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10.5.2. Relationship of plants

Relationship function in plants

Although it may seem that plants have no relationship function, this is not the case. Plants also react to environmental stimuli, such as light, temperature, humidity, etc. Thus, although they cannot move, they adapt to the changes that occur in their internal environment and remain stable.

When the plant captures a stimulus, some cells detect it and react or send signals to other cells so that a coordinated response of the entire plant to this stimulus occurs.

The most typical plant responses are of two types:


The nastias are rapid and reversible movements of some parts of the plant that occur in response to an external factor.

There are several types of nastias:

  • Photonasty: The plant responds to light, such as when some flowers open at dawn or dusk or when leaves or flowers orient themselves following the movement of the Sun.

  • Geonasty or gravinasty: response produced by the force of gravity.
  • Thigmonast: response by contact, as for example, the tendrils of many plants that allow them to attach to other places.
  • Hydronasty: response to humidity in the environment, as in the opening of the sporangia in ferns.
  • Chemonasty: response to chemicals, changes in pH, etc.
  • Nyctinasty : They respond to changes between day and night by changing the position of the leaves .

  • Seismonasty: when they detect a shock or jolt, like the movement of some mimosas and carnivorous plants.

  • Thermonasty: They respond to variations in temperature, such as when the tulip flower closes.

  • Traumatonasty: response produced as a result of an injury.


Tropisms are permanent movements of the plant, which grows or changes direction, in response to an environmental stimulus.

Plants, or some of their parts, grow by tropisms or responses given by plants according to the direction of the stimuli they receive. If the organ of the plant approaches the stimulus, it is said to be a positive tropism, and if it moves away, it is a negative tropism.

The main stimuli that cause tropisms are:

  • Phototropism: is the response of the plant to a light stimulus. Plants exhibit positive phototropism in stems and leaves, which grow towards light, and negative phototropism in roots, which grow towards darkness.
  • Geotropism or Gravitropism: response produced when the stimulus is the force of gravity. The stems, which grow in the opposite direction to the force of gravity, show negative geotropism. On the other hand, the root presents positive geotropism because it grows towards the same direction from which the stimulus (gravity) comes.

  • Hydrotropism: and occurs in response to the stimulus of the presence of water. The roots have positive hydrotropism, as they grow towards where there is water.
  • Thigmotropism: occurs in some plants, in response to contact with a solid, which grow around the objects they touch, as occurs with the stems of vines.

  • Chemotropism: it is the response that plants produce when detecting some chemical substances, which reacts to it by growing towards them (positive chemotropism) if they are necessary, or growing in the opposite direction (negative chemotropism) if they are not harmful.


Hormones are substances produced by the plant itself that act as internal stimuli, causing different responses in the plant. They are the ones that control growth, reproduction, ripening of fruits, etc.