Thallus variety (from Albert Schneider's A text-book of general lichenology)
These drawings (of thallus cross sections) show some of the variety in thallus structure. In all cases the photobiont cells are dark and rounded.
1. Haematomma ventosa - The drawing shows the cortex, photobiont layer and medulla.
2. Urceolaria scruposa - This crustose species has no lower cortex and anchoring hyphae grow downward directly from the medullary hyphae.
3. Heppia despreauxii - Both upper and lower cortices, with rhizines growing from the latter.
4. Speerschneidera euploca - The upper and lower cortices are very well-developed, with little in the way of medullary tissue.
Note: not all to the same scale
Albert Schneider's A text-book of general lichenology, published in Binghamton in 1897 by Willard N. Clute & Co. illustrates the structural features of various lichens from the north-eastern United States.