GROWTH DYNAMICS IN CLADONIA-GRAYI

The erect secondary thallus (podetium) of the lichen Cladonia grayi de velops in distinct stages that can be traced to the development of its fungal meristem tissue. In early ontogeny, while the contiguous meris tem grows unilaterally both outward and vertically, the gross morpholo gy of the podetium resembles other cup-forming species. In later ontog eny, when the divided meristem grows vertically, the podetium forms a subtly articulated spiral. The spiral growth form is most apparent in branching proliferations that grow along the cup margin. The meristem tissue at the apex of these proliferations is curved, maturing sequent ially in a spiral pattern. The proliferations curve in a spiral around the vertical axis of the podetium. Apothecia at the apex of prolifera tions curve in a similar manner. Evidence for spiral growth along the cup margin can also be inferred from the sequential development of pyc nidia that emerge from it. Similar to early land plants, the growth dy namics of C. grayi may represent an evolutionary advance from an ortho tropous growth pattern to a branched, photosynthetic thallus.