The subclass Alismatidae consists of 16 families and scarcely 500 species. They are considered to be a near basal side branch of monocots, and are a relictual group that has retained a number of primitive features. The subclass is typically apocarpous and aquatic.
Flowers can be showy or highly reduced.
The vascular system is usually not well lignified and often much reduced, with vessels restricted to roots if present. It has been customary, although not without considerable debate, to link the Alismatidae to the Magnoliidae via the dicot Nymphaeales - the water lilies. Adaptation to aquatic, brackish, and even marine environments and subsequent reduction or loss of characters is a common trend. This trend is correlated with a diverse set of adaptations to water pollination. Recent molecular phylogenetic analyses indicate that the Alismatidae are indeed the basal group in monocots.