Fluorescence micrograph of a transverse section through a hornwort (Anthoceros agrestis) thallus. Nostoc cyanobacteria colonies (large red areas in lower frame) are embedded in the thallus. In the upper frame are hornwort thallus cells with single chloroplasts that appear red due to chlorophyll autofluorescence. The presence of one chloroplast per cell is a unique feature in land plants, and hornworts share it with green algae, the progenitors of land plants. Plant tissue is observed in ultraviolet (UV) light. Magnification: x300 when printed at 10 x 10 cm.