Chapter 37, Problem 1TYK

Summary Introduction

Introduction:

Phototropism is a mechanism that is mostly seen in the plants. In this mechanism, the plant responds to the light stimulus.

Answer to Problem 1TYK

Correct answer:

Phototropism is the growth of an organism toward a light source (positive phototropism) or away from a light source (negative phototropism). In phototropism, the cells located on the shaded side of a stem elongate more than cells on the illuminated side of a stem.

Explanation of Solution

Explanation/Justification for the correct answer:

Option (c) is given that cells on the shaded side of a stem elongate more than the cells on the illuminated side of a stem during phototropism. The plant tip cells when exposed to light source sense light and release a chemical known as auxin. This results in the process of phototropism. It leads to the elongation of the cells that are far from the light. The shoot bends toward light and cells present in the shaded region of the stem elongate more. Hence, Option (c) is correct.

Explanation for incorrect answers:

Option (a) is given that cells on the illuminated side of a stem elongate more than the cells on the shaded side in phototropism. The cells present on the illuminated side, that is, those cells that are receiving the light, do not extend more. Actually, the cells that are present in the shaded region and are far from the light elongate more than those on the illuminated side due to the chemical auxin released by the cells, which causes phototropism. So, it is a wrong answer.

Option (b) is given that phototropism is triggered by red/far-red light. The bending of the plant occurs only toward the light that includes blue wavelength. Plants have a phototropic response when the blue wavelength is available, which falls in the visible range. Therefore, it is not necessary that phototropism is triggered due to the red or far-red light. So, it is a wrong answer.

Option (d) is given that phototropism is triggered by phytochrome. Phototropism is triggered by a chemical named auxin. This chemical is produced by the cells that are present in the shaded region of the stem, thereby inducing phototropism. So, it is a wrong answer.

Hence, options (a), (b), and (d) are incorrect.

Conclusion

Phototropism refers to the orientation of the plant in response to the light. Cells that are present on the shaded side of the stem extend more than cells that are present on the illuminated side of the stem.