English botanist and writer. Specialised in the growth and cultivation of orchids. Later in his life he studied ferns.
Studied at University of Cambridge, studies were interrupted by WWI. He volunteered in France with the Friends' Ambulance Service. Returned to his studies at Cambridge in 1920 received a degree in Natural Sciences Tripos.
From 1920-1922 he worked at Cambridge University as a junior demonstrator. In 1922, he was appointed by the Colonial Office as assistant director at the Singapore Botanical Gardens. Promoted to Director in 1925, a role he would keep for the next 20 years.
During WWII, Japan occupied Singapore. Holttum was placed under house arrest while he continued to work at the Garden but he was relieved of his administrative duties. During this time he was able to concentrate on research. He wrote a number of drafts for books that would be published after the war. These include: "Gardening in the Lowlands of Malay" and the first two volumes of "A Revised Flora of Malaya."
Founding member and president of the Singapore Gardening Society. Was also a founding member of the Malayan Orchid Society.
In 1949, he left the Botanic Garden and became the first Chair of Botany at the University of Malaya until his retirement in 1954.
After retirement Holttum returned to England and began spending time at Royal Botanic Gardens, Kew. Here he continued his research and contributed to the Pteridophyta in the Flora Malesiana project.
Holttum penned about 500 scientific texts, largely on ferns and orchids, but also dealing with bamboos, gingers and the Marantaceae family. Many plant taxa are named in his honour, including the fern genus Holttumiella Copel.