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Abstract
The fruticose thallus of mature Cladina subtenuis is characterized by a variable branching pattern. While the shape of the mature thallus is not easily predictable, early branch initiation follows a highly predictable pattern. Branching is determined by meristemlike bundles of fungal cells located at the apices of the thallus. When the bundles split, branching is initiated. Meristem growth is limited to a uniform developmental pattern of dichotomous, sequential divisions of the fungal bundles. Filial meristem bundles, the result of a single division, comprise a developmental unit. Their subsequent divisions, which are sequential, produce meristem initials of three or five bundles. Branching is constrained by this pattern, but variability is introduced by the differential elongation and orientation of certain branches. Variability may arise in response to growth conditions such as the proximity of other branches or environmental conditions, for example light availability. The ability to elaborate on the basic plan of development permits radial expansion and increased photosynthetic opportunities for the thallus of C. subtenuis.