[Campbell Biology P.896] The text states that positive phototropism results from "a differen... | Practice Question

The text states that positive phototropism results from "a differential growth of cells on opposite sides of the shoot; the cells on the darker side elongate faster than the cells on the brighter side." If a grass coleoptile is illuminated uniformly from all sides, what would be the expected growth response?

  • A: The coleoptile would bend sharply towards the strongest light source due to increased cell elongation on one side.
  • B: The coleoptile would exhibit no phototropic bending but would grow straight upwards, as cell elongation would be uniform around the circumference.
  • C: The coleoptile would show negative phototropism, bending away from the light, as light inhibits cell elongation.
  • D: Growth would cease completely because differential light perception is necessary for any elongation.

Explanation

Phototropic bending occurs due to differential cell elongation caused by uneven light distribution. If light is uniform, there is no 'darker' or 'brighter' side, so cell elongation would be equal around the circumference, leading to straight upward growth without bending. Growth itself would not cease as elongation is a general growth process.