Alisma L.
Family: Alismataceae
Alisma image
from USDA Plants Website  

Key to Wisconsin Alisma

Author: Christopher D. Tyrrell

    • 1a. Achenes with 2 abaxial grooves; leaves of mature plant submersed, ribbon-like, if emersed, lanceolate to narrowly elliptical, less than 2 cm wide A. gramineum

    • 1b. Achenes with 1 abaxial groove; leaves of mature plant emergent or floating, elliptical to ovate, up to 15 cm wide 2

    • 2a. Petals 3.5--6 mm long, petals distinctly longer than the sepals; mature fruiting heads 4--7 mm wide; achenes 2--3 mm long A. triviale

  • 2b. Petals 1--3 mm long, petals and sepals more or less equal in length; mature fruiting heads 2--5 mm wide; achenes 1.5--2.5 mm long A. subcordatum
Recognition of the rank of North American Alisma taxa (species vs. subspecies) has previously been the subject of controversy. Molecular and morphological evidence has shown Alisma plantago-aquatica, a name formerly in widespread use in North America, is native to Eurasia and the North American taxa are A. subcordatum and A. triviale. Alisma gramineum and A. subcordatum are diploid, while A. triviale is tetraploid.