5.1.1  Introduction

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Water loss by plants accompanies photosynthesis and nutrient acquisition. In most plants, this water loss reaches a maximum during daylight hours, as gas exchange peaks. Leaves are the primary evaporating surfaces of shoots but stems, fruits and flowers can be important sites of water loss too. Roots must therefore extract large amounts of soil water and deliver it to heights of up to 100 m in a transpiration stream. This trans-piration stream carries with it nutrients into the shoot canopy (Section 5.1.5; see Case study 5.1).

Water is difficult to extract from plants in any quantity, but as Australian Aborigines found, long tree roots can be detected as raised soil contours and can be dug up fairly easily to yield water (White 1994). If such a root is cut or broken into short lengths and held vertically, watery sap runs from the lower end where it can be collected to drink. The tenacious hold of plant tissues on entrained water has been broken by wounding, allowing air to enter and displace the captive sap.

Such release implies water-filled conduits, but does not reveal the mechanism for long-distance movement of sap in plants. Xylem flow has been the subject of endless debate for experimentalists trying to identify forces involved. A summary below of the early history of experiments dealing with vertical lift of sap into shoots shows that some accord has been reached but details of the mechanism of sap transport are still being debated.