David Thoday, British botanist, was the son of David Thoday and his wife Susan Elizabeth Bingham. He studied at the Tottenham Pupil Teachers' Centre for some time, but a scholarship enabled him to enter Trinity College, University of Cambridge, in 1902, while also attending the Day Training College for Teachers in Cambridge. In 1905 he was awarded both the BA degree in the natural sciences and the Cambridge University Teachers' Certificate in Theory and Practice. He continued his studies in botany at Trinity College and from 1909 to 1911 was a demonstrator in botany there. His research at this time was mainly on photosynthesis in leaves. From 1912 to 1918 he was a lecturer in physiological botany at the University of Manchester, where he did research on respiration and water relations in plants.
In 1917 Thoday was appointed professor and head of the Department of Botany at the South African College, Cape Town, which became the University of Cape Town in 1918. Presumably as a result of the disruption caused by World War I (1914-1918) he arrived in South Africa only in March 1919 and remained at the university until 1922. From 1921 he was also honorary keeper of botany at the South African Museum in Cape Town. His research during this period included a study of the structural development of the sunflower stem and the water relations of ericoid shrubs (xerophytes). He also revised the genus Passerina (Kew Bulletin, 1924), described its distribution both in South Africa (South African Journal of Science, 1921) and elsewhere (Annals of Botany, 1925), and published an account of the behaviour of the leaves of two Cape Species of Passerina during drought (Annals of Botany, 1921). He was a mild mannered person who proved to be an indifferent lecturer, but a good organiser. While in South Africa he collected some plants in the Cape Province, often with Miss E.M. Delf. His specimens went to the Compton Herbarium in Cape Town and the Selmar Schonland Herbarium at the Albany Museum, Grahamstown. During his stay he visited many places in South Africa, as well as Zimbabwe and Mozambique. He visited South Africa again from August to November 1953.
In 1923 Thoday was appointed professor of botany at the University College of North Wales at Bangor, where he remained until his retirement in September 1949. He then spent two years at the University of Alexandria but returned to Bangor in 1952. His research dealt mainly with the water relations of plants, especially of the South African species Kleinia articulata. For his work in this field he developed the Thoday potometer and the Thoday respirometer. During his career he also wrote two textbooks: Botany: A textbook for senior students (1915, 1921, 1923) and Botany: A senior textbook for schools (1935, 1938).
Thoday was elected a Fellow of the Royal Society of South Africa in 1921. He was a member of the South African Biological Society and served on its council in 1922. He also became a member of the South African Association for the Advancement of Science and in 1922 served as president of Section C (which included botany) at the association's annual congress in Lourenco Marques (now Maputo), Mozambique. His presidential address dealt with 'Carbon assimilation' (South African Journal of Science, 1922, Vol. 19, pp. 52-63). In 1933 he was awarded the degree Doctor of Science (ScD) by the University of Cambridge and in 1942 was elected as a Fellow of the Royal Society of London. The University of Wales conferred an honorary DSc degree upon him in 1960. He was married in June 1910 to Mary Gladys Sykes, a fellow-student at Cambridge, who was also a botanist.