5.2.5  Water extraction from the pipeline

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Vessels are not just pipes to carry water, they are pipes with holes in them (pits) through which water can leak out, fulfilling a principal leaf function of water distribution through the transpiration stream to places where it will evaporate. The branching network of vessels is beautifully adapted to achieve this.

Think about flow in a leaky vessel. The volume of forward flow varies as the fourth power of the radius (Poiseuille’s Law). The frequency of leaks through vessel walls varies with surface area of the walls, that is, as the first power of the radius (2πr). So if the vessel is wide, forward flow is much larger than leakage. The wide vessels in large veins supply water all over the leaf without losing much on the way. As the width of a vessel becomes smaller, the forward flow (a function of r4) is reduced much more strongly than the leaks (a function of r). The proportion of extracted water increases in relation to forward flow. Indeed, for a fixed pressure gradient there is a critical radius at which all water entering the vessels supplies leaks, and there is no forward flow at all. The finest veins of leaves have vessels of a diameter that is close to this critical value. As sap disperses into the fine ramifications of the network, it moves more and more slowly forward, and leaks increasingly outwards through the sheath to the mesophyll. This is the rationale of the distributing networks of the small branched veins of both dicotyledons and grasses.


Figure 5.9    Fresh paradermal hand-section of a leaf of Eucalyptus crenulata which had been transpiring for 80 min in a solution of sulphorhodamine G. The dye solution is present at low concentration in the vessels of the larger veins, but is not visible at that concentration. In the smallest veins the dye has become so concentrated by loss of water to the symplasm that dye crystals have formed inside vessels (cf. Figure 5.8). Sectioned in oil, bright-field optics. Scale-bar = 100µm (photograph courtesy M. McCully and M. Canny)

Extraction of water from fine veins can be readily demonstrated. Stand a cut leaf in an aqueous solution of dye, such as 0.1% sulphorhodamine G, and allow it to transpire for an hour. The solution moves rapidly through large veins all over the leaf in a few minutes. Then it moves increasingly slowly into the network of distributing small veins. By the end of an hour it has reached the ends of the finest veins and, as water is extracted from them, the dye becomes more and more concentrated (Figure 5.8). Sparingly soluble dyes crystallise out as solids in these small vessels. You may see these dye deposits by cutting hand-sections of leaves under paraffin oil and mounting these sections in oil (O’Dowd and Canny 1993). Solid dye is confined to the smallest veins, often in localised deposits (Figure 5.9). Whether dye movements give a reliable picture of the movement of natural solutes will be taken up below.

This experiment reveals something else fundamentally important about water movement. Separation of dye from water is evidence of ultrafiltration. Water passes out of the pits in vessel walls and enters the plasma membrane of bundle sheath cells; dye is excluded from these cells. This experiment shows that water was extracted from vessels into living cells.