The Collaborative International Dictionary of English v.0.48:

mahon stock \ma*hon" stock`\ (Bot.)
   An annual cruciferous plant with reddish purple or white
   flowers (Malcolmia maritima). It is called in England
   Virginia stock, but the plant comes from the Mediterranean.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Malcolmia maritima
    n 1: erect branching herb cultivated for its loose racemes of
         fragrant white or pink or red or lilac flowers; native to
         sands and sea cliffs of southwestern Greece and southern
         Albania [syn: Virginian stock, Virginia stock,
         Malcolmia maritima]