The Collaborative International Dictionary of English v.0.48:

Exogen \Ex"o*gen\, n. [Exo- + -gen: cf. F. exog[`e]ne.] (Bot.)
   A plant belonging to one of the greater part of the vegetable
   kingdom, and which the plants are characterized by having c
   wood bark, and pith, the wood forming a layer between the
   other two, and increasing, if at all, by the animal addition
   of a new layer to the outside next to the bark. The leaves
   are commonly netted-veined, and the number of cotyledons is
   two, or, very rarely, several in a whorl. Cf. Endogen.
   --Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exogen
    n 1: flowering plant with two cotyledons; the stem grows by
         deposit on its outside [syn: dicot, dicotyledon,
         magnoliopsid, exogen]