Photo of Rhomboid Mercury (Acalypha rhomboidea)

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This photo specifically shows the relative length of the petioles (stem connecting each leaf blade to the main stem) to the leaf blade. This species, rhomboid mercury (Acalypha rhomboidea), tends to have longer petioles relative to the leaf blade, a feature that is usually reliable for telling it apart from the visually-similar Virginia threeseed mercury (Acalypha virginica), and even more dramatically, from slender threeseed mercury (Acalypha gracilens), both of which overlap in range. This characteristic is less reliable for distinguishing between asian copperleaf (Acalypha australis).

When looking at petiole length, it is important to focus on the mature leaves lower down on the plant, as the young leaves towards the tip tend to have short petioles. This is in part an adaptation of the plant to maximize light capture. The newer leaves capture light falling towards the center of the plant and then the mature leaves lower down project farther outward to capture light farther from the center.

This overall growth habit also serves other purposes; the open space in the center of the plant reduces drag both from wind and water. The reduction of water drag can reduce the chance this plant is uprooted or loses leaves in a flooding event, which is a pressure this species is sometimes subjected to because it can grow in floodplains. The reduction of wind drag helps reduce water loss and thus allows it to grow in drier conditions.