If the stimulus signal is coming from the environment, it is an external stimulus. Weather is an example of an external stimulus. Hot weather causes sweating and cold weather causes shivering. Light and water are external stimuli for plant growth.

Homeostasis is the maintenance of a stable internal environment. Homeostasis is a term coined to describe the physical and chemical parameters that an organism must maintain to allow proper functioning of its component cells, tissues, organs, and organ systems.

Examples of stimuli and their responses:

• You are hungry so you eat some food.
• A rabbit gets scared so it runs away.
• You are cold so you put on a jacket.
• A dog is hot so lies in the shade.
• It starts raining so you take out an umbrella.

An example of external stimuli is your body responding to a medicine. An example of internal stimuli is your vital signs changing due to a change in the body.

An organism will respond to a stimulus by modifying its behavior, or what it’s doing. Stimuli can be broken into either good or bad. If something is good, a living thing will behave in a way that increases the stimulus. If something is bad, a living thing will behave in a way that decreases the stimulus.

Plants are known to respond to a number of external stimuli like light, gravity, touch, chemicals, etc. Plants respond to the external factors with the help of receptors and hormones. The receptors help the plants to sense the external stimulus and act accordingly. They control the growth of plant in response to light.

Growth Responses. A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially every cell in a plant can produce plant hormones.

Plants produce hormones in response to external stimuli. They exhibit this by tropism in which they grow towards sources of water and light. In positive tropism, plants grow in the direction of stimulus and in negative tropism plants grow in the direction opposite to stimulus.

Plants respond to 3 main stimuli:

• water.
• gravity.
• light.

Touch, gravity and light are important stimuli that trigger growth response. Produce by a plant is a chemical that affects how the plant grows and develops.

The ways in which plants respond to the environment include; Thigmotropism which the plant’s growth response to touch, Phototropism which is the direction growth of a plant towards light, geotropism which is response of a plant to gravity, hydrotropism which is the growth of plant towards a source of water ( mainly the …

Plants are like humans and animals, at least in the sense that they constantly adapt to their environment to preserve their well-being. Phototropism, thigmotropism, gravitropism, hydrotropism, and thermotropism are common tropic responses in plants.

Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. They change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins: cells in roots grow less.

Summary

• Tropisms are growth toward or away from a stimulus.
• Types of tropisms include gravitropism (gravity), phototropism (light), and thigmotropism (touch).

Plants respond directly to Earth’s gravitational attraction, and also to light. Stems grow upward, or away from the center of Earth, and towards light. Plants’ growth response to gravity is known as gravitropism; the growth response to light is phototropism. Both tropisms are controlled by plant growth hormones.

Forms of tropism include phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular substances), hydrotropism (response to water), thigmotropism (response to mechanical stimulation), traumatotropism (response to wound lesion), and galvanotropism, or electrotropism (response …

Negative tropism is the growth of an organism away from a particular stimulus. Gravitropism is a common example which can be used to describe negative tropism. Generally, the shoot of the plant grows against gravity, which is a form of negative gravitropism.

A tropism is the innate ability of an organism to turn or move in response to a stimulus. As opposed to a learned ability, innate reactions are genetically programmed. Organisms with a tropism will naturally turn toward a stimulus.

This response to gravity is called geotropism, or gravitropism. The roots are exhibiting positive geotropism, or growth with gravity, while the stem is exhibiting negative geotropism, or growth against gravity. Different parts of a plant grow in opposite directions because they have different functions.

The response may be positive or negative. A positive hydrotropism is one in which the organism tends to grow towards moisture whereas a negative hydrotropism is when the organism grows away from it. An example of positive hydrotropism is the growth of plant roots towards higher relative humidity level.

A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant. Positive = the plant, or a part of it, grows in the direction from which the stimulus originates. Negative = growth away from the stimulus.

positive tropisms – the plant grows towards the stimulus. negative tropisms – the plant grows away from the stimulus.

(a) Positive tropism is the movement or growth of plants toward the direction of the stimulus. For example, the roots of a plant always grow towards the pull of gravity. Negative tropism is the movement or growth of plants away from the direction of the stimulus.

→ Nastic movements are non directional responses to stimuli such as temperature and heat. → The most important feature of this type of movement is that movements are not governed by the direction of the stimulus. → Tropic movement is the response to stimuli that comes from one direction. → It is a directional movement.

Tropisms are growth responses of plants that result in curvatures of plant organs toward or away from certain stimuli. Within hours, the shoot of a plant placed on its side will usually bend upward and the roots will bend downward as the plant reorients its direction of growth in response to gravity.