Meristem growth dynamics and branching patterns in the Cladoniaceae

Branching patterns in the lichen family Cladoniaceae are varied and taxonomically important. Branching occurs on the podetium, the erect secondary thallus that characterizes most species in the Cladoniaceae, and is influenced by growth dynamics of the fungal meristem tissue at the apex of the podetium. Branching is primarily the result of meristem divisions, and branching patterns are modified by meristem enlargement, deformation, and torsion. Branching processes are conserved, and early branch ontogeny provides information from which to determine relationships in the Cladoniaceae. Branching is characterized by two major patterns. In one pattern, branches arise from the relatively late divisions of a large meristem (≥100 μm in diameter), whose shape changes during ontogeny. In a second pattern, branches arise from small meristems (<100 μm in diameter), which split early in ontogeny but whose shape does not change. The trend toward reduced meristems that split early in ontogeny is seen as an evolutionary advance in the Cladoniaceae. Some “small meristem” species retain aspects of the “large meristem” habit in early ontogeny, and this provides a clue to their relationships. Patterns of meristem growth dynamics provide a basis for interpreting phylogeny in mycobionts of the Cladoniaceae. Meristem activities in four genera of the Cladoniaceae were compared in order to determine trends in growth dynamics within the family.