Crystalline fibrils and complexes of membranes in the parietal layer in sieve elements.

Fibrils are described which were found in the parietal layer in well differentiated sieve elements under the electron microscope. The fibrils were usually about 0.1 micrometers wide and appeared, in longitudinal section, to be composed of closely parallel filaments. In transverse section they appeared as a lattice, possibly of closely packed tubules. Some of the fibrils were joined to complexes of membranes in the parietal layer and to plasmatic filaments. Their relationship to these structures and to fibrils which may cause movements in other kinds of cells is discussed.