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squamule (noun, pl. squamules) – a small (up to 5 mm long and wide; larger in basal squamules of Cladonia), more or less complanate, scale-like thallus or thallus segment, generally shorter than a thallus lobe or foliole, with an entire to flexuous or crenate margin, with or without a lower cortex. Squamules represent an intermediate growth form between crustose and foliose. They are generally more leaf-like than an areole, i. e., they have a distinct lower side and are at least partly ascending or lifted from the substrate. In contrast to areoles, they are thus sometimes removable intact. Areoles generally remain flattened, squamules always have distinctly lifted edges. However, some areoles can be described as subsquamulose, i. e., they have edges that show at least some tendency to turn upwards and become scaly. Some definitions restrict the term squamule to structures lacking a lower cortex or rhizines. Others definitions describe a squamule as corticate on both sides. In many lichens, a lower cortex is, however, only partially developed and these concepts, whether a lower cortex is present or not, are therefore too restrictive. Squamules may generally be fairly broadly attached and thus lack a distinct stipe or umbilicus. However, there is a continuum from flattened, broadly attached areoles to peltate or stipitate squamules, and, finally to subfruticose thalli. Related terms: primary thallus, horizontal thallus, primary squamule.

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