Penhallow was the first botanist appointed as the Macdonald Chair of Botany in 1883. Born in Maine, USA, he died at sea in 1910.
He was trained in botany at Harvard and was recommended to Sir William Dawson for a lectureship in botany at McGill by Asa Gray in 1882. Anatomy, paleobotany and physiology were his special interests, publishing a leading textbook on North American Gymnosperms in 1907.
Under Penhallow's leadership, the McGill Herbarium underwent considerable expansion thanks to an exchange system that secured new botanical material from abroad. His collections from Shelburne and Mount Washington, NH, as well as in Hakkaido, Japan, also contributed to the herbarium's growth.