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a cytoplasmic thread that connects adjoining plant cells. Protoplasts are connected by plasmodesmata. The number of plasmodesmata, which have a diameter from 180 to 680 Å (most often 300 to 400 Å), varies from cell to cell. They are distributed in the tiny canals that pass through the primary cell membrane on the primary pit fields. In cells with a secondary membrane they are found only in the pit membranes. The cavities of the tiny canals are lined with the external membrane of plasmodesmata—the plasma membrane. Plasmodesmata ensure transmission of stimuli and movement of matter from cell to cell.