The apoplast is one of three methods of radial transport of water and solutes in plant tissues.
The apoplast consists of all of the interstices of cell walls and dead cells of the xylem. Water and solutes pass through this channel to reach the organs without penetrating cells.
Water and solutes can also move through this channel over a certain distance before passing through the symplast or passing through the plasmic membranes and pectin-cellulose walls several times.
Diagram of radial transport channels in plant tissues. The apoplast channel (shown in pink) moves through the cell walls. © Jackacon, Wikimédia public domain