In I/GCSE Biology, The growth response of a plant to a directional stimulus (stimulus that act in one direction such as light and gravity) is called a tropism. If the growth response is towards the direction of the stimulus, it is a positive tropism, and if it is away, then it is a negative tropism.
Phototropism is the response of growth towards light. Every seed has a protective sheath called the coleoptiles. The coleoptile detects light and releases growth hormones called auxins, which diffuse down the shoot and grows at the dark side of the shoot. This makes the shoot bend towards the light.
Geotropism is the response of growth towards gravity. The auxins accumulates on the lower side of the shoot to stimulate growth, causing the shoot to bend upwards. The hormone also accumulates on the lower side of the root, causing it to bend downwards.
An experiment to show this is to put germinating seeds on a clinostat. Repeat with another clinostat. Leave one clinostat standing on one side, and another spinning.
The result: Roots of the seeds in the still clinostat would grow downwards, whereas, seeds on the spinning clinostat would grow straight out horizontally because they cannot detect gravity due to the constant spinning.
That's the end of the topic!
Drafted by Joey (Biology)