Distribution and structure of the plasmodesmata in mesophyll and bundle-sheath cells of Zea mays L

In leaf blades of Zea mays L. plasmodesmata between mesophyll cells are aggregated in numerous thickened portions of the walls. The plasmodesmata are unbranched and all are characterized by the presence of electron-dense structures, called sphincters by us, near both ends of the plasmodesmatal canal. The sphincters surround the desmotubule and occlude the cytoplasmic annulus where they occur. Plasmodesmata between mesophyll and bundle-sheath cells are aggregated in primary pit-fields and are constricted by a wide suberin lamella on the sheath-cell side of the wall. Each plasmodesma contains a sphincter on the mesophyll-cell side of the wall. The outer tangential and radial walls of the sheath cells exhibit a continuous suberin lamella. However, on the inner tangential wall only the sites of plasmodesmatal aggregates are consistently suberized. Apparently the movement of photosynthetic intermediates between mesophyll and sheath cells is restricted largely or entirely to the plasmodesmata (symplastic pathway) and transpirational water movement to the cell walls (apoplastic pathway).