Question

Which one of the following statements is true?
(a) The opening of the petals of dandelion flowers in bright light and closing in the dark at night is an example of phototropism.
(b) The roots of a plant do not respond to the stimulus of touch.
(c) The bending of the root of a plant away from light is caused by the plant hormone, auxin.
(d) Thigmonasty is not a growth movement.

Answer 1

Hint: This is the movement or growth towards a stimulus. It is the innate ability of an organism to turn or move in response to a stimulus such as the temperature, light, humidity. The movement can be positive or negative.

Complete answer:
An example of photonastic movement is the opening of the petals of dandelion flowers in bright light and the closing in dark at night. The opening of petals in the morning light in response to light is called positive photonastic movement while closing at night is called a negative photonastic effect.
Nastic motions are non-directional responses to stimulus such as temperature, humidity, light irradiance, and are typically associated with plants. The movement may be due to turgor shifts or growth changes. Reduced turgor pressure causes shrinkage while increased turgor pressure causes swelling.
Nastic movements vary from tropical movements. In tropical movements, the direction of tropical reactions depends on the location of the stimulus, whereas the location of nastic movements is independent of the position of the stimulus. Tropical movement is a growth movement but nastic movement may or may not be a growing movement.
Types of nastic movements
-Epinasty: In some plants, the growth of the leaf at the top outgrows the leaf at the bottom portion and as a result, the leaf bends down. A large flower bending down is an example of epinasty.
-Photonasty: The response of the plants due to light.
-Nyctinasty: The movement of plants at night or in the dark.
-Chemonasty: The response of plants to chemicals or nutrients.
-Hydronasty: The response of plants to water.
-Thermonasty: The response of plants to temperature.
-Thigmonasty: The response of plants to contact.

So, the correct answer is ‘The opening of the petals of dandelion flowers in bright light and closing in the dark at night is an example of phototropism’.

Note:
Photonastic movements are advantageous to the plants. It helps in maintaining the warmth and the optimum light during the day. This will attract various pollinating agents during the daytime. During the night these pollinating agents will not be active and hence the plants conserve their energy by closing their petals and leaves.