Plant Hormones: Tropisms

Plants have developed responses called tropisms. A tropism is a growth in response to a stimulus; so light and water in the plant’s case. There are different types of tropisms: Positive tropisms are when growth is towards the stimulus - so the plant growing towards the light to maximise the stimulus for photosynthesis. Negative tropisms are when growth is away from the stimulus - so roots growing away from the light, and deeper into the soil, so there is less chance of them being dried out. These are both phototropisms: growth in response to light. There are also geotropisms which is growth in response to gravity. The stem undergoes negative geotropism, because it goes against gravity, and grows upwards towards the light. Whereas roots undergo positive geotropism, because they grow in the same direction as gravity - downwards. So unsurprisingly, plants grow in response to light and water, and grow towards or away from light (Phototropism) and with or against gravity (Geotropism). But something must control this growth. Just like humans, plants have growth hormones. We will look at these plant hormones in our video called ‘plant hormones: auxins and gibberellins’ and we will see how they are used in weed killers, fruit ripening and more.