Sap movements in trees

Abstract

Water and dissolved mineral nutrients are taken up by the roots and ascend via the xylem (the wood) into the leaves, where most of the water is lost by transpiration. Transpiration from leaves pulls up the water; movement is presumably along a gradient of negative pressures (tensions). The conducting tissues are uniquely adapted to negative pressures, for they contain rigid tubes which acquire a strong cellulose and lignin re-inforced wall during growth. Conducting cells die before they become functional. The whole water-conduction tissue is continuous throughout the plant. Rapid movement takes place through xylem tubes, slow movement through walls of all cells, even living ones. The transpiration stream is very low in concentration (< 0.5% w/v). However the rapid rate of movement (up to around 50 m/hr), and loss of water by transpiration from leaves, carry sufficient quantities of mineral nutrients into the leaves.—Growth of flowers, fruits, branches, stem and roots can only take place if building materials (photosynthetic products) are translocated from the leaves to these places. This type of long-distance transport takes place in the phloem (the inner bark) mainly in a downward direction. The conducting channels are living but enucleate cells. Velocities of translocation are of the order of 1 m/hr, the concentration of the moving solution is much higher than in the xylem (10–20% w/v, mostly sugars), and the pressures in the system are positive. Cross transfer between the two transport systems enables plants to circulate materials throughout their body.