Henry Horatio Dixon 1869-1953

When Henry Dixon retired from the Chair of Botany in Trinity College Dublin in 1949, he had occupied it for 45 years. Dixon entered Trinity College in 1887 and won a foundation scholarship in classics in 1890 but subsequently changed to natural science in which he graduated with a gold medal in 1891.

Henry Horatio Dixon

The change to natural science appears to have been stimulated by his friendship with John Joly, the geologist and physicist. The other influence on his scientific career was his two years in Strasburger’s laboratory at Bonn. At Strasburger’s suggestion, Dixon investigated the fertilisation of Pinus silvestris and, following this, his interest in cytology continued throughout his career.

In 1892, on one of their joint visits to the continent, Dixon and Joly saw some of Strasburger’s experiments which demonstrated that tall trees continued to draw up water even after the stem had been killed.

It was this which sowed the germ of an idea which was to link the names of Dixon and Joly in the scientific literature and to associate them forever with the Theory of the Ascent of Sap in Plants. The significance of this theory was that it laid one of the cornerstones for the understanding of why and how water plays such an important part in maintaining the functioning of plants.

According to the Guinness Book of Records, the tallest tree ever measured was an Australian gum tree (Eucalyptus regans) which in 1872 was 132.6m high. Vast amounts of water are continuously being evaporated from the surfaces of leaves on trees and the source of this water is the soil surrounding the roots. Water must be moved from the roots to the uppermost leaves, vertical distances of well over 100m. in some cases, but the mechanism by which these trees supply their upper foliage with water posed one of the most intractable problems of plant physiology at the turn of the 20th century.

An explanation of this mechanism was first proposed by Henry Dixon and John Joly in 1895. The hypothesis was that evaporation of water from the leaves caused a suction which was transmitted down the plant in the continuous water columns in the xylem, the main conducting tissue, and which drew water through roots from the soil. The energy to drive this process came solely from the sun which evaporated water from the cell walls within the leaves. The theory was greeted with much scepticism in 1895 as the general consensus at the time was that the cells in the xylem somehow pushed the water up through the plant against the forces of gravity using an energy-requiring ‘vital’ process. It was also difficult for many plant physiologists to understand how the evaporation of water from the leaves could produce suction strong enough to pull water to the top of the tallest trees. A mechanical suction pump can only raise water to a height of about 10m before the water column breaks or ‘cavitates’. To raise water to the top of the tallest trees requires a suction ten or more times the maximum achieved by a suction pump.

Dixon and Joly then carried out a series of experiments which answered many of the criticisms and in 1914 Dixon published the conclusions a book entitled ‘Transpiration and the Ascent of Sap in Plants. In 1908, Dixon, at the relatively early age of 39, was elected to Fellowship of the Royal Society. By this time, the theory of ascent of sap had become widely accepted.

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