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Water plantain is another plantain that's not a plantain, but it does look like one. This plant is an emergent aquatic with a basal rosette of large (up to 7"x 4"), hairless leaves with long petioles that are filled with distinctive spongy tissue (like a cellular foam); the leaves are (largely) parallel-veined. A tall (up to 3') highly-branched, whorled inflorescence rises from the center of the basal rosette; at the tip of each branch is a single 1/4" wide flower with three white petals (with a spot of yellow at their bases), three green sepals (with transparent margins) about half the length of the petals, and six stamens surrounding a greenish center that bears 1-2 dozen pistils. Water plantain is apparently popular with muskrats. Small-flowered water plantain (Alisma subcordatum) is more common in Illinois, but not known from Jackson Park; water plantain and small-flowered water plantain are nearly identical, but the latter has smaller flowers (1/8") and the petals and sepals on the flower are the same length.

Alisma

Alisma triviale

northern water-plantain [Blooms: Jun-Aug]