The Collaborative International Dictionary of English v.0.48:

Alcanna \Al*can"na\, n. [Sp. alcana, alhe?a, fr. Ar.
   al-hinn[=a]. See Henna, and cf. Alkanet.] (Bot.)
   An oriental shrub (Lawsonia inermis) from which henna is
   obtained.
   [1913 Webster]