditions that affect germination of seeds: - requirement for water and oxygen and a suitable temperature. - State that seed and fruit dispersal by wind and by animals provides a means of colonising new areas.
- Describe, using named examples, seed and fruit dispersal by wind and by animals