Henry Horatio Dixon (1904-1949)

Dixon had been appointed as assistant to the professor of Botany, E. P. Wright, in 1892 and over the next sixty years he  dominated the subject of Botany in Trinity College, becoming particularly well known to generations of medical students who referred to him as 'Botany Dick'. However, he was not a botanist in the traditional sense, neither in his academic background nor in his major areas of botanical research. Indeed, one of the older generation of botanists when visiting Dixon's laboratory asked what he was doing. On being told he is alleged to have remarked 'Ah yes, it may well be physics or chemistry, but it is not botany'. Dixon was, in fact,  strongly influenced by botanical developments on the  Continent, especially in Germany, where governmental concern for the improvement of agriculture resulted in strong support for botanical investigations, stimulating experimental research into plant physiology. By contrast, in Britain the major botanical effort at this time was concentrated on systematics based on the prodigious supply of material from diverse foreign floras of the empire.

Dixon entered Trinity College in 1887 and won a foundation scholarship in classics in 1890 but subsequently changed to natural science which comprised botany, zoology and geology in which he graduated with a gold medal in 1891. The change to natural science appears to have been stimulated by his friendship with John Joly, the geologist and physicist, who was 12 years his senior. This friendship started when Joly, a neighbour and
friend of the Dixon family, invited the 18 year-old Henry to accompany him on a walking-tour of Switzerland in 1888. The friendship lasted until Joly's death in 1933 and must have influenced Dixon's research interests as
these were really founded in the disciplines of physics and chemistry.

Indeed Dixon in later years was said to have regretted his lack of basic training in these subjects. However the combination of Dixon, the astute observer and experimenter, and Joly the physicist was exactly right, for it
resulted in the application of the physical sciences to biological organisms. Dixon and Joly then were amongst the first to see that this was to be one of the major routes for biological progress in the twentieth century.

The other major influence on the scientific career of Dixon was his two years in Strasburger's laboratory at Bonn. This laboratory was, at that time, one of the most active research schools in Botany in Europe.
Strasburger's numerous discoveries between 1880 and 1890 led to the establishment of cytology, the study of the organisation and functioning of cellular material, as a distinct field of botanical research. At Strasburger's
suggestion, Dixon investigated the fertilisation of Pinus silvestris and, following this, his interest in cytology continued throughout his career.

However, in 1892 Dixon and Joly, on one of their joint visits to the continent, also 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 the idea which was to link the names of Dixon and Joly in the scientific literature and to associate them for ever 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. Most herbaceous plants consist of
more than 80% water and evaporate several times their own weight in water every day. They have a great demand for water during their life and as a result the availability of water is probably the most important factor in determining the distribution 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 132.6 m high. The tallest tree currently standing is a Sequoia
sempervirens; it is in California and measures 110.3m. Vast amounts of water are continuously being evaporated from the surfaces of leaves on these trees and the source of this water is the soil surrounding the roots. Water must move from the roots to the uppermost leaves 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 twentieth century.

An explanation of this mechanism was first proposed by Henry Dixon and John Joly in 1895 in a paper published in the Philosophical Transactions of the Royal Society entitled On the Ascent of Sap. The
hypothesis was that evaporation of water from the leaves caused a suction which was transmitted down the plant in the continuous water columns inthe xylem, the main conducting tissue, and which drew water through the
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 skepticism 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 achievedby a suction pump.

Following the first formulation of the theory, Dixon and Joly carried out a series of experiments which answered many of the criticisms and in1914 Dixon published a book entitled Transpiration and the Ascent of Sap
in Plants, which was a masterly account of the physical explanation of the rise of water in trees. In this book he described in detail many of the experiments he had carried out to provide evidence in support of his
theory and to refute others.

The key to resolving the problem was the recognition of a very special property of water. Water molecules have strong mutual attractions or 'cohesive' forces which are so great that the continuous columns of water in the xylem vessels hold together even under suctions in excess of 300 atmospheres. This however only occurs when the water is confined to very narrow tubes, as in the xylem. The movement of water up a treedepends therefore firstly on the unique physical properties of water and secondly on the structure of the conducting tissue, the xylem, which consists of long narrow-bore tubes. The foundation of the explanation of water movement in plants was therefore laid by Dixon and Joly in what is still today referred to in standard plant physiology textbooks as the 'Dixon and Joly cohesion theory'.

