Fine cytoplasmic strands that connect the protoplasts of adjacent plant cells by passing through their cell walls. Plasmodesmata are cylindrical in shape (about 20–40 nm in diameter) and are lined by the plasma membrane of the two adjacent cells. The endoplasmic reticula of the two adjacent cells are connected by a narrower structure, the desmotubule, which runs through the centre of a plasmodesma. Plasmodesmata tend to occur in groups, forming distinct areas called primary pit fields (see pit). They permit the passage between cells of substances including ions, sugars, amino acids, and macromolecules.