Alismatidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used (there are many such systems); the only requirement being that it includes the family Alismataceae. It is a relatively new name: earlier systems, such as the Engler and Wettstein systems, used the name Helobiae for a comparable unit.
The Takhtajan system treats this as one of six subclasses within the class Liliopsida (=monocotyledons). It consists of:
superorder Alismatanae
order Butomales
order Hydrocharitales
order Najadales
order Alismatales
order Aponogetonales
order Juncaginales
order Potamogetonales
order Posidoniales
order Cymodoceales
order Zosterales
The Cronquist system treats this as one of four subclasses within the class Liliopsida (=monocotyledons). It consists of (1981):
order Alismatales
order Hydrocharitales
order Najadales
order Triuridales
This subclass comprises less than five hundred species total: many of these are aquatic or semiaquatic plants (seeAlismatidae info).
The APG II system does not use formal botanical names above the rank of order; it assigns most of the plants involved to the (expanded) order Alismatales, in the clade 'monocots', although the plants in Cronquist's order Triuridales are assigned to quite different placements.