thallogen

from The Century Dictionary.

• noun In botany, same as thallophyte.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) One of a large class or division of the vegetable kingdom, which includes those flowerless plants, such as fungi, algæ, 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany Any of a large number of plants that consist of a thallus only, with no flower, leaves etc.