A plasmodesma is a tunnel through the cellulose-pectin wall of plant cells that connects the plasma membranes with the cell cytoplasm.
Linked by this cellular connection, the cells form a continuous channel : the symplastic pathway.
Plasmodesmata are used by water, solutes, proteins and some viruses to circulate through the plant.
Structural diagram of a plasmodesma. The plasmodesma crosses cell walls (CW) and links the membranes (PM). A desmotubule (DM) connecting two endoplasmic reticula (ER) controls the movement of molecules. © Smartse, Wikimédia CC by-sa 3.0