Ludwig Diels, son of the classical scholar Hermann Alexander Diels, was a prominent German expert in systematic botany and plant geography who started botanising while still a pupil. He studied botany and geography, but particularly botany, at the University of Berlin and was influenced by H.G.A. Engler*. His doctoral thesis dealt with the botany of New Zealand.
Diels's early research pertained to certain families of African plants. For example, his first paper, in 1897, (in German) was a contribution to the knowledge of African Scrophulariaceae in Engler's Botanische Jahrbücher... (Vol. 23, pp. 471-496). In Part 2 of this paper (1898, Vol. 25, pp. 120-123) he described five new South African species collected by Dr. Friedrich Wilms*. The same year, in a paper on African Campanulaceae, he described eight new South African species collected by Wilms, F.E. Bachmann*, and Rev. William Tyson* (Vol. 25, pp. 111-119). During the next two years he published on living and fossil plants of the families Cyatheaceae, Polypodiaceae, Parkeriaceae, Gleicheniaceae, Schizaeaceae, and Osmundaceae, in six separate papers. He was also Engler's co-author for two monographs on the Combretaceae in 1899 and another on the Annonaceae in 1901, in the series Monographieen afrikanischer Pflanzen-Familien und -Gattungen.
In 1900 Diels and his friend Ernst Pritzel undertook a botanical journey to Western Australia. On their outward journey they visited the Cape from August to mid-October 1900. Diels met the local botanists H.W.R. Marloth* and H. Bolus*, and travelled north to the Hantam Mountains, collecting there and in the districts of Clanwilliam, Vanrhynsdorp, and Calvinia. Continuing his journey he landed at Perth on 30 October. During his stay in Australia he took time to visit Tasmania (in January 1902) and New Zealand, returning to Germany later in 1902. He collected some 4700 specimens in Western Australia and his observations were written up in a book, Die pflanzenwelt von West-Australiën (Leipzig, 1906).
Meanwhile Diels had heen inaugurated as a lecturer at the University of Berlin in 1900. In 1906 he was appointed associate professor of botany at Marburg, but in 1914 returned to Berlin as assistant director of the Berlin-Dahlem Botanic Garden and Museum. In 1921 he succeeded Engler as director of the garden and received an additional appointment as professor of botany at the University of Berlin. He was an honorary or corresponding member of many scientific societies and editor or co-editor of the Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie, Das Pflanzenreich and Syllabus der Pflanzenfamilien. As an active researcher he produced numerous publications on plant taxonomy, morphology and geography. He also continued publishing on African plant families for many years. Three of his papers related directly to southern Africa: "Formationen und Florenelemente im nordwestlichen Kapland" (1909); "Das Verhältnis einiger Wuchsformen in der Flora von Deutsch-Südwest-Afrika" (1940); and "Zwei neue Scrophulariaceen aus dem Kapland" (1942). In addition to his extensive contributions to the taxonomy of the African flora, he published many papers on the flora of Australia between 1905 and 1908. The species Crassula dielsii was named in his honour by S. Schonland*.