It has been fully establish that mineral salts absorbed by the roots are mostly translocated to young leaves and other growing organs of the plant through xylem. Stout and Hogland (1939) using radioactive k42 have confirmed that translocation of inorganic salts in an upward direction was effected largely through xylem elements.
Studies of the sap from xylem vessels show that the concentration of inorganic constituents in the xylem sap is usually higher. Further more; appreciable concentrations of mineral salts are commonly present in the sap of vessels at seasons when upward flow of water is occurings at its most rapid rates.
Ringing experiments where xylem was removed have shown complete prevention of the upward translocation of inorganic ions which suggests that minerals are translocated through xylem. Some minerals are also transported through pholem tissues of plants.
The movement of minerals in the phloem has been shown to be both upward and downward. These are the minerals which are exported from the leaves immediately before abscission. Salts also move laterally in tile cambium and pholem. Thus the direction of inorganic salt movement in plants may be upward, downward, lateral or outward.
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In primary roots in which the casparian strip of endodcrmal cells is intact and continuous around the stelar region, the major pathway of mineral ions across the root is through the protoplasm of root cells i.e. from the protoplast of one cell to the next cell. The protoplasts of all cells of the primary root arc interconnected and united by plasmodesmata.
The continuum of protoplasts of many cells, together with plasmodesmata that connect them is called symplast. Having entered the symplast, inorganic ions diffuse inward because of cytoplasmic streaming and inward flow of water. The symplast extends from the cortex into the stele and penetrates the endodermis.
Thus the movement of ions from cortex to stele does not require them to be transported across the plasma membrane at the endodermis.
The ions leave the symplast after reaching the xylem parenchyma cells and get deposited into xylem tracheids and vessels, where they are translocated upward to leaves with the aid of transpiration stream. Salts entering the main vascular stream of leaf sources move primarily in a downward direction in the pholem tissue.