Cell-wall differentiation during growth of electrically polarised protoplasts of Physcomitrella

Protoplasts of Physcomitrella patens have been grown in continuous electric field of 50 V cm(-1), resulting in a predictable pattern of filament emergence. The events preceding the visible formation of a polar axis have been examined by electron microscopy. The first sign of polarity is the formation of a thickened inner wall layer over the potential growth site. Elongation of the filament is preceded by the appearance of a layer of heavily stained amorphous material at the external surface of the thickened wall. This material marks the region of initial extension of the filament, but it is not produced once extension has begun, and further growth of the filament results in the retention of the material as an annular ring at its base. The wall of the filament has a complex thickened structure which is a result of the osmotic conditions under which the protoplasts are grown. These results are discussed in terms of the development of the polar axis.