This video illustrates clearly why you have to be careful using YouTube to find information for revision. It is produced by a company in Australia and is clearly presented, includes the right level of detail for GCSE in the UK and is easy to follow. But….
The video reinforces one of the commonest areas of confusion in this topic by its choice of images to accompany the text. In the section when the voice over is describing the role of animals in seed-dispersal, it has an image of a bee feeding on pollen in a flower. This is NOT seed dispersal!
Later in the video when the voice over talks about wind-pollinated flowers, there is an image of the seeds of a dandelion being blown by the wind. This is NOT pollination!
It might seem like a small point but when you have marked exam questions on this topic for 20 years and seen many students confuse these two separate processes, it starts to take on more significance. So please watch YouTube for science videos – there are some great resources on there…. But be critical and remember, just because it is on a video, it doesn’t mean it is correct!
In the topic of sexual reproduction in plants, the final stage is often overlooked. I think it is helpful for students to think of this topic in several distinct stages.
- Flower Structure (hermaphrodite nature of most plants)
- Pollination (self v cross pollination; wind v insect pollinated flowers)
- Fertilisation (how does the pollen tube reach the egg cell to fertilise it?)
- Seed and Fruit formation (what forms what after fertilisation)
- Seed Dispersal (by animals, by wind, by water, by explosive means)
Once the seed has been dispersed there then follows a period of dormancy when nothing happens. In latitudes such as the UK this often is there to delay germination until the following spring when growing conditions become more favourable. The process of taking an inert seed and growing a new plant from it is called germination.
You don’t need to worry too much about the details of germination but there are a few vital parts of the process that GCSE candidates need to appreciate for A* marks. Firstly you should know the structure of a typical seed.
The seed coat or testa surrounds the seed and provides a tough waterproof container. Inside there are the embryonic plant (composed of a plumule and radicle), one or two seed leaves called cotyledons and a storage tissue called endosperm.
Germination starts when the seed starts to take up water by osmosis. There is an opening in the testa called the micropyle that allows water to move into the seed causing it to swell and thus rupture the seed coat to allow the embryo plant to emerge.
Water entering the seed also activates the embryo plant such that it starts to release digestive enzymes such as amylase. Amylase catalyses the digestion of starch into a simple sugar maltose. The endosperm and cotyledons contain energy stores in the form of starch, lipids and proteins and as these get broken down by the various enzymes, they provide the energy for the early growth of the seedling. The radicle emerges first and grows downwards (positive geotropism) and then the plumule or shoot grows upwards towards light (positive phototropism). Remember that throughout the early stages of this growth the energy required comes from stored food molecules in the seed. If you measure the mass of the plant during this phase, it would be decreasing. Only when the first leaves emerge above ground and the plant can start the process of photosynthesis will the overall mass start to increase.