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, algae,
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.
[1913 Webster]