Question

By looking at a plant externally can you tell whether a plant is $C_3$ or $C_4$. Why and how ?

Answer 1

No, it is not possible to distinguish whether a plant is C₃ or C₄ by observing it externally (leaves and other morphological features). However, the leaves of C₄ plants have a distinguishing anatomy called Kranz Anatomy and it is only observed at the cellular level. C₃ plants do not showcase this.

Example: Both wheat and maize are grasses, but wheat is a C₃ plant, while maize is a C₄ plant.