Question

The discovery of auxin is traced back to the work of Charles and Francis Darwin on
A. Photonastic movement
B. Photoperiodism
C. Phototropism
D. None of these

Answer 1

Hint: Auxins are hormones that are found in the shoot and root tips that are responsible for stem elongation and promote plant growth.

Complete answer:
To solve this question, first we need to know about plant movement. Movement in pants can take place either naturally or due to the response to an external stimulus. There are different kinds of movements based either on spontaneous or induced movement.

Now, let us find the solution from the given options.
Photonastic movements are induced by changes in the intensity of light. In such plants, flowers will open when there is increased light during the day and close up with decrease in light intensity.
Phototropism occurs when light is provided to plants in only one direction. The plants tend to move or to bend towards the light source. Charles Darwin described the effects of light on canary grass coleoptile movements. He suggested that a substance transduced to the lower portion from the tip of the oat coleoptile would induce bending toward directional light. When unidirectional light falls on the coleoptile, it bends in the direction of light. If the coleoptile tip were covered with aluminium foil, no bending would occur. Darwin and his son's proposal were among the first scientific descriptions of the action of auxin.
In photoperiodism flowering and other phases of growth are controlled in response to the photoperiod, or day-length. Short-day plants flower when the length of the day is below a certain threshold, while long-day plants flower when the length of the day is above a certain limit.

So, the correct answer is option C i.e., phototropism.

Note: Darwin's experiment suggested that the tip of the coleoptile was the tissue that perceived the light and produced some signal that was transported to the lower region of the coleoptile where the physiological response of bending occurred. When the coleoptile tip was cut and unidirectional light was exposed to the rest of the plant body, bending did not occur.