SUMMARYPrevious taxonomic vicissitudes of the family are examined and the form and variation of the vegetative organs described. Detailed accounts are given of the comparative anatomy of all the plant members (except the flower) ofButomus umbellatus, Hydrocleys nymphoidesandLimnocharis flava.Special facets of structure are discussed in an attempt to correlate its organization with the influence of environment and to interpret its taxonomic significance.