Dixon, Henry Horatio (1869–1953), botanist, was born 19 May 1869 in Dublin, youngest son among seven sons and two daughters of George Dixon, soap manufacturer, and his wife Rebecca, daughter of George Yeates, scientific-instrument maker. His father had an interest in science and his uncle Robert Vickers Dixon was Erasmus Smith professor of natural philosophy at TCD, later becoming archdeacon of Armagh. Henry's father died when he was 2, and the nine children were brought up by their mother at 30 Holles St. (1869–80) and then 4 Earlsfort Terrace (1881–8). Henry was educated at the Rev. Benson's school, Rathmines, before entering TCD (1887). He obtained a classical scholarship (1890) and received prizes in Italian. However, with the influence of his great friend, neighbour, and lifelong mentor, physicist John Joly (qv), he changed to natural science, achieving a gold medal (BA, 1891). That same year he wrote his first paper on the locomotion of arthropods (Nature, xliii (1891), 223). For almost a decade from 1888, he went on annual walking tours in Switzerland with Joly and a group of friends, which, he said, opened his eyes to the wonder of nature. After graduating he spent a year in Bonn, where he studied under Eduard Strasburger (cytologist), who was working on the structure and function of the cell, especially nuclear division (meiosis). This period had a direct influence on his work for the rest of his life, particularly Strasburger's demonstration of sap rising in a tree even after the death of the stem. This stimulated Dixon's imagination and led to further work on this topic on his return to Dublin (1892), where he was appointed assistant to Edward Perceval Wright (qv), professor of botany at TCD. He succeeded Wright on his retirement (1904) and became director of the TCD botanic garden (1906) and, after Wright's death, keeper of the herbarium (1910). Two of his brothers were also professors at TCD, in engineering and anatomy.
Dixon's most notable scientific contribution, influenced by his earlier experience in Germany, was his work on the transport of water in plants. A seminal paper written with the help of John Joly, ‘On the ascent of sap’, (Proc. Royal Society, lvii B (1894), 3–5 abstract, Philosophical Transactions, clxxxvi B (1895), 563–76), combined physics and biology and gave the first explanation of the processes behind the upward movement of fluid in plants. This explanation, which later became known as the ‘cohesion theory’, was that internal water movement from root to leaves was largely driven by the physical process of evaporation. Evaporation from the leaves creates water tension in the xylem vessels, which, due to the cohesive strength of water, allows water to rise. Some say that it was probable that Joly was the chief originator of the idea and that Dixon carried out the detailed experiments (McDowell (1982), 410). This theory was elaborated during the 1890s with additional work done on transpiration, osmosis, and the tensile strength of water. There were many sceptics of the theory and it took till c.1914 before it was generally accepted and then included in standard textbooks.
A resourceful and imaginative researcher, he also wrote numerous papers, some with TCD co-authors, on a variety of topics, some of them pioneering: seedling growth in sterile culture thirty years before this became common practice; measurement methods of respiration by manometer; and the mutagenic effects of cosmic radiation. However, some of his ideas were not pursued and were later developed and credited to others. In his researches there was always a close association with form and function. In his last publication, at the age of 83, he suggested the involvement of a hormone in the process of meiosis (‘Mitotic hormone’ Proc. 7th International Botanical Congress (1953), 783). A full list of his publications is found in his Royal Society obituary.
In 1907, through a generous donation by the first earl of Iveagh (qv), a new school of botany was built in TCD. The successful design owes a large part to the careful planning of Dixon. A herbarium was later added (1910), and although not a taxonomist he spent two years carefully cataloguing plants. He also introduced over 7,000 new specimens to the TCD botanic gardens. An excellent teacher who was the first to introduce the experimental methods of teaching botany to TCD, he developed and later published a successful course, Practical plant biology (1922; 2nd ed. 1943). His two other books were Transpiration and the ascent of sap in plants (1914) and The transpirational stream: three lectures delivered before the University of London (1924).
During his academic career he received many accolades; FRS (1908), the Boyle medal of the RDS (1916); Croonian lecturer, Royal Society (1937); RDS vice-president (1929–44) and president (1944–47); MRIA (1947); honorary fellow, TCD; and honorary president of International Botanical Congress, Stockholm (1950). He was a visiting professor to the University of California (1927) and an honorary life member of the American Society of Plant Physiologists. Outside of his botanical career he was a representative of several institutions: commissioner of Irish Lights, trustee of the NLI, and a member of the Council of the International Institute of Agriculture.
For most of his TCD career he was an elected representative of the non-fellow professors. Although he had little real interest in politics he voted, as a unionist, against a resolution by the board of TCD supporting the Anglo–Irish treaty (1922). The college board took the view that the interests of the college were best served by peace in the country. At the time Dixon wrote to Frederick Orpen Bower, professor of botany at the University of Glasgow, that ‘this country [Ireland] is done as a habitation for civilised man’ (letter to Bower, 20 May 1922). That same year Dixon applied for the chair of botany at Edinburgh University, but decided not to go for a position in Bangor, north Wales, stating it was hard to sever his ties with Ireland.
A genial personality, in college he was remembered as courteous and reserved, mixing little with non-scientific colleagues. Outside academia he enjoyed tennis and playing the pianola. He particularly loved the music of Wagner. After initiating Joly to sailing on his small boat, he made many trips around the coasts of Ireland and Scotland on Joly's larger craft. On Joly's death Dixon was left his house, contents, and family heirlooms, and was asked by the Royal Society and Nature to write Joly's obituary.
He married (1907) Dorothea Mary Franks, a graduate of the RUI, who had entered TCD to study medicine. Her father was Sir John Franks (CB 1907). They had three sons, two of whom became scientific fellows of King's College, Cambridge. After his retirement (1949) Dixon and his wife spent a lot of time at their second home, Dooks, Co. Kerry, where they enjoyed swimming, golf, and sketching. Several trips were made abroad: Norway, Sweden (1950), and Italy (1952). Dixon died in his Dublin home, Somerset, Temple Road, 20 December 1953, aged 84.