Source : Webster's Revised Unabridged Dictionary (1913)

Thallogen \Thal"lo*gen\, n. [Gr. ? young shoot or branch, frond
   + -gen.] (Bot.)
   One of a large class or division of the vegetable kingdom,
   which includes those flowerless plants, such as fungi,
   alg[ae], and lichens, that consist of a thallus only,
   composed of cellular tissue, or of a congeries of cells, or
   even of separate cells, and never show a distinction into
   root, stem, and leaf.