His father's family were bankers in Oxford and Banbury, his mother's shoe manufacturers in Street, Somerset. On both sides he was descended from Quaker businessmen and industrialists, with strong liberal and sometimes radical political views. His mother (a granddaughter of the Quaker statesman John Bright) was an ardent pro-Boer and accompanied Emily Hobhouse to South Africa in 1903 to undertake relief work among Boer families, which led to her meeting General Jan Smuts and his wife, who became lifelong friends. It was after Smuts that Jan was named.
He was educated at the Dragon School, in Oxford, and at Leighton Park School, Reading. He won a scholarship to King's College, Cambridge, in 1929, and took First Class honours in both parts of the Natural Sciences Tripos. After obtaining a diploma in Education at London University he taught at the Warehousemen & Clerks School, Cheadle Hulme, until he joined the Army in 1941.
Influenced by his mother, herself a keen amateur botanist, Jan Gillett was also inspired by his biology master at Leighton Park, F.W. Flattely (co-author of The Biology of the Seashore, 1922). In 1928 he joined John Hutchinson, a distinguished Kew botanist, on a collecting tour in South Africa. After assisting Hutchinson at Kew for a few weeks with the Flora of West Tropical Africa and a visit to Munich to learn German (when he also saw something of Nazism) he returned to South Africa in mid-1929. There, with his parents, Hutchinson and Smuts, he undertook a collecting expedition into the Rhodesias, as far north as Lake Tanganyika. The botanical results were substantial and fully described in Hutchinson's A Botanist in Southern Africa (1946).
In 1932, while still at Cambridge, he was invited to join the British Somaliland / Ethiopia Boundary Commission and made a fine collection of plants, accompanied by an astute survey of the vegetation, published in the Kew Bulletin for 1941.
Despite his keen interest in botany and blandishments from the Director of Kew, Gillett's political interests were keener still. He took up schoolmastering, feeling that research would take up more time than he felt able to spare from radical politics. He had joined the Communist Party in 1932 and remained a member until 1946, though his sympathy for its policies had virtually vanished in 1939.
Conscripted in 1941, he was commissioned into the Royal Armoured Corps and went to India in 1942 in the Reconnaissance Regiment of the 2nd (British) Division. After participating in the relief of Imphal and Kohima he transferred to "V" Force, a cadre of intelligence officers operating in the jungle and between the lines. He was mentioned in despatches and recommended for the MC. He returned to England after the Japanese surrender at the end of 1945.
In 1946, thanks to Smuts's influence at the Foreign Office, he was appointed botanist to the Iraq Department of Agriculture. He was based at the department's research station at Abu Ghraib, near Baghdad. He made extensive collections in remote parts of the country which later proved of great value for work done at Kew on the Flora of Iraq. He had married Gertrude Spector in 1937 and had three sons and a daughter. His wife being Jewish they found life in Iraq increasingly uncomfortable after the Arab-Israeli war of 1948 and in 1949 he returned to England to take up a job with the Colonial Office as a Principal Scientific Officer at Kew on the newly instigated Flora of Tropical East Africa.
He worked under Edgar Milne-Redhead and as the senior member of the research team he undertook revisionary work for a large part of the legumes, including not only the largest and most complex genera, but laying the foundation of a new classification for the subfamily of peaflowers, now universally accepted. He also did important work on indigo plants. His theoretical concepts, notably the effects of pest pressure as a factor in natural selection, were wide-ranging and well ahead of his time.
In 1952-53, he made an expedition to the hitherto little-known parts of the Kenya-Ethiopia border on a further Boundary Commission. He brought back not only an outstanding collection of plants, with numerous species new to science, but also the highest commendation of R.G. Turnbull, a future Governor of Tanganyika, with special interests in the development of arid zones. In 1963 he accompanied an ecological survey mission of Jordan at the invitation of King Hussein; this was organised by the ornithologist Guy Mountfort and the party included Sir Julian Huxley, Max Nicholson and the bird photographer Eric Hosking. The mission's achievements are racily described in Mountfort's Portrait of a Desert (1965).
In 1959 Gillett had been nominated as Botanist in Charge of the East African Herbarium, but the colonial authorities vetoed the appointment of an ex- Communist to a government job just as Kenya was coming out of the Mau Mau emergency. There were other small incidents to ruffle the authorities, such as his locally publicised arrest in Richmond Park in 1959, proving the ponds were safe to skate on in winter. However, Kenya became independent in 1963 and Bernard Verdcourt, successively Assistant Botanist and then Botanist in Charge since 1959, made way for him, shortly after appointing Christine Kabuye as his Assistant.
The next 20 years until 1971 were spent in East Africa as Botanist in Charge of the Herbarium, and thereafter adviser to his successor Christine Kabuye. He provided a major impetus to the Herbarium, seeing its transfer from the East African High Commission to the National Museums of Kenya, maintaining the high standards of the largest herbarium in tropical Africa, and training up a new generation of local botanists. It was a matter of great satisfaction to him that he was able to arrange for Christine Kabuye's nomination, as a Ugandan national, to what was now a Kenya government appointment.
Freed of administrative duties, and with support from the Overseas Development Administration, Gillett began to spend more time in the field, taking up an interest in commiphoras (the source of myrrh) in Kenya and undertaking surveys in Somalia. He made many contributions in a self-effacing way to the compilation of books on local plants and with the production of the Flora of Tropical East Africa. Overtaxed however by the demands put on him and with failing health he returned to England in 1984.
He was at once, somewhat to his family's concern, a daily visitor to Kew Gardens, becoming a much-loved father-figure of African botany in the Herbarium. He became a regular attender of the Friends meeting in Isleworth, and an elder. With the change of altitude his health improved and in 1989 he was given a new lease of life with a by-pass operation. He completed his landmark account of the commiphoras for the Flora of Tropical East Africa in 1991 and continued his studies of arid-land plants for the new Flora of Somalia. He helped his friend the distinguished settler leader Sir Michael Blundell with his Guide to the Wild Flowers of East Africa and also advised African protgs on their books.
In his last years he turned more to ideas about the role of diet, fire, speech and religion to their origins and spread of human culture and customs. His astuteness, his extraordinary breadth of reading left him unrivalled in any argument he liked to raise, even as he struggled with problems of a failing memory. His faith as a Quaker, his love of life and his social concern contributed greatly to the richness of Kew and even in his last days at home he rallied to talk to his botanical friends with all his old enthusiasm.
Jan Bevington Gillett, botanist: born 28 May 1911; married 1937 Gertrude Spector (three sons, one daughter); died Kew 17 March 1995.