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Parenchyma Cells - Parenchyma cells are thin-walled cells that contain living substances for a few to many years - over 150 years in some trees. Parenchyma cells may be axial, aligned vertically or radial, aligned in radial ray- like arrangements. Radial parenchyma makes up structures called rays. Sapwood has an interconnected network of living axial and radial parenchyma. These Parenchyma cells make up a web work in sapwood, xylem, cambium zone, inner bark (phloem) and bark cambium (phellogen) which collectively we call the symplast. The symplast is held in place by the apoplast (dead fibers and tissues). The symplast stores starch and the apoplast stores bound water, water bound to the walls of cellulose. Trees only store starch in living cells. Not all wood contains living cells, e.g. "protection wood". More info with diagram ... Click here for some pictures.