WHAT IS IRRITABILITY IN PLANTS? - BIOLOGY - 2026

The irritability of plants or sensitivity is the ability of plant cells to respond to stimuli. Stimuli are understood to be changes in the environment that can affect their activity.

All living things respond to stimuli. Plants as living beings have the ability to respond by reacting to various elements of the environment. This reaction of cells to the environment is known as irritability.

Mimosa pudica

Irritability occurs in correspondence to the degree or level of the stimulus. If the stimulus is not so intense, the response may be local, that is, only a part of the plant reacts, but if the stimulus is strong, the irritability spreads throughout all the cells and tissues of the plant.

The Mimosa Pudica is the best example of the ability to respond to a stimulus. It is a very fragile plant with small and thin leaves, it is well known because we play with them since we were little. When we touch them, their leaves retract as if the plant is sleeping, but this mechanism has a reason.

When it rains, the thick droplets can damage plant leaves, breaking them or affecting their structure. When the Mimosa Pudica feels the rain, it hides its leaves to prevent these drops from damaging it, since it is so fragile that one drop can damage several leaves.

At the end of the rain, this plant re-unfolds its leaves free of damage while other plants are abused by the intensity of the water.

What are the types of responses that a plant can have due to irritability?

Each cell of the plant has a complete genetic program for growth and development. All plants are highly receptive to internal and external stimuli.

All parts of the plant are sensitive, however, some are more than others. From the root that reacts by sending the order of the direction of growth to the flowers and leaves that react to light, temperature and movement.

Plants particularly perceive factors such as light, temperature, humidity, ventilation and the levels of salt, acidity and alkalinity in the soil.

Although plants do not have a very complex range of motion, this does not mean that they do not move. Plants, like humans, react to stimuli through movement.

These have three types of responses: tropisms, nastias, and circadian rhythms.

- Tropisms

They are the specific and permanent responses that occur in a plant when having a stimulus. These affect the movement of the plant in two ways: If you are looking to get closer to the stimulus, we speak of positive tropism. However, if you are looking to get away, we are talking about negative tropism.

Phototropism

It is the best known tropism of all. This tropism explains the behavior of plants with respect to sunlight; plants grow where the light reaches.

It can be negative, like the roots that grow in the opposite direction, or positive like that of the sunflower, considered the most notorious example of phototropism.

The sunflower when growing has a very peculiar phototropism. These flowers seek the sun throughout the day. When dawn breaks, the sunflowers look east and slowly chase the sunlight until nightfall; then they return their journey to the east and there they await another sunrise.

This ends when the sunflowers reach all their splendor, when they are already "adult sunflowers" they adopt the phototropism of the rest, keeping facing east to wait for the light.

Geotropism

It is the movement of plants in response to gravity, according to their needs. The very growth of plants is an example of geotropism, they grow against the law of gravity looking for the sun; for which it is a negative answer.

The roots grow downward, seeking nutrients so they are geotropically positive.

Thigmotropism

Explain the use of a solid object as a support for its growth, when coming into contact with it. A great example is creepers.

Hydrotropism

It is the movement of the plant with respect to the water. The roots are positively hydrotropic because they grow looking for water, the leaves and flowers are not.

- Nastias

They are temporary responses to stimuli that occur in the plant. Unlike tropisms, these are not directed towards or against the stimulus, they simply react until they return to their original shape or position.

Sismonastias

It is the reaction that plants take to friction or blows, such as Mimosa Pudica or Carnivorous Plants.

Chemo nastia

They are all the movement responses that plants take to chemical stimuli.

Photonastia

It is said thus to the reactions to light stimuli. It differs from phototropism because the stimuli due to luminosity are temporary.

The "Morning Glory" flower is an example of this, they open their petals at dawn with sunlight and when the sun goes down they close their petals; returning to its original state. It is a nastia, because the reaction lasts only as long as the sunlight lasts.

Unlike the sunflower whose way of growth is affected by the direction of the sun, the Morning Glory is only affected for a few hours and returns to its original state without affecting its growth.

- Circadian rhythm

It is the ability to react according to the internal clock of each plant. Plants like any living being have a clock that indicates the cycle of the seasons and the day / night.

That is why plants flourish in certain seasons or bear fruit in certain times, all of this is related to your internal clock. Growers must understand the circadian rhythm to get the most out of their harvest.

Plant irritability and homeostasis

Although they are usually confused, homeostasis and irritability do not refer to the same concept.

The irritability of plants is closely connected with homeostasis, in fact, it is considered a homeostatic capacity. But, this does not mean that it is correct to use one term as a synonym for the other because they are both different.

Homeostasis is the ability to maintain a balance in the internal structure of the plant, in order to cope with the conditions that arise.

For its part, irritability is the response that the plant has to be able to maintain that internal balance. That is, irritability helps fulfill the purpose of homeostasis.

References

1. "Classwork Series and Exercises (Biology- SS2): Cell reactions to its environment (Irritability)" Retrieved on July 3, 2017 from passnownow.com
2. Texas Education Agency "Plant Responses to Stimuli." Retrieved on July 2, 2017 from texasgateway.com
3. Weber, D. "Tropisms: Phototropic, Geotropic and Thigmotropic Plant Growth" Retrieved on July 2, 2017 from study.com
4. Armitt, S. "Irritability in Plants" Retrieved on July 2, 2017 from amblesideonline.org
5. Bose, J. "Researches on irritability of plants" Retrieved on July 2, 2017 from archive.org
6. ABC Digital (2009) "Tropismos y Nastias" Retrieved on July 2, 2017 from abc.com.py.