The Collaborative International Dictionary of English v.0.48:

Telegraph plant \Telegraph plant\ (Bot.),
   A tick trefoil (Meibomia gyrans formerly Desmodium
   gyrans), native of the East Indies; it is a leguminous plant
   whose lateral leaflets jerk up and down like the arms of a
   semaphore, and also rotate on their axes.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

telegraph plant
    n 1: erect tropical Asian shrub whose small lateral leaflets
         rotate on their axes and jerk up and down under the
         influence of sunshine [syn: telegraph plant, semaphore
         plant, Codariocalyx motorius, Desmodium motorium,
         Desmodium gyrans]