Dixon was an inveterate experimenter and perhaps the best example of the originality of his approach was the construction in 1922 of a 'pressure chamber' or perhaps more aptly named the 'pressure bomb' which he used
to extract sap from living leaves. Dixon constructed this from glass and there was the inevitable accident. When watching the reflection of the experimental vessel in a mirror he observed that its walls had begun to
bulge and finally it exploded under pressure. However, over forty years later this piece of apparatus was 'reinvented' in 1965 by Scholander and co- workers in the United States, but this time it was made from metal.
Subsequently it became the standard instrument for measurement of plant water status. Recently a leading plant ecophysiologist, professor Hal Mooney of Stanford University, stated that 'One might speculate that this
instrument, and its extensive use by physiological ecologists during the past twenty years, helped invigorate the field of whole-plant water relations, an area that was receiving diminishing attention by plant physiologists'. It can
be claimed that the development of plant water relations and the understanding of the effects of water stress on plants was delayed for more than forty years by the failure of other scientists to appreciate the significance of Dixon's construction of a pressure chamber in 1922.

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 and virtually all of Dixon's experimental work in support of the theory had been completed. Although Dixon continued to publish articles which explained his theory, including the Croonian Lecture to the Royal society in 1937, after 1910, he moved to other areas of
physiological research. Dixon had a series of collaborators employed as 'assistants to the professor of Botany' between 1910 and the 1930s. Several of these became eminent scientists in their own right and all
benefited from the stimulation provided by Dixon's continuing interests in several areas of plant physiology. His collaborators included W. R. G. Atkins who carried out with Dixon a series of valuable investigations on
osmotic pressures in plants. This work was interrupted by the first World War when Dixon investigated the microscopic characteristics of different varieties of mahogany with the aim of using these characteristics for
identification purposes. Mahogany of commerce is produced from many different genera and species of trees and Dixon attempted to classify them according to their cellular structure. This was important because the
increased demand for mahogany for military purposes during the war meant that it was obtained from a wider range of less well known sources. Following this, Dixon worked with T. G. Mason on another problem, the
transport of organic substances, mainly sugars, in plants. He was less successful here, but Mason later collaborated with E. J. Maskell to show that phloem tissue was the main route for this movement, and that the rate of translocation of sugars through plants was proportional to the concentration gradient. Later Dixon worked with T. A. Bennet-Clark on the variation of permeability of leaf cells and they published a series of papers
on their findings in this area between 1927 and 1932. Dixon was an extremely practical man. D. A. Webb, who succeeded him as Professor of Botany remembers his interests outside work being 'few and simple'. They included photography, the annual cruise of inspection as Commissioner of Irish Lights and 'the making
and mending, whenever opportunity offered, of mechanical contrivances and household devices of all kinds'. Dixon also played a large part in the planning of the new School of Botany which was completed in 1907. It
was designed by Mr. Marshall, of London, from sketch plans drawn up by the Professor of Botany. Marshall was also the designer of the Botany School, Cambridge. The building was initiated after the growing needs of
the science departments had been drawn to the attention of College by Professor Joly. This was part of a long battle for the adequate recognition of the place of science and its professors in the University. However funds
were eventually found and the cost of building and fitting, amounting to £7950, was generously borne by Viscount Iveagh who was, at the time, Chancellor of the University. The funds for maintenance, giving an annual
income of £280, was subscribed by friends of the College' and, in addition to this, the Board of Trinity College allowed the princely sum of £50 per annum.

Dixon was involved in the detailed planning of the new School including the construction of visual aids in the lecture theatre which, even today may be judged as sophisticated. For example, the screen for lantern
display was counterpoised by weights and could be placed at any required height, and at any angle to the wall, while the horizontal pivot in the frame allowed it to be inclined with the vertical. The lecture theatre still exists
much in its original form; even the floor is still covered in the original cork carpet. The Botany School was probably Dixon's main legacy to the College because, following his retirement, research in plant physiology fell
into abeyance. The Botany Department became once again best known for its contribution to the more traditional areas of botany, including taxonomy.

Dixon had actually laid the foundation for this when, in 1910, an annex was added to the School of Botany to house the herbarium, Dixon then spent the best part of two years in filing and indexing the specimens although he had no real interest in taxonomy and often referred to the herbarium contents as 'hay'.

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Photo: Botany Department, Trinity College Dublin