The Collaborative International Dictionary of English v.0.48:

Epinastic \Ep`i*nas"tic\, a. [Pref. epi- + Gr. ? pressed close.]
   (Physiol.)
   A term applied to that phase of vegetable growth in which an
   organ grows more rapidly on its upper than on its under
   surface. See Hyponastic.
   [1913 Webster]