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Figure 3.10 Cross section from a seminal wheat root stained with toluidine blue, showing the cortex, endodermis (black arrow), late metaxylem (LMX) central vessel, and peripheral xylem (red arrow). Many root hairs can be seen. Scale bar, 100 µm. (Image courtesy H. Bramley)

In vascular plants, water absorbed by roots is transported up the plant in the mature (dead) tracheary elements (xylem vessels and tracheids) of roots and stems (Figure 3.10).

Plants are capable of rapidly transporting water to heights in excess of 100 m, even from extremely dry soils and highly saline substrates. They can transport water from soils to leaves at velocities of up to 16 m per hour (4 mm per second) if they have wide xylem vessels in the range of 100 µm. With the more common xylem vessel size of 25-75 µm, peak velocities are 1-6 m per hour. What biophysical mechanism allows plants to achieve this? We know that plants do not possess a pump to move water to the canopy under positive pressure. Instead, plants suck!

Plants have evolved a transport system that relies on water sustaining a tensile force while under suction. The xylem sap in transpiring plants is under negative pressure. This elegant, but counter intuitive mechanism, described by the Cohesion-Tension theory, allows plants to move large quantities of water from the soil to the transpiring leaf surface with little input of metabolic energy. The following section describes the experimental history of how the Cohesion Theory came to be accepted.