William Harold Pearsall was born on 23 July 1891, at Stourbridge in Worcestershire. He came of an old Worcestershire family, some of whom ran into trouble in Cromwellian times because of their Royalist sympathies. He is said to have been the fourteenth bearer of the name William H. Pearsall. His father, William Harrison Pearsall, was a schoolmaster who moved to Dalton-in-Furness when Harold was quite a small boy to become headmaster of Broughton Road School. W. H. Pearsall senior was an excellent teacher, and Mrs T. G. Tutin writes: ‘When I was a schoolgirl in Barrow I knew people who had been pupils of the elder Pearsall in Dalton, and they still spoke of what a kind man and wonderful teacher he had been and how he had made them look at plants.’ He had very definite views on the way to bring up his own and other children. It was his belief that one should ‘never do for a child what a child can with reasonable effort do for itself’, and the playroom in the Pearsall house had a large printed notice bearing the three words THINK TRY ASK. Apart from his competence as a teacher he was also a good organist and trainer of choirs, a Methodist laypreacher and a first-class naturalist who devoted all his spare time to an intensive study of the natural history of the English Lakes. He was a member, and for a time Honorary Secretary, of the Botanical Society and Exchange Club (now the Botanical Society of the British Isles) and became a leading British expert on several genera of aquatic flowering plants and especially on the pondweeds, starworts and water buttercups, publishing many descriptions and keys. His key to British grasses was recently still in use at the Brathay Field Centre near Ambleside.