As plants grow, they physically adapt to environmental changes that occur in order for them to continue surviving, even without being able to move. Read to find out more!
All living organisms in ecosystems are always seeking ways to continue being able to carry out the functions necessary for their survival. This requires access to resources, such as food and water. Animals will constantly adapt to be able to maintain desirable conditions, including by migrating when the seasons change or moving to areas with access to more prey and greater source of water. But, how do plants adapt when they can't move?
The answer is: plant tropisms! A tropism is defined as response to a stimulus, and plants do this by getting closer or further away from a factor in their environment when they're growing as environmental changes occur. This is caused by something called differential growth, which describes how cells in certain parts of the plants that are exposed to stimuli, such as light and water, grow faster than cells in other areas. Auxins, which are plant hormones, regulate this differential growth in plant organs.
Plants have positive tropisms, where they grow toward the stimulus, and negative tropisms, where they grow in the opposite direction of the stimulus. The following tropisms described below are all common behaviors in plants as they adapt to their environments.
Phototropism
Phototropism refers to growth behavior that is a result of light. Stems are one of the organs of the plant that respond to light stimuli, as they will grow toward the direction of the light source. This is because plants perform photosynthesis to produce their own energy to survive, and to do so, the chloroplasts in their leaves must be able to absorb light. In plant cells, the hormones photoreceptors are able to sense light, and auxins are signaled to move to the parts of the stem that are the furthest proximity away from the light, which causes the plant to curve in the opposite direction from where the auxins are in its stem. While stems exhibit positive phototropism, another organ of the plant that responds to light stimuli, the plant's roots, exhibit negative phototropism. The roots of a plant serve to absorb water and other nutrients present in soil, so they would grow in the direction opposite of light to reach underground.
Heliotropism
A specific type of phototropism is heliotropism. This refers to the behavior of some plant structures, specifically stems and flowers, of moving in the direction of the sun as it rises in the east and sets in the west. For instance, young sunflower plants would move their flowers to face the east during nighttime so that they can directly capture the sun's light as soon as it starts rising. However, as plants grow older, they become more stationery to face the east at all times, and are no longer heliotropic.
Gravitropism
Another plant tropism is that in response to gravity, which is called gravitropism. Regarding this behavior, the response for stems and roots are also opposite. Roots experience positive gravitropism, as they grow downward in the same direction as gravity, while stems experience negative gravitropism, and grow upward in the opposite direction of gravity. At the tip of roots, root caps help roots navigate in the direction of gravity as they grow through soil. Statocytes, which are cells in root caps, are involved in sensing gravity, as they contain amyloplasts, which are organelles where starch is stored. When amyloplasts sediment due to gravity, the root cap signals to cells in the plant's elongation zone, where cells that are in charge of the growth of the plant's root would direct its differential growth in the appropriate direction. As for the stems of plants, auxins tend to accumulate near the lower areas of the stem as a result of the downward gravitational force, which causes it to bend in the upward direction.
Thigmotropism
Thigmotropism refers to plants' response to touch, such as how they grow around solid objects. For instance, climbing vines have tendrils, which are structures that help the plant grow in a circular pattern when in contact with objects, such as how vines climb vertical beams. The surfaces of tendrils have sensory epidermal cells that get stimulated when they touch other objects, which would then produce signals for the plant to twine around them.
For roots, making contact with objects will signal for them to grow in the opposite direction, experiencing negative thigmotropism. This avoidance of objects in the soil helps roots avoid encountering obstacles as they spread out, and allow it to access more nutrients in the soil.
Other Plant Behavior
Besides from the three listed above, plants also exhibit other tropisms to sustain themselves in their environment based on the resources there, including hydrotropism, which is response to water concentrations, thermotropism, response to changes in temperature, and chemotropism, response to the presence of chemicals in the soil.
As you can see, plants don't visibly move, but they still have mechanisms to adapt to their environments and silently persevere through their changing conditions. Thank you for reading!
References:
“How Plants Respond to Light, Touch, and Other Stimuli.” ThoughtCo, 28 Feb. 2018, www.thoughtco.com/plant-tropisms-4159843.
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