Apoplast and symplast are two related concept in plant biology.
Apoplast is the extracellular space consisting of cell walls and spaces between cells. Water and solutes can move freely in the apoplast, although in roots it is interrupted by the Casparian strip, forcing water to flow to symplast.
Symplast is the space within plants formed by the cytoplasms of individual cells connected by plasmodesmata . Water, solutes, and metabolites can move in the symplast, but cells usually take up compounds they need and export those available in excess.