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.
© Webster 1913.