Question

How do auxins promote the growth of a tendril around a support?

Answer 1

The word auxin is derived from Greek word 'auxin' which means to grow. The first discovered hormone was Auxin. Charles Darwin and his son Francis Darwin Observed that the stimulus of light was perceived by the coleoptile tip of canary grass but the bending response to unilateral light was produced at a distance in the growth zone i.e.., subapical part.

Complete answer:

Thigmotropism is the contact or touch stimulus induced growth movement like the growth of tendril, twiner twisting around a solid support. When the tip of the tendril comes in contact with a solid object of irregular surface, it coils around it. It occurs due to differential growth in the tendril.

Tendrils do not have any sensory structures but still they are able to find their support just as we gropen the dark for finding the switch-board.

Tendrils perform circumnutation from their apical regions. In this the terminal parts of tendrils move in together directions.

Wherever these tendrils come in contact with a support, they stop performing circumnutation that is growth property. Instead, the contacted region shows little growth while the opposite side grows rapidly in order that the tendril coils over the support.

When a tendril is in contact with support we can say a wall or any other plant more auxin diffuses towards the side of tendril away from the support. This auxin concentration stimulates the cells to grow longer on the tendril side which is far from the support.

Auxin is synthesised and produced at the tips of shoots. This process occurs when a growing plant detects light and helps the cells to grow longer.

When light is entering from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is far away from light. Thus, the plant is observed to bend towards light.

Note: After a series of experiments conducted by other scientists, it was concluded that the sensation picked up by the coleoptile tip is transmitted to the subapical part which bends towards the direction of light. As it was found that in an experiment if the coleoptile tip was removed then there was no bending of shot towards light because the stimulus of light was not perceived.