Homoiomerous thalli (from Albert Schneider's A text-book of general lichenology)
In a homoiomerous thallus the photobiont is distributed more or less uniformly throughout the thallus. These four illustrations show cross sections through such thalli. Of the four species pictured the Collema lacks cortices, the Pannaria has an upper cortex (indicated by the red bar) and the remaining pair have both upper and lower cortices, albeit just one cell thick in each case. In all four species the photobiont cells are in short chains and are shown darker than the hyphae.
1. Collema pulposum; 2. Leptogium tremelloides; 3. Pannaria lurida; 4. Mallotium saturninum.
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.