▶️ Answer/Explanation
Since the seedling is growing underground, there is no light stimulus available to trigger a response. Therefore, phototropism (response to light) is not involved for either the root or the shoot. The movement is entirely directed by gravity. The root shows positive gravitropism by growing downward, while the shoot shows negative gravitropism by growing upward toward the surface. Thus, gravitropism is active in both parts, but phototropism is not.
✅ Answer: (A)
✅ Answer: (A)
▶️ Answer/Explanation
When light shines from the right, the plant hormone auxin migrates from the light side toward the shaded side of the shoot tip. Consequently, a much higher concentration of auxin diffuses into block $X$ (the shaded side) compared to block $Y$ (the light side). When these blocks are placed off-center on decapitated stems, the auxin diffuses down into the side of the stem beneath the block, stimulating cell elongation. Since block $X$ contains a high concentration of auxin, it causes the cells on that side to elongate rapidly, resulting in a curvature. Block $Y$, however, contains little to no auxin, meaning no significant elongation occurs and the stem remains straight.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
The question specifies that the plant is growing “in the dark,” which immediately eliminates phototropism (response to light) as a cause for the current movement. Gravitropism is the growth response of a plant to gravity. Shoots exhibit negative gravitropism by growing away from the gravitational pull, while roots exhibit positive gravitropism by growing toward it. Even without light, gravity acts as a constant stimulus that guides the vertical orientation of the plant. Therefore, the observed growth is caused by gravitropism, but not phototropism.
✅ Answer: (C)
✅ Answer: (C)