Dixon, Henry Horatio

Henry Horatio Dixon (1869 – 1953)


Henry Dixon was born in Dublin , the youngest son of a large and talented family. He graduated BA (Natural Sciences) from Trinity College Dublin in 1891 and served there as Professor of Botany from 1904 to 1949. He was appointed Director of the TCD Botanic Gardens in 1906.

Professor Dixon had an alert, imaginative mind and made many original contributions to botany. These included bivalent chromosomes at meiosis, thermocouples for measuring osmotic pressure, the mutagenic effect of cosmic rays and the transport of organic substances in plants.

He is best noted for the cohesion theory of the ascent of sap in plants. How water is transported to the leaves at the top of tall trees had long intrigued botanists.   Atmospheric pressure alone could not propel a column of water up through the plant’s vascular system further than about 10 metres. Root pressure and capillarity were also inadequate for the purpose. In cooperation with his good friend and colleague, the TCD physicist John Joly, he elaborated a theory that the motive force was primarily the evaporation from the leaves; this produces a tension in the water filling the vascular tubes and, thanks to the cohesive strength of a continuous column of water, draws up the water from the roots.   Although it took some years to convince the sceptics, the Dixon and Joly cohesive theory of the ascent of sap in plants, first published in 1995 by the Royal Society, has become generally accepted and has stood the test of time.

His various awards and distinctions included Fellowship of the Royal Society (FRS) (1908), the Boyle Medal of the Royal Dublin Society (1917), President of the RDS (1944 – 1947), Membership of the Royal Irish Academy (1947), Honorary Fellow TCD (1950) and Honorary President of the International Botanical Congress, Stockholm 1950.

Professor Dixon was instrumental in the design and equipping of the School of Botany building in TCD in the early 1900s (with generous funding by Lord Iveagh of the Guinness family). The valuable herbarium was properly housed and indexed, and the new building facilitated botanical research to be conducted to 20th-century standards.

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