The symplast concept. A general theory of symplastic transport according to the thermodynamics of irreversible processes.

The movement of solutes through the symplasm (from cell to cell via the bulk cytoplasm and cytoplasm filled plasmodesmata) is examined in the steady state according to the thermodynamics of irreversible processes. This analysis together with a survey of the biological literature has yielded the following tentative conclusions: o (1) Plasmodesmata are commonly found in a fairly wide range of plants (Laminaria, Tamarix, Allium, Viscum, Avena, Lycospersicum, Salix, Nitella, Zea, etc.). (2) Plasmodesmata occupy anywhere from 0·08 to 0·002 times the area in common between adjacent cells; a relative area of about 0·01 (found in the mature cortical cells of Allium cepa roots) is not unusual. (3) The plasmodesmata constitute the pathway of least resistance for the diffusion of all small solutes. (4) Diffusion will be the predominant mechanism of transport across the pores for small solutes. (5) Solute distribution within the bulk cytoplasm of each cell ought to be by a combination of self diffusion and cyclosis. (6) There ought to be nearly perfect mixing within the bulk cytoplasm of each cell. (7) In the course of the symplastic transport of solutes in the steady state there can be a significant coupling of water transport under proper conditions. (8) The concentration drop of salts between adjacent cortical cells of onion roots necessary to transport all the observed salt flux into the xylem is 0·1 m n per cell or about 1 m n over the entire width of the cortex.