thumb|300px|The apoplastic and symplastic pathways
The symplast (from Greek sym "together" + plasma "formed or moulded substance") is the continuous, living network of cytoplasm that extends through most plant tissues. Its continuity is established by thousands of plasmodesmata — plasma-membrane-lined nanoscopic tunnels that pierce the cell walls and join the cytosol and endoplasmic reticulum of adjacent cells. Because those channels also open into the phloem sieve elements, the symplast provides both short-range and long-range conduits for water, nutrients, metabolites, proteins and RNAs to move between cells along concentration or pressure gradients. By contrast, the apoplast comprises the porous cell-wall matrix and extracellular spaces through which solutes diffuse outside the plasma membrane.
Structure and continuity
Electron microscopy shows that a single plasmodesma consists of a narrow cylindrical sleeve of cytosol bounded by plasma membrane and traversed by a central rod of endoplasmic reticulum called the desmotubule. and the term plasmodesmata was coined by Eduard Strasburger in 1901. Johannes von Hanstein introduced the word symplast in 1880, and Ernst Münch contrasted apoplast and symplast in his 1930 monograph on phloem transport. Modern fluorescence microscopy and dye tracer studies during the late twentieth century proved cytoplasmic continuity at the whole-plant scale and revealed that plasmodesmata are actively regulated conduits rather than passive holes.