Freeze-fracture observations on the plasma membrane, the cell wall and the cuticle of growing protonemata of Adiantum capillus-veneris L.

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Abstract
Using freeze-fracture electron microscopy we have examined the morphology of the plasma membrane and the cell wall of single-celled protonemal filaments of the fern Adiantum capillus-veneris grown under continuous red light. The surface of the protonemal cell wall is completely covered by a multilayered, lipid-like coat, probably consisting of cuticular waxes. The rhizoid seems to lack this type of coat. The cell walls of the protonemata contain 8-nm thick, randomly oriented fibrils. In rapidly growing protonemata the P-face of the plasma membrane contains both randomly distributed particles and distinct particle rosettes. The rosettes consist of six 8–9-nm-wide particles in a ring-like configaration and have an outer diameter of 24 nm. They closely resemble the particle rosettes seen on the P-face of the plasma membrane of green algae and of higher plants, which recently have been implicated in the synthesis of cellulose fibrils. Within 20 μm from the tip of the protonemata, and coinciding with the region of maximal cell-wall growth and expansion and thus cellulose-fibril synthesis, the greatest density of rosettes (20/μm2) is observed. Beyond 20 μm from the tip this number drops rapidly to near zero at 50 μm. The rosettes have a tendency to form small, irregular clusters, but only very rarely are three or more rosettes found in a row or in a geometrical pattern. Our measurements of the size and the density of the randomly distributed plasma membrane particles indicate that the tip region must be specialized with respect to other plasma-membrane activities as well. Thus the tip region contains not only the highest density of randomly destributed intramembrane particles, but also particles of different sizes than those found elsewhere in the plasma membrane.