Definition of: hypothallus

(hī′pə·thal′əs, hip′ə-) noun plural ·li (-ī) Bot.
1. The delicate hyphal filaments upon which a lichen thallus is first developed; a mycelial or hyphal outgrowth from the margin of the thallus of a crustaceous lichen.
2. In fungi, an under thallus on which spore cases are produced. [<HYPO- + Gk. thallos young shoot]
—hy′po·thal′line (-ēn, -in) adjective