Alisma is a genus of plants in the Alismataceae or water-plantain family. The genus consists of aquatic plants with leaves either floating or submerged, found in a variety of still water habitats. The flowers are hermaphrodite, and are arranged in panicles, racemes, or umbels. Alisma flowers have six stamens, numerous free carpels in a single whorl, each with 1 ovule, and subventral styles. The fruit is an achene with a short beak.
The nineteenth century British art and social critic John Ruskin believed that the particular curve of the leaf-ribs of Alisma represented a model of 'divine proportion' and helped shape his theory of Gothic architecture.