William Henry Harvey, botanist, was the youngest of the eleven children of Joseph M. Harvey, one of the prominent merchants of Limerick, Ireland, and his wife Rebecca. He showed an early interest in natural history and was taught botany at school. While working in his father's counting house he extended his knowledge of particularly botany and conchology by reading and undertaking biological excursions, during which he collected plants, algae and shells. In 1831 he discovered two new localities of the rare West Indian moss species Hookeria laetevirens in Ireland and reported his finds by letter to the prominent English botanist, Sir William J. Hooker, after whom the genus had been named. Hooker met the young man and provided encouragement and lifelong friendship which had a determining influence on Harvey's career. He became particularly keen to devote more time to natural history and to study the flora of other countries.
Soon after the death of his father in October 1834 his brother Joseph was appointed to the post of colonial treasurer of the Cape Colony. William decided to accompany him in order to study the Cape flora. They arrived in September 1835. Joseph's health soon failed, with the result that they sailed for Britain in April 1836. Joseph died during the voyage and in August that year William left Britain again for the Cape to succeed his brother as colonial treasurer and accountant general. However, he did not do well in this position. Late in 1838 he too had to return to Britain, because of mental illness, but he was back at the Cape again in October 1840. After little more than a year the state of his physical and mental health caused him to resign and leave the Cape for treatment in London. From 1838 until he left he served on the council of the South African Literary and Scientific Institution.
Harvey spent all his spare time at the Cape studying the local flora, often collecting plants or algae early in the morning before going to work. He befriended Baron C.F. von Ludwig*, who grew many indigenous plants in his famous Ludwigsburg garden, and went on short botanical excursions with Reverend James Backhouse* and Sir Charles J.F. Bunbury*. Though his duties prevented him from travelling far (the furthest seems to have been a visit to Paarl in April 1838), he was able to extend his knowledge of more distant regions by studying the collections of others, such as James Bowie*, then curator of von Ludwig's garden, Carl Zeyher*, and Dr C.W.L. Pappe*. In response to his request for additional specimens from inland regions he received material from Reverend Wallace Hewetson* and John Peddie* in the eastern Cape, and Reverend William Elliott* at Paarl. His intention all along had been to produce the first comprehensive work dealing specifically with the South African flora and in a surprisingly short time he compiled a most useful work, The genera of South African plants, arranged according to the natural system, published in Cape Town in 1838. The book was dedicated to Baron von Ludwig. A second edition, edited by J.D. Hooker* was published after Harvey's death in 1868. The purpose of the work was to provide an introduction to the flora of the Cape, in preparation for a more comprehensive Flora Capensis. However, it served also to inspire local plant collectors, such as Mary E. Barber*, as it contained an introduction to botany and a glossary of botanical terms. Harvey was also an accomplished botanical artist. In October 1838 the book's publisher, A.S. Robertson, solicited subscriptions to finance the publication of a series of lithographic plates, in monthly numbers, under the title Illustrations of South African botany. It was to be edited by Harvey, but it seems that insufficient interest was shown by the public.
Upon leaving the Cape Harvey arrived in London in the spring of 1842 and by March 1844 had recovered sufficiently to accept a post as curator of the herbarium at Trinity College, Dublin. In order to legalise his additional appointment as professor of botany the college conferred on him the honorary degree Doctor of Medicine (MD). However, his application was not successful. He spent the next twelve years developing the herbarium, incorporating his own collection of some 10 000 sheets in it. He also studied marine algae (seaweeds), on which he became an authority and wrote several major works. His Phycologia Britannica; or, a history of British seaweeds (3 vols, 1846-1851) included coloured illustrations of all the species known to occur along the shores of the British Isles. In Nereis Australis (1847) he dealt with the algae of the southern ocean represented in the Dublin herbarium, including those he had collected at the Cape. After a visit to the United States from July 1849 to May 1850, during which he lectured and collected algae along the Atlantic coast, he wrote Nereis Boreali-Americana (3 vols, 1852-1858). In 1853 he obtained long leave and travelled via the Red Sea and Ceylon to Australia, Tasmania and the South Sea Islands. Returning via Chile in 1856 with a large collection of marine algae, he published Phycologia Australica; or, a history of Australian seaweeds (5 vols, 1858-1863). His later publications included Index generum algarum; or, a systematic catalogue of the genera of Algae, marine and frewhwater (1860). Meanwhile he had described several new genera of South African plants in the Journal of Botany (1842), contributed descriptive remarks to Specimens of the flora of South Africa (1849) by Arabella E. Roupell*, and contributed a note on fossil plants collected at Sundays River by R.N. Rubidge* to the Report of the British Association... (1851). In 1848 he was appointed professor of botany of the Royal Dublin Society. He was elected a Fellow of the Linnaean Society in 1857 and a Fellow of the Royal Society of London the next year, and was one of the outstanding British botanists of his time. In addition to his books he published more than 40 botanical papers.
In 1856 the post of professor of botany in the University of Dublin again became vacant and he was appointed to it. Around this time he returned to the study of the Cape flora, as part of a larger effort to compile various colonial floras, supervised by Sir William Hooker. With Otto W. Sonder* as co-author he compiled Volumes 1-3 of the Flora Capensis: being a systematic description of the plants of the Cape Colony, Caffraria and Port Natal. The work was published in parts between 1859 and 1865, in both Dublin and Cape Town, with financial support from the British government. Harvey wrote about two thirds of the text himself. The work was continued by others only in the late 1890's, and finally completed in 1933. The Flora Capensis has played a very important role in the development of South African botany, enabling botanists to identify plants and establish their recorded distribution in a single source. Harvey's initial work on this monumental publication constituted a contribution of extreme importance in the development of South African botany.
While working on the Flora Capensis Harvey also worked on a parallel publication containing illustrations of representative plants of each genus - along the lines of his unsuccessful attempt twenty years earlier. He was able to complete some 200 illustrations of new or rare South African plants, doing all the work himself, including most of the engraving. The resulting two volumes of the Thesaurus Capensis; or, illustrations of the South African flora, being figures and brief descriptions of South African plants, selected from the Dublin University Herbarium were published in 1859 and 1863.
In April 1861 Harvey married Elizabeth L. Phelps of Limerick. From around that time his health was periodically poor and finally deteriorated to such an extent late in 1865 that he and his wife moved to Torquay to benefit from a milder climate. Harvey died there of tuberculosis a few months later. Sir Charles Bunbury remembered him as a gentle, modest and amiable person. Charles Darwin regarded him as a first-rate botanist, even though Harvey had criticized his Origin of species. While at the Cape he was kind and generous to anyone interested in plants, naming their specimens and encouraging their involvement in botany. The genus of African root parasites Harveya (fam. Scrophulariaceae) and the tree species Albizia harveyi were named after him.