Intercalary growth causes geometric length expansion in Methuselah's beard lichen (Usnea longissima)

In sharp contrast to many slow-growing lichens, some epiphytic pendulous and foliose lichens display striking biomass accumulation in favorable habitats. Whereas lichen growth rates have received considerable attention, comparatively few detailed studies of growth patterns have been carried out. Generally, most lichens seem to grow apically, with only pseodomeristomatic tissue confined to lobe margins and branch tips. However. some species appear to retain the capacity to expand throughout the thallus, thereby increasing their growth potential. Such intercalary growth processes have proved difficult to confirm in the field for two- and three-dimensionally growing folious and fruticose forms. Using transplants of the conspicuous, one-dimensionally growing Usnea longissima Ach., we document that intercalary, growth actually does occur, with thalli expanding geometrically in length with a doubling time of less than a year under favorable conditions.