Paul Andries van der Bijl, botanist, was the youngest son of Andries Christoffel van der Bijl and his wife Aletta Catharina Johanna, also born van der Bijl. He attended the boys' high school in Wynberg, Cape Town, and in Stellenbosch, continued his studies at Victoria College (later the University of Stellenbosch) and was awarded the degree Bachelor of Arts (BA), with honours in botany, by the University of the Cape of Good Hope in 1909. After a short period as a science teacher in Bethlehem, in the Free State, he joined the staff of the Division of Plant Pathology and Mycology of the Department of Agriculture in Pretoria on 1 August 1911. During the next four years he continued his studies and obtained the degrees Master of Arts (MA) in botany in 1913 and Doctor of Science (DSc) in 1915 from the University of the Cape of Good Hope, the latter with a thesis entitled A study of the dry-rot disease of maize caused by Diplodia zeae. The thesis was published by the Department of Agriculture in 1916. In September 1915 he was transferred to Durban as mycologist in charge of the phytopathological laboratory and Natal Herbarium, where he worked on the destruction of trees by fungi and the diseases of citrus, sugar-cane and other subtropical crops.
In August 1921 van der Bijl became the first professor of phytopathology and mycology in the Faculty of Agriculture, University of Stellenbosch, the first and for many years the only such position in South Africa. He was a clear and capable lecturer and was tireless in his efforts to do the best for his students. His post-graduate students later filled a number of senior positions in their field. From 1927 he served as dean of the Faculty of Agriculture and he was a member of the university council from 1927 to his death. He did his best to extend the university's influence on agriculture and in particular promoted the amalgamation of the Faculty of Agriculture and the agricultural school at Elsenburg to form the Stellenbosch-Elsenburg College of Agriculture. In 1928 he succeeded Professor C.K. Brain* as principal of the college, a post he held until his death eleven years later. His work was appreciated by the agricultural community of the Western Cape and at their special request the Department of Agriculture appointed him chairman of the newly established Dried Fruit Board just before his death. He was also one of the first and strongest advocates of advanced forestry training in South Africa, until the first chair of forestry was instituted at the University of Stellenbosch in 1932.
Van der Bijl specialised in the collection and study of South African fungi and lichens and with his students described many new species of especially plant parasitic fungi. He soon became interested in the larger fungi, especially the Polyporaceae found on indigenous trees. With J.D.M. Keet, the Director of Forestry, he made an extensive collection of fungi in the forests of the Eastern Cape Conservancy in 1915. During his years at Stellenbosch he and A.V. Duthie* studied the fungi of the Western Cape and later those of the Knysna forests. After that he gave attention to the taxonomy of South African fungi. Altogether he produced some 60 publications, many of them in Afrikaans. The latter included the first local textbook of its kind, Plantsiektes; hul oorsaak en bestryding (Plant diseases, their causes and control; 1928), and all his scientific papers after 1929. He also wrote The fungus flora of the Western Province of the Cape (1929). His first scientific paper dealt with a mottled disease of the black wattle and many later papers with the decay of indigenous forest trees. A number of his papers were published in the Transactions of the Royal Society of South Africa and dealt with topics such as heart rot of Sneezewood (1917), South African Xylarias occurring around Durban (1921), a fungus which causes powdery mildew on the leaves of the pawpaw plant (1921), a new species of fungus on a spider (1922), fungi of the Stellenbosch district (1922), and notes on some South African Xylarias (1924). Many more papers appeared in the South African Journal of Science, on topics such as die-back of apple trees (1915), fungi affecting the wood of the willow (1916) and the wood of black ironwood trees (1917), the fungus causing root disease in sugar cane (1919), host plants of the Loranthaceae (1919, 1920), and descriptions of new South African fungi (1925, 1926, 1927, 1928, 1929). A number of papers, most with descriptions of the South African species of several different families, were published in the Annale van die Universiteit van Stellenbosch during the period 1923 to 1935. These were all written in Afrikaans.
Van der Bijl was elected a Fellow of the Royal Society of South Africa in 1924 and served on its council for some years. He was a Fellow also of the Linnean Society (London), a member of the Suid-Afrikaanse Akademie vir Wetenskap en Kuns, and a member of the South African Association for the Advancement of Science, serving as president of Section C in 1926. During 1914-1915 he delivered three papers before the Transvaal Biological Society and in 1916 became a foundation member of its successor, the South African Biological Society.
Van der Bijl left his collection of fungi and lichens to the University of Stellenbosch. The university named the Paul van der Bijl Laboratory for the study of plant diseases after him. He is also commemorated in the names of the fungus genus Byliana and the species Physalospora bylii, Acarospora bylii, Cerospora byliana, Lecanora bylii, Uromyces byliana and Puccinea byliana.
In January 1923 he married Anna Elisabeth Schreuder, but they had no children.