5.1.2  Experimental history: how the Cohesion Theory came to be accepted

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Around 1905 great plans were made to resolve the mystery of the ascent of sap in trees by Professor E.J. Ewart in Melbourne, using eucalypts as a model plant. At that time, Australian mountain ashes (Eucalyptus regnans) vied with American dawn redwoods (Sequoia gigantea) as the tallest trees in the world, being well over 100 m tall. Using special scaffolding, Ewart climbed eucalypt trees, removed lengths of branch and measured the pressures required to push water through these stems. These investigations led him to conclude ‘The ascent of water is, therefore, a vital problem in so far as it depends upon conditions which hitherto can only be maintained in living wood’. If water transport required living cells, it could not be supported by discovery of a pump akin to that in animals. Even roots, which sometimes could pump water by root pressure (Section 3.6), lacked the necessary positive pressures to push water so far aloft, especially around midday when water was most needed.

Suction from the shoots was an alternative explanation but manmade suction pumps could not fulfil the task without inducing formation of air bubbles (embolisms) in the xylem and blocking flow. One clue to the solution came from Dixon and Joly (1894) who claimed that very pure water molecules would be held together by powerful cohesional forces provided the water was especially clean (much cleaner than in manmade pumps).

Ewart did not agree with the unorthodox proposal that the suction of pure water through xylem vessels underpinned transpiration. However, Dixon (1914) ultimately postulated the Cohesion Theory, based on those properties of water which distinguish it as an ideal biological solvent — cohesion, adhesion to walls of the vessels and surface tension being central features. In short, in the absence of microscopic gas bubbles water could withstand quite enormous tensions. Through the evaporative power of the atmosphere, a continuous ‘chain’ or ‘catena’ of water, well below atmospheric pressure, could be drawn up to a leaf canopy. The tensions created in this way could even suck water from the surrounding soil. We now recognise that the evaporative energy is supplied as the latent heat of vaporisation ultimately derived from solar power. This cohesive property of water gave rise to the ‘Cohesion Theory for the Ascent of Sap’.

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Figure 5.1 (a) Mercury sucked into tracheids of pine (Pinus radiata) by transpirational pull generated in the shoots. The water—mercury interface is powerful enough to hold this vertical column of mercury in stems. The height to which the dark column of mercury rises is used to calculate suctions created in xylem vessels. Note the generally small heights, reflecting the high specific gravity of inercury. About 2 MPa suction is produced in these xylem vessels. (b) Mercury enters bordered pits but remains connected to the vertical column of mercury in xylem vessels. While mercury can pass through the pit apertures, it cannot pass the finely porous ‘pit membranes’ because it is much more cohesive than water. Seen laterally, the half-aspirated bordered pits appear as discs

Two other properties of water are also essential for long-distance water transport: surface tension, and the adhesion of water to solid surfaces such as the xylem vessels within trees. Dixon claimed that if water could ‘hang together’, the enormous evaporative energy of the air (the same power which dries the washing hanging on a line) could be harnessed to lift sap, which is mainly water, vertically. This would entail no metabolic energy on the part of the plant. This theory of sap flow accorded with earlier experiments by Strasburger (1893) showing that a tall oak tree trunk, severed at the base, could draw poisons and dyes up to the leaves by some wick-like action. If metabolism energised sap flow, poison should have inhibited it. This was well illustrated in later experiments (Figure 5.1a) in which mercury was drawn through fine tracheids of pine stems purely through the suction created by transpirational water loss from the shoot above. The tension required to achieve this is about 2 MPa.

However, the physical properties of plants had to be more complex than those of simple pipes conducting water. As mentioned, manmade pumps failed through embolisms if used to suck water higher than 10 m, whereas hundreds of litres of water reaches the canopies of tall trees daily. Even overlapping sawcuts in tree trunks, which should allow a massive quantity of air to flow into xylem vessels when under suction and cause trees to die from embolisms, did not stop all water flow to leaves. If water was under such suction, how could trees keep air bubbles out of the sap when the trunk was cut? This additional problem was not resolved until the very complex anatomical structures of trunks were much better understood. Xylem (Figure 5.2) is not composed merely of pipes: it is made up of partially sealed units (technically vessels, tracheids and fibres, called collectively conduits) which most effectively limit the spread of introduced gases and thus maintain water flow in some conduits despite very severe disruption from embolisms in others.

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Figure 5.2 (a) Scanning electron micrograph (SEM) of the finely sculptored scalariform perforation plates in poplar (Populus italica) xylem. Water passes easily from one xylem vessel to another by this route. (b) SEM illustrating bordered pits connecting two vessels in Eucalyptus. The pits can be seen in surface view (top right) and the borders of pits can be seen when this surface is broken away (bottom left).The borders appear as cup-shaped depressions. (c) SEM of poplar wood illustrating the close connections between ray cells (note three adjacent cells) and xylem conduits (xylem wall with pits lie beside these ray cells). Simple pits can occur in fields allowing transfer of ions from xylem sap into ray cells or release of photosynthates into the xylem stream. Magnifications appear on the micrographs

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