Punctelia colombiana

The thallus of Punctelia colombiana attains a diameter of up to 18 cm (7.1 in). It is tightly attached to its bark substrate, and has a greenish-gray to ash-grey colour. Pseudocyphellae are white, small (measuring 0.1–0.2 mm in diameter) and evenly distributed throughout the thallus surface. Isidia are present over much of the surface. The medulla is white, and it measures 80.0–130.0 μm thick. The thallus undersurface is mostly black with a dark brown zone around the margin. Rhizines are dark brown to black, with a branching pattern ranging from unbranched to irregular; they are abundant and evenly distributed around the margin. The upper cortex is paraplenctenchymatous; this refers to a type of tissue in which the hyphae are oriented in all directions, and is analogous to the parenchyma of plants. The upper cortex is covered by a thin epicortex, although it is ruptured on the areas above the pseudocyphellae. The pseudocyphellae are formed from the rupture and disintegration of the upper cortex cells, which exposes the hyphae of the medulla. The size of the pore gradually increases as the cells around the inside perimeter disintegrate and the medullary hyphae grow into the pore area. Air pockets, called aeroplectenchyma, occur under the upper cortex in the region in contact with the algal layer. …