Pickerelweed (Pontederia cordata)
A common sight around the margins of ponds and lakes; this stand grew in North Park, where it was blooming at the beginning of July. The spikes of blue flowers and stiff arrowhead-shaped (or elongated-heart-shaped) leaves are distinctive: you will find nothing else that looks like this standing in shallow water.
Gray describes the genus and the species:
PONTEDÈRIA L. PICKEREL-WEED. Perianth funnel-form, 2-lipped; the 3 upper divisions united to form the 3-lobed upper lip; the 3 lower spreading, and their claws, which form the lower part of the curving tube, more or less separate or separable to the base; tube after flowering revolute-coiled. Stamens (3; the 3 anterior long-exserted; the 3 posterior (often sterile or imperfect) with very short filaments, unequally inserted lower down; anthers versatile, oval, blue. Ovary 3-celled; two of the cells empty, the other with a single suspended ovule. Utricle 1-celled.— Stout herbs, with thick creeping rootstocks, producing erect long-petioled leaves, and a 1-leaved stem, bearing a spike of violet-blue ephemeral flowers. Root-leaves with a sheathing stipule within the petiole. (Dedicated to Pontedera, Professor at Padua in the 18th century.)
P. cordata L. Leaves heart-shaped, blunt; spike dense, from a spathe-like bract; upper lobe of perianth marked with a pair of yellow spots (rarely all white); calyx-tube in fruit crested with 6-toothed ridges. — N. S. to Ont., Minn., and Tex. July-Sept. (Trop. Am.) Var. Angustifolia Torr. Leaves lanceolate or triangular-attenuate, roundish or truncate at base. — Same range.