The Collaborative International Dictionary of English v.0.48:

Jalap \Jal"ap\, n. [F., fr. Sp. jalapa; -- so called from
   Jalapa, a town in Mexico, whence it was first obtained.]
   (Med.)
   The tubers of the Mexican plant Ipom[oe]a purga (or
   Exogonium purga) of the family Convolvulaceae, a climber
   much like the morning-glory. The abstract, extract, and
   powder, prepared from the tubers, are well known purgative
   (cathartic) medicines, and are also called jalap. Other
   species of Ipom[oe]a yield several inferior kinds of jalap,
   as the Ipom[oe]a Orizabensis, and Ipom[oe]a tuberosa.
   [1913 Webster]

   False jalap, the root of Mirabilis Jalapa, four-o'clock,
      or marvel of Peru.
      [1913 Webster]