The Collaborative International Dictionary of English v.0.48:

Acrogen \Ac"ro*gen\, n. [Gr. 'a`kros extreme, high + -gen.]
   (Bot.)
   A plant of the highest class of cryptogams, including the
   ferns, etc. See Cryptogamia.
   [1913 Webster]

   The Age of Acrogens (Geol.), the age of coal plants, or the
      carboniferous era.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acrogen
    n 1: any flowerless plant such as a fern (pteridophyte) or moss
         (bryophyte) in which growth occurs only at the tip of the
         main stem