Intercellular communication in Azolla roots: I. Ultrastructure of plasmodesmata

A model is proposed for the structure of the plasmodesmata of Azolla root primordia, based on micrographs obtained by a combination of fixation in glutaraldehyde/p-formaldehyde/tannic acid/ferric chloride, digestion of cell walls and the use of stereo pairs. Unlike the model for plasmodesmatal structure proposed by Robards (1971), the desmotubule is depicted as a virtually closed cylindrical bilayer providing little or no open pathway for transport. In this respect it is similar to the model of López-Sáezet al. (1966). An analysis of the molecular packing of types of lipids found in endoplasmic reticulum (of which the desmotubule is an extension) indicates that the model is geometrically feasible. Details cannot be discerned with accuracy, but material, possibly particulate, occupies much of the space between desmotubule and plasma membrane, the cytoplasmic lumen being reduced to inter-particle spaces of cross-sectional area comparable to that of the bore in a gap junction connexon. Implications for intercellular transport are discussed.