Answer:
The expected observation would be **elongation of the stem** due to the disruption of normal tropic responses caused by the absence of cues from gravity and light.
Explanation:
If a third plant were placed on a wound clinistat and covered with a light-proof box with no opening, we would expect to observe elongation of the stem. Clinistats are devices used to study the effects of gravity on plant growth. By providing a clinostat, which constantly rotates the plant, it disrupts the plant's perception of gravity, leading to changes in growth patterns. Additionally, covering the plant with a light-proof box would eliminate the influence of light on phototropism, the growth response of plants to light. Without the cues from gravity and light, the plant's stem growth would likely become elongated and exhibit a more randomized pattern due to the disruption of normal tropic responses.