What is a tropism, specifically phototropism and geotropism?

• A. A growth curvature caused by internal hormonal signals only
• B. A random movement of stems
• C. A response to drought stress
• D. A growth curvature caused by an external stimulus such as light (phototropism) or gravity (geotropism)

Answer: (D)

Tropisms are directional growth responses in plants where bending occurs toward or away from a stimulus. Phototropism is the growth toward light, while geotropism is growth in response to gravity. Both rely on the stimulus being external and cause uneven growth on opposite sides of the plant organ, typically via hormone redistribution that makes one side elongate more than the other.

The best description is a growth curvature caused by an external stimulus such as light or gravity because it captures the essential idea: the direction of growth is guided by an outside cue, not by internal signals alone or by random movement. Options describing purely internal hormonal signals, random movements, or drought stress miss the key element of directional bending toward or away from an external stimulus.