The Collaborative International Dictionary of English v.0.48:

Pieplant \Pie"plant`\, n. (Bot.)
   A plant (Rheum Rhaponticum) the leafstalks of which are
   acid, and are used in making pies; the garden rhubarb.
   [1913 Webster] Piepoudre

The Collaborative International Dictionary of English v.0.48:

Rhubarb \Rhu"barb\, n. [F. rhubarbe, OF. rubarbe, rheubarbe,
   reubarbare, reobarbe, LL. rheubarbarum for rheum barbarum,
   Gr. ??? (and ??) rhubarb, from the river Rha (the Volga) on
   whose banks it grew. Originally, therefore, it was the
   barbarian plant from the Rha. Cf. Barbarous,
   Rhaponticine.]
   1. (Bot.) The name of several large perennial herbs of the
      genus Rheum and order Polygonaceae.
      [1913 Webster]

   2. The large and fleshy leafstalks of Rheum Rhaponticum and
      other species of the same genus. They are pleasantly acid,
      and are used in cookery. Called also pieplant.
      [1913 Webster]

   3. (Med.) The root of several species of Rheum, used much
      as a cathartic medicine.
      [1913 Webster]

   Monk's rhubarb. (Bot.) See under Monk.

   Turkey rhubarb (Med.), the roots of Rheum Emodi.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Rheum rhaponticum
    n 1: long cultivated hybrid of Rheum palmatum; stems often
         cooked in pies or as sauce or preserves [syn: pie plant,
         garden rhubarb, Rheum cultorum, Rheum rhabarbarum,
         Rheum rhaponticum]