What are plant tropisms?

Tropisms are growth movements of some parts of plants like roots, stems and shoots in response to external environment. The plant part movements are usually in response to directional stimuli. Plants can respond positively or negatively. The most common stimuli responded to by plants are water, gravity, light and touch. These positive responses affect certain parts of plants only as stated above and affect all other parts of the plants negatively. Negative response is when plant parts grow or move in opposite direction to stimuli.

Types of tropisms

Plant responses to stimuli are of four main types. These are:

• Geotropism
• Phototropism
• Hydrotropism
• Haptotropism

Geotropism

This is the positive response of plant roots to gravity such that they grow towards the force of gravity into the earth. For this reason, roots are termed positively geotropic.

On the other hand, plant roots grow away from light and are termed negatively phototropic. Plant roots respond negatively to sunlight.

Experiment to demonstrate geotropism

1. Germinate bean seeds rolled in wet paper 
2. When the radicles are at least one centimeter long, re-arrange the seedlings such that the radicles are horizontal to the ground and the moisture regime is maintained.
3. Leave set-up for two days and make observations on the set-up.

 On observation, the radicles will be found to have bent to face downwards towards the force of gravity.

Hydrotropism

Hydrotropism is the respond by plant roots to water.

Plant roots respond positively by growing towards water. This ensures plant survival. The plant roots are therefore hydrotropic in their behavior. Roots are the parts of plants that absorb water from the soil. Water is needed by the plant for such functions as photosynthesis, cooling, transpiration and germination.

Phototropism

This is the positive response of shoots to the presence of light by growing towards it. The apical meristem of the plant produces a hormone, Indole-acetic acid (IAA) which causes plant cells to elongate. The hormone is produced in increased amounts when the cells in question are deprived of light. Therefore, the dark side of the shoot has a higher concentration of the IAA, causing the cells to elongate faster than the lit side of the plant. The plant stem therefore bends towards the slower growing side and by so doing, facing the light. When the shoot faces the light, there is an even distribution of the IAA around the stem of the plant and therefore the growth rate of the whole stem is the same.

Experiment to demonstrate phototropism in shoots Several ways can be used to demonstrate phototropism.

1. Bend a small plant such as covo plant in such a way that it lies horizontal to the ground.
2. Observe how the plant responds to light over several days or weeks.

The growth response to touch by some parts of plants like tendrils of climbing plants is called haptotropism.

Examples of plants that grow by climbing on others are cucumber, peas, pumpkin and beans. The methods by which these plants climb on structures are by growing and winding around the stems of other plants or stakes that are provided for them. They also hold onto other plants and structures by use of tendrils that grow from the stems of climbers at the nodes. Trellising is a farming method that is used to provide supports for climbing plants. The method bases on plant haptotropism mechanism. 

The plant’s response to a given stimulus is achieved through the effects of a growth substance called auxin or indole-acetic acid, (IAA). The presence of auxin in the plant cells in large amounts causes cells to elongate fast, and a restricted amount of auxin retards growth. This growth response by plants is also termed sensitivity.

Plants are sensitive to stimuli such as light, gravity, water and touch. The same hormone indole-acetic acid affects all plant responses but differently on different plant parts.

Growth substances in plants

The plant’s response to a given stimulus is achieved through the effects of a growth substance called auxin or indole-acetic acid, (IAA). The presence of auxin in the plant cells in large amounts causes cells to elongate fast, and a restricted amount of auxin retards growth. This growth response by plants is also termed sensitivity.

Plants are sensitive to stimuli such as light, gravity, water and touch. The same hormone indole-acetic acid affects all plant responses but differently on different plant parts.