We studied the petiole structure of twelve American species of Pteris: P. ciliaris, P. cretica, P. deflexa, P. denticulata, P. ensiformis, P. exigua, P. inermis, P. multifida, P. mutilata, P. quadriaurita, P. tristicula and P. vittata, using both light and scanning electron microscopy. We found that petioles are characterized externally by the presence of an adaxial groove and two dorsal-lateral ventilation areas. Internally, petioles are monostelic V-, U- or inverted-Ω-shaped axes. The vascular system is surrounded by one endodermis cell layer and 1–3 pericycle strata. We propose a classification of the vascular bundles into four types considering their shape, the shape of the xylem ends, the number of protoxylem zones and the presence of parenchyma bands in the xylem.