The minimum radial translocation rate of sugars has been determined from the starchaccumulation rate for the wood rays of Populus x canadensis Moench 'robusta', and related to ultrastructural peculiarities of the cell walls to be passed. The minimum radial flux or flow of sugars through the tangential walls, the pit fields, and per plasmodesma was 80.7 pmol · cm(-2) · s(-1), 400 to 800 pmol · cm(-2) · s(-1), and 1.0 to 1.7 · 10(-7) pmol · plasmodesma(-1) · s(-1), respectively. These values exclude a transmembrane flux mechanism and indicate that the radial translocation in this tissue must proceed via plasmodesmata. In the isolation cells of the ray center we found 39 plasmodesmata per μm(2) of pit field, 8.0 per μm(2) of tangential wall, and 1.98% of the wall occupied by plasmodesmata. Cells of the ray margins show plasmodesmata on only 1.16% of their tangential wall area and thus appear to be slightly inferior for radial translocation. As judged from both the observed plasmodesmatal frequencies and the translocation rates, the ray parenchyma cells are comparable to cells specialized in short-distance translocation.