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- 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.
Here’s a diagram of a typical dicot, insect-pollinated flower:
Just so that you get a better idea of how these parts actually look in real life – which would probably be a good idea if you’re writing the practical exam – it’s a good idea to pluck an actual dicotyledonous flower and compare its parts to the ones given on the diagram. Cut it up and dissect it, if it’ll help you identify all the parts.
If you’re confused as to whether the flower is insect-pollinated or not, if it has petals, it’s probably insect pollinated. If you don’t know whether it’s dicot or monocot, the leaves on dicotyledonous plants tend to be veiny, and the veins branch out towards the edges of the leaf. In monocots, however, the veins tend to run parallel to each other.
This image is of different types of pollen grains – the image was constructed using an electron microscope, and the colours were added digitally.
Here are a few diagrams of different types of pollen grains under the light microscope:
- 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.
The sepals are a hard layer that protects the flower while it is a bud.
Petals come in different, often vibrant, colours to attract insects for pollination.
Anthers contain pollen sacs. This is where pollen grains are formed. Pollen grains contain the male gametes (sex cells) required for fertilisation.
The stigma is a sticky surface that catches the pollen during pollination.
The ovaries contain ovules. These develop into seeds when they are fertilised.
- Candidates should expect to apply their understanding of the flowers they have studied to unfamiliar flowers.
Again, studying real dicot flowers will help you with this.
- 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).
Pollination is 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.
Animals, including insects; the wind; water.
- Compare the different structural adaptations of insect-pollinated and wind-pollinated flowers.
- Investigate and state the environmental conditions that affect germination of seeds: requirement for water and oxygen and a suitable temperature.
Before telling you the required environmental conditions, it’ll be useful for you to know the basic structure of the seed.
The tough outer coat is called the testa. The cotyledon serves as a food store. The radicle grows to become a root, and the plumule grows to become a shoot. According to Cambridge, the radicle, plumule, and cotyledons are all part of the embryo.
Seeds mostly require three environmental conditions for germination: oxygen, water and growth.
Oxygen is required for respiration, which provides the seed with the energy required for germination.
Water is required to make the food in the food stores of the seed soluble so that they can be transported to the seed embryo and used in respiration. It is also required for the seed to swell and burst so that the root and shoot can emerge.
Most seeds require warmth to germinate, which is why most plants only grow in spring and summer.
Investigating these conditions:
First, I’ll describe the investigation of temperature:
Take 5 or more transparent containers. Stuff them with kitchen tissue and spray adequate water in each (so that the tissue in each container is damp, but not a soggy a mess). Put the same number of seeds in each container (e.g. 4 seeds in each), making sure that each of the seeds are visible from outside the container. Make sure the containers are open to the air, so plenty of oxygen reaches each seed.
Place each of the containers in different incubators at different temperatures, for three weeks. Maintain the dampness of the tissue in each container for the duration of the experiment. Take pictures of the containers (so that we can view all the seeds) at the same time each day, every day for three weeks. Note which seeds sprout the fastest, and which temperature they germinate at. You will notice that the seeds at warmer temperatures sprout faster, but if the temperature is too high or low, they end up not sprouting.
To investigate water:
Use 2 transparent containers stuffed with tissue, and place the same number of seeds in each. Make sure that there are enough air spaces for each seed to receive plenty of oxygen. Spray one of the containers with water, and leave the other one dry.
Place the two containers in two different incubators for three weeks, at the same temperature (25oC). Take pictures every day, at the same time of day, throughout the experiment and note which seeds germinate.
Use 2 transparent containers. Fill one with wet sand (this will reduce the air supply to the seeds in this container greatly), and set up the other with damp tissue. Place seeds in each so that they are visible from outside the container. Incubate both containers at the same temperature for three weeks, taking pictures every day at the same time of each day. Note which seeds germinate first.
- 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.
The structure of the seed has already been described. Seeds are usually present inside a fruit, which protect the seed from the environment by forming a physical barrier between the seed and its surroundings.
- State that seed and fruit dispersal by wind and by animals provides a means of colonising new areas.
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 animals.
Note that seed dispersal is important because it ensures that there is sufficient space between plants, so that competition for food, water, light and space is reduced.
- Dandelion fruit have a group of fine hairs called a pappus
- Pappus acts as a parachute and catches the wind
- The fruit counterbalances the pappus.
- Sycamore plants have wings with large surface areas
- When the fruit falls from the tree, the shape of its wings causes it to spin. This slows down its descent.
- If caught by the wind, it’ll be carried away from the parent plant, and thus seed dispersal is achieved.
Fleshy fruits, e.g. blackberries:
- These tend to be colourful, juicy, sweet and nutritious, so they attract animals.
- Once ingested, the seeds pass through the alimentary canal without being digested, and is removed from the animal in its faeces somewhere far away from the parent plant.
Hooked fruits e.g. bur:
- As animals brush by plants with hooked fruits, these fruits become caught on the animal’s fur, because of the hooks on their surface.
- Because animals can move from location to location, these fruits tend to be carried away from their parent plant, and eventually fall off the animal.
Notes submitted by Sarah.
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