REITERATIVE PRODUCTION AND DEFORMATION OF CELL-WALLS IN EXPANDING THALLUS NETS OF THE LICHEN RAMALINA MENZIESII (LECANORALES, ASCOMYCETES)

The thallus of the lichen Ramalina menziesii Tayl. is composed of net- like units that develop by diffuse expansion of perforated tissue prod uced at the net apex. Study of net tissue with transmission electron m icroscopy reveals that the cortical cells are surrounded by a successi on of cell walls alternating with layers of an electron-transparent ma trix substance. In the course of thallus growth the cortical cell wall s are continually deformed and new ones constructed. The deposition of new walls and matrix layers displaces the older walls centrifugally f rom the cell. Electron-dense boundaries develop at the interfaces amon g cells where the remains of the oldest walls are compressed against t hose of neighboring cells. As new branch cells are inserted through th e concentric accumulations, the dense boundaries appear to enclose fas cicles of cells, visible in cross section with light microscopy. Corti cal organization in Ramalina menziesii is contrasted with that reporte d in other lichens, and a functional relationship to diffuse growth of the thallus is suggested.