The Collaborative International Dictionary of English v.0.48:

Bladderwort \Blad"der*wort`\, n. (Bot.)
   A genus (Utricularia) of aquatic or marshy plants, which
   usually bear numerous vesicles in the divisions of the
   leaves. These serve as traps for minute animals. See
   Ascidium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bladderwort
    n 1: any of numerous aquatic carnivorous plants of the genus
         Utricularia some of whose leaves are modified as small urn-
         shaped bladders that trap minute aquatic animals