Plant Cell Picture: Plasmodesmata; this image is 500 pixels across the original is 4,000 pixels across.
Plasmodesmata are the connecting channels that link plant cells together. They consist of a break in the cell wall that is lined by cell membrane. Through this channel, passes a strand of membrane called the desmotubule.
Desmotubules are derived from rough endoplasmic reticulum that has been stretched to form a rod like structure. Molecules pass along the channel that surrounds the desmotubule. The cytoplasm of all cells in a plant is connected via the plasmodesmata. Plant viruses spread via plasmodesmata (please see Ed
Rybicki's page on virus entry into plant cells).