O.Henry tells of a lawyer with amnesia on a train with
a pharmacist, who mentions "jalap". I was dumbfounded at how much ancient information is available about this
long outdated botanical purgative.

Webster's Revised Unabridged Dictionary (1913)

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), a climber much like the morning-glory.
The abstract, extract, and powder, prepared from the tubers,
are well known purgative medicines. Other species of
Ipom[oe]a yield several inferior kinds of jalap, as the I.
Orizabensis, and I. tuberosa.

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