German botanist whose work on osmotic pressure made him a pioneer in the study of plant physiology.

After earning his Ph.D. from the University of Göttingen in 1865 Pfeffer continued his studies at the universities of Marburg and Bonn. He then held teaching positions at Bonn (1873), Basel (1877), Tübingen (1878), and Leipzig (1877), where he remained until his death. Pfeffer's work on cell metabolism led to his work in 1877 in devising a semi-permeable membrane that he used to study osmosis. He developed a method for measuring osmotic pressure and showed that pressure depended on the size of the molecules too large to pass through the membrane. Pfeffer was then able to measure the size of giant molecules. His findings were published in Osmotische Untersuchungen, Studien sur Zellmechanik (1877; “Osmotic Research Studies on Cell Mechanics”). His best publication is Pflanzenphysiologie. Ein Handbuch des Stoffwechsels und Kraftwechsels in der Pflanze (1881; The Physiology of Plants; A Treatise upon the Metabolism and Sources of Energy in Plants, 1900–06), which was for long a standard handbook.