I think plasmodesmata is the organelle (or process) that allows things to pass from cell to cell. Hang on, let me get my books.

Okay;

Plasmodesma (or plasmodesmata, plural) are minute strands of cytoplasm which extend between adjacent cells through pores in the cell walls.

Ok, here is some BASIC stuff about plasmodesmata(remember, basic)
– present in all LIVING higher plant cells
– much longer than their corresponding junctions in animals(gap junctions) at a 0.1 micrometers. Also wider: 20-40 nm.
– running through the center is a cylindrical desmotubule which is continuous with the smooth ER. between the outside of the desmotubule and the inner face of the cylindrical channel formed by the plasma membrane is an annulus of cytosol through which small metabolites can pass. they allow passage of similar molecules as GAP junctions: with a weight <800
– they are inserted as the cell wall forms in cytokinesis. they can also be inserted through preexisting cell wall, when they are found in dense clusters called pit fields. when no longer required, they can be removed.
– transport through them CAN be regulated, as through GAP junctions. for example, some gene regulatory proteins involved in the process of cell fate determination bind to the plasmodesmata and override the size exclusion mechanism which otherwise would prevent their passage. In some rare cases, mRNA can also pass through them.