photo-epinasty

from The Collaborative International Dictionary of English v.0.48

Photo-epinasty \Pho`to-ep"i*nas`ty\, n. [See {Photo-}, and
   {Epinastic}.] (Bot.)
   A disproportionately rapid growth of the upper surface of
   dorsiventral organs, such as leaves, through the stimulus of
   exposure to light. --Encyc. Brit.
   [1913 Webster]