Roots and the delivery of solutes to the xylem

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Abstract
The structural features of the pathways followed by solutes and water are described. The porous nature of the cell walls comprising the apoplasm is described and the difficulties in verifying the passage of water through different parts of the apoplasm are discussed. The endodermis is of ubiquitous occurrence and has two invariant characteristics, a girdle-like wall thickening, the Casparian band, and the attachment of the plasma membrane to the band. Suggestions are made concerning the constraints placed on the passage of materials in the stele by these structures. The hypodermis is also a very common structure which shares a number of properties seen in the endodermis. The implications of an apoplasmic barrier in the hypodermis are discussed. The plasmodesmata are the key structural feature of the symplasmic pathway and recent information makes it clear that the size of the pores in the neck region can vary with the physiological state and position of tissues. The symplasmic pathway seems not to be interrupted by structural developments which make the endodermis an apoplasmic barrier of high resistance. Recent information from transpiring plants indicates that the turgor pressure in cortical cells increases centripetally: there is, therefore an outwardly directed hydrostatic pressure gradient. The implications of these new findings for water and solute flows in the symplast are considered. The final step in the radial transfer of materials is their release into the xylem. There is evidence that stelar tissues contain an H$^{+}$-translocating ATPase whose activity can be influenced by physiological factors. It is pointed out that there may be major changes in the concentration of K$^{+}$ in xylem sap during a day-night cycle which may influence the polarization of the cell membranes of xylem parenchyma and the opening of ion-channels. The xylem elements themselves are not always fully conductive, even when their final diameter has been reached. The protoplasts and cross walls may be more persistent than is usually assumed, especially in soil-grown roots. Because of the low activity of Ca$^{2+}$ in the cytoplasm and the discontinuity of compartments within cells which contain abundant free Ca$^{2+}$, this ion probably moves radially primarily by diffusion in the apoplasm. The transfer of Ca$^{+2}$ across the endodermis is shown to depend on the activity of Ca$^{2+}$ ATPase in the plasma membrane of the stelar side of the endodermis, emphasising once again the epithelial nature of this cell layer.