auxin

from WordNet (r) 3.0 (2006)

auxin
    n 1: a plant hormone that promotes root formation and bud growth

from The Collaborative International Dictionary of English v.0.48

auxin \aux"in\ n.
   1. a substance which, in small concentrations, promotes root
      formation, bud growth, or certain other processes such as
      fruit ripening or leaf drop in plants.
      [WordNet 1.5 +PJC]