\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.