From 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]
From 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