Plants grow in response to different stimuli like light, gravity, or touch. Growth in response to these stimuli are known as tropisms. Phototropism is when a plant grows in response to light. This benefits the plant by allowing it to absorb more sunlight for photosynthesis.
Gravitropism, also called geotropism, is when a plant grows in response to gravity. This causes the roots to grow down and the shoots to grow up. Hydrotropism is when a plant grows in response to water. This allows plants to respond to their environment to absorb this vital resource.
Thigmotropism is when a plant grows in response to touch. This is especially important for climbing plants like vines that grow around a surface. Plant hormones, like auxin, allow plants to grow in response to these stimuli. Auxins cause plant cells to elongate, which bends the shoots. Here you can see that the cells on the side of the plant that are getting less sunlight are producing more auxin.
That causes those cells to grow and elongate, which bends the plant towards the sunlight. Plants show a positive tropism when they grow towards the stimuli. Plants show a negative tropism when they grow away from the stimuli. Some plants can perform rapid movements in response to a stimuli like light or touch.
These are called nastic movements and they are not the result of growth. Rapid movements in plants happen because of osmosis. Plants release ions like potassium and calcium to affect the cell's turgor pressure.
This causes water to rapidly leave or enter the central vacuoles of plant cells, causing them to stiffen or droop quickly. The purple shamrock exhibits photonasty and quickly responds to light. When it becomes dark, the turgor pressure in the leaves drop and they sag.
Touch-me-nots exhibit thigmonasty and quickly respond to touch. When touched, the turgor pressure in the leaves drop and they sag. Figma nasty is an important defense mechanism for plants.
The quick movements can startle large herbivores and knock smaller herbivorous insects off the leaves. Figma-nastic movements in carnivorous plants, like the Venus flytrap, can be used to trap insects in modified leaves. Now let's change gears and talk for a moment about how plants respond to losing water. When water reaches the leaves, Some of it evaporates through the stomata.
Evaporation of water from plants is called transpiration. Losing water can be a problem for plants, especially in arid climates, like the desert. To prevent transpiration, leaf cells send a hormone to the guard cells around the stoma to signal them to stay closed during the heat of the day. This response from the plants allows them to conserve water which may be hard to come by in an arid climate.