A student compares cross-sections of C3 and C4 leaves. Which measurable parameter most reliably distinguishes Kranz anatomy?

Correct answer: (a) Bundle sheath cell chloroplast area relative to the mesophyll

Explanation

Kranz anatomy ("Kranz" means "wreath" in German) is the mark of C4 plants such as maize and sugarcane. The veins are surrounded by a ring of large bundle sheath cells with many large chloroplasts, and outside them is a ring of mesophyll cells.

In C3 leaves, the bundle sheath cells have few or no chloroplasts. So the chloroplast area of the bundle sheath compared with the mesophyll is the most reliable measure for telling the two apart. Stomatal density, palisade thickness and cuticle thickness vary with the environment and do not distinguish C4 plants.