Plant responses

Uh, Kim, your plant is weird. Why is it growing towards the window rather than straight up? Huh, I’m not sure... Different organisms need to get different things from their environment to survive. Animals, for example, need food and water for their bodies to function, while plants need light and water to carry out photosynthesis.

If something in an animal’s environment changes and there isn’t enough food or water for it to survive, an animal can move to where living conditions are better. But a plant can’t do that. A plant can’t just move to a new place. Because of this, plants have developed special responses to help them grow towards sources of light and water. These responses are called tropisms.

When a plant grows towards a feature of the environment, a stimulus, such as light or water, we call it a positive tropism. If a plant grows away from the stimulus, we call it a negative tropism. Let’s look at the plant on Kim’s windowsill. The stem and the leaves aren’t growing upwards in a straight line. Instead they are leaning heavily towards the window.

The plant is growing towards the source of intense light, which is coming from the outside! This response is phototropism, and because the plant is growing towards the stimulus, it’s a positive type of tropism. Positive phototropism allows the leaves to absorb the maximum amount of sunlight and carry out lots of photosynthesis. This helps the plant make the most of its environment! The part of the plant above ground responds with positive phototropism.

The part below ground, the roots, responds with negative phototropism, as roots grow away from the light. But what about the plant’s response to water? Well, imagine Kim replants the plant from the round pot it’s in now, into a long, rectangular pot. Then Kim only waters the soil at the end of the pot furthest away from the plant. The plant would sense the lack of water in its corner and adapt!

The roots would start growing towards the part of the pot that is being watered. This response is positive hydrotropism. Negative hydrotropism can happen too, when there is too much water. In that case roots would start growing away from that area. Plants respond to other stimuli from their environment as well, not only light and water.

Roots and stems respond to the same force that is stopping you from floating off the ground. Roots and stems respond to gravity! The majority of roots grow downwards, in the direction of gravitational force. This response is positive geotropism. Because of positive geotropism roots grow deep down into soil, and that is how roots anchor the plant.

Stems on the other hand grow upwards, in the opposite direction of gravitational force. This is negative geotropism. If Kim and Philip placed the plant pot on its side, the stems would still respond with negative geotropism and the roots with positive geotropism. This means eventually, the roots would start curving downwards, while the stems would start curving upwards. So Kim and Philip would end up with a rather strange looking plant.

Tropisms are plant responses that allow them to adapt to their environment and make the most of it. Philip and Kim, you could do experiments with different plants to see how different environments impact their growth!