The Collaborative International Dictionary of English v.0.48:

Istle \Is"tle\, n.
   Same as Ixtle.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ixtle \Ix"tle\, Ixtli \Ix"tli\([i^]x"tl[-e]), n.
   1. (Bot.) A Mexican name for a variety of Agave rigida,
      which furnishes a strong coarse fiber; also, the fiber
      itself, which is called also pita, and Tampico fiber.
      [Written also istle.]
      [1913 Webster] Ixtle