What is thallus sessile?

A thallus is the body of a fungus or non-vascular plant such as algae or lichen, which is not differentiated into roots, stems and leaves. A sessile thallus is one that is attached to a substrate such as a rock, tree or soil, and does not have a stalk. Sessile thalli are common in algae and lichens, and may be crustose, foliose or fruticose in form. Crustose thalli are crust-like and closely appressed to the substratum, foliose thalli are leaf-like, and fruticose forms have a branched, shrub-like appearance.