Dixon, Henry Horatio

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Dixon, Henry Horatio

(b. Dublin, Ireland, 19 May 1869; d. Dublin, 20 December 1953)


Dixon was the youngest of seven brothers and two sisters; his father, George Dixon, was owner of a soap works; his mother was the former Rebecca Yeates. In 1887 he entered Trinity College, Dublin, with a classical scholarship and prizes in Italian; but in 1891 he graduated with a first-class degree and gold medal in botany, geology, and zoology. He was stimulated by a very active botany department and also by friendship with the physicist John Joly, with whom he collaborated in fruitful research. He then studied in Bonn with Eduard Strasburger until 1893, when he was appointed assistant to E. Perceval Wright, professor of botany in Dublin, and occupied himself reorganizing the botanical gardens and herbarium. Almost all of Dixon’s working life was spent in Dublin: he became professor of botany in 1904 and was active in the Royal Dublin Society, receiving the Boyle Medal in 1916 and serving as president from 1944 to 1947. The Royal Society elected him to fellowship in 1908, and he was the Croonian lecturer of 1937. In 1927 he was visiting professor at the University of California. He resigned his chair in 1949 and was elected an honorary fellow of Trinity College, Dublin. He married Dorothea Mary Franks, a medical student, in 1907; they had three sons.

Dixon’s early research was suggestive of his potential rather than significant in botany: work on the cytology of chromosomes and first mitosis of spore mother cells of Lilium showed that the appearance of bivalents is due to the approach of chromosomes rather than to splitting of some preexisting structure, thus giving the first indication of reduction division.

He started original work soon after graduating and by 1892 had shown how to grow seedlings in sterile culture, foreshadowing later tissue and root culture. He also suggested mutagenic effects to be expected from cosmic radiation. Work on the resistance of seeds to heat, cold, and poisons developed thermoelectric methods of cryoscopy that he was to use again in work on osmotic pressure.

In 1894 Dixon and Joly read their classic paper “On the Ascent of Sap”; they gave an outline of the theory to the Trinity College Experimental Science Association in March and read the full paper to the Royal Society in October. The tension theory of the ascent of sap in trees arose from the combination of Dixon’s knowledge of Strasburger’s work on transpiration in high trees and Joly’s knowledge of work by François Donny and Berthelot on the tensile strength of columns of sulfuric acid and water. This idea was worked out in experiments on transpiration of various trees; and they showed that since leaves can transpire even against high atmospheric pressures, this tension can be maintained. Transmission of internal stress was found to be due to the internal stability of the liquid when mechanically stretched and to the additional stability of minutely subdivided connective tissue; this is unaffected by dissolved gas, and free gas is restricted by the size of the vessel. The tensile stress in the sap is transmitted to the root, where it establishes in the capillaries of the root surface menisci competent to condense water rapidly from the surrounding soil and thus complete the process of transpiration.

Dixon, working alone, continued to perfect the theory in all details. He published a standard account in 1909 and, in 1914, Transpiration and the Ascent of Sap in Plants, a comprehensive monograph bringing together theories and experimental work on the subject in a well-argued account. The transpiration stream is raised either by secretory actions in leaf cells, using energy from respiration in the leaves, or by evaporation from the surfaces of leaves, according to the degree of saturation of surrounding cells. The osmotic pressure of turgid mesophyll cells, calculated from cryoscopic measurements of the freezing point of sap, was shown to be enough to raise sap by tension in even the highest tree, through passive vessels, not living cells.

During World War I, Dixon worked on the microscopic identification of different kinds of mahogany. Later he returned to problems of transport of organic substances in the phloem, variation in the permeability of leaf cells, and the mechanisms of transpiration. He wrote a textbook on practical plant biology in 1922 and gave three lectures on the transpiration stream at the University of London in 1924. Some of his manuscript notebooks are held at Trinity College, Dublin.


I. Original Works. The classic paper “On the Ascent of Sap,” written with J. Joly, was recorded in abstract in Proceedings of the Royal Society, 57B (1894), 3–5, and published in full in Philosophical Transactions of the Royal Society, 186B (1895), 563–576. The standard account of the theory was published as “Transpiration and the Ascent of Sap,” in Progressus rei botanicae, 3 (1909), 1–66, reprinted in abbreviated form in Report of the Board of Regents of the Smithsonian Institution, 1910 (1911), 407–425. The work on transpiration was brought together in Transpiration and the Ascent of Sap in Plants (London, 1914). The other two volumes published by Dixon are Practical Plant Biology (London, 1922; 2nd ed., Dublin, 1943); and The Transpiration Stream (London, 1924).

II. Secondary Literature. Two assessments of Dixon’s scientific importance are by W. R. G. Atkins, who worked with him on the cryoscopy of sap, in Obituary Notices of Fellows of the Royal Society of London, 9 (1954), 79–93, with portrait and comprehensive bibliography of Dixon’s works; and by T. A. Bennet-Clark, in Nature, 173 (1954), 239.

Diana M. Simpkins