Section of the lichen Parmelia, a genus of foliose lichens. Lichens are formed by a symbiotic association between a fungus (the mycobiont) and an alga or a cyanobacterium (the photobiont). Only the fungal partner reproduces sexually, this occurs in the two saucer shaped structures called apothecia. The vertically orientated cells form the hymenium which consists of asci interspersed with sterile hyphae (the paraphyses). Magnification: X 28 at 10cm wide.