Question

33 a plant was placed in a bell-jar under warm and brightly lit conditions. the air in the jar contained carbon dioxide made with radioactive carbon atoms.

after one hour, thin slices were cut from the stem and placed on photographic film which becomes dark when exposed to radioactivity.

which diagram shows the areas where the film became dark?

Explanation:

Analyze the experimental setup

The plant is placed in a bell-jar under warm, brightly lit conditions. The air inside contains radioactive carbon dioxide (\(^{14}\text{CO}_2\)). However, the leaves of the plant are covered by a black polythene bag.

Determine the effect of the black polythene bag

Because the leaves are covered by a black polythene bag, light cannot reach them. Light is a necessary requirement for photosynthesis. Without light, the leaves cannot perform photosynthesis to fix the radioactive carbon dioxide into radioactive sugars (such as sucrose).

Evaluate translocation of radioactive sugars

Since no radioactive sugars are synthesized by the leaves, there are no radioactive organic substances to be translocated through the phloem to the rest of the plant.

Relate to the photographic film detection

The question states that the photographic film becomes dark when exposed to radioactivity. Since no radioactive sugars were produced or translocated down the stem, a cross-section of the stem will show no radioactivity. Therefore, no areas on the photographic film will become dark.

Match with the correct diagram

• Diagram A shows radioactivity in the outer region of the vascular bundles (phloem).
• Diagram B shows radioactivity in the inner region of the vascular bundles (xylem).
• Diagram C shows radioactivity in both xylem and phloem.
• Diagram D shows a completely blank circle, indicating no radioactive areas.

Since no radioactivity is present in the stem, Diagram D is the correct representation.

Answer:

• (A) Dark spots in the phloem region of the vascular bundles
• (B) Dark spots in the xylem region of the vascular bundles
• (C) Dark spots in both xylem and phloem regions
• (D) No dark areas on the film (Correct answer)