The Collaborative International Dictionary of English v.0.48:

Apetalous \A*pet"al*ous\ ([.a]*p[e^]t"al*[u^]s), a. [Pref. a-
   not + petal.] (Bot.)
   Having no petals, or flower leaves. [See Illust. under
   Anther].
   [1913 Webster]

WordNet (r) 3.0 (2006):

apetalous
    adj 1: (of flowers) having no petals [syn: apetalous,
           petalless] [ant: petaled, petalled, petalous]