pitfall traps are derived from rolled leaves, which evolved several independent times through convergent evolution. the vascular tissues of "sarracenia" is a case in point. the keel along the front of the trap contains a mixture of leftward- and rightward-facing vascular bundles, as would be predicted from the fusion of the edges of an adaxial (stem-facing) leaf surface. flypapers also show a simple evolutionary gradient from sticky, non-carnivorous leaves, through passive flypapers to active forms. molecular data show the "dionaea"–"aldrovanda" clade is closely related to "drosera", and evolved from active flypaper traps into snap traps.