Abstract
Foliicolous lichens complete their life cycles upon non-deciduous leaves in humid tropical and subtropical environments. The anatomy of these lichens and its adaptations to this specialized niche have received only limited attention. The present work examines the structural organization in seven species of the Gomphillaceae that colonize palm leaves in south-west Florida, USA. Thalli with their leaf substratum were embedded in resin and examined with SEM in backscattered electron detection mode. All species showed a continuous, covering epilayer of fungal origin that consisted of relatively sparse, scattered cell lumina of reduced diameter connected by cell wall-derived material. Fungal cells were intermixed below this layer, with algal cells often resting directly on the substratum. Interactions between symbionts were mainly limited to wall-to-wall contacts without penetration. The epilayer showed continuity with the fungal prothallus at the perimeter of the thalli. Crystalline deposits in the upper portions of thalli were common, particularly in the species of Gyalectidium examined. All thalli were entirely epicuticular; some covered the stomata of the leaf beneath. It is suggested that the prominence of non-living wall materials and mineral crystals in thallus structure might serve to minimize the metabolic cost of fungal tissue in climates where warm temperatures at night result in higher rates of respiration.