The Collaborative International Dictionary of English v.0.48:

Gamopetalous \Gam`o*pet"al*ous\, a. [Gr. ? marriage + E.
   petalous: cf. F. gamop['e]tale.] (Bot.)
   Having the petals united or joined so as to form a tube or
   cup; monopetalous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gamopetalous
    adj 1: having a corolla composed of partially or wholly fused
           petals forming a corolla shaped like a tube or funnel
           [syn: gamopetalous, sympetalous]