By looking at a plant externally can you tell whether a plant is C₃ or C₄? Why and how?

One cannot distinguish whether a plant is C₃ or C₄ by observing its leaves and other morphological features externally. Unlike C₃ plants, the leaves of C₄ plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C₃ plant, while maize is a C₄ plant.

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