Horace Mann, Jr. (1844-1868) was a noted American botanist and the eldest son of the well-known educational reformer, Horace Mann (1796-1859). He was born in Boston on February 25, 1844, and beginning in the early years of his childhood, his father fostered an interest in nature study. As a young man, he studied botany and became friends with Henry David Thoreau. They frequently consulted each other about the identification of plants and animals, and traveled together to Minnesota in the spring of 1861.
In the fall of 1861, he enrolled in the Lawrence Scientific School at Harvard and took courses in zoology with Louis Agassiz and in botany with Asa Gray, both prominent American scientists. In 1864, he accompanied William T. Brigham to the Hawaiian Islands, where he studied Hawaiian plants for a year. On his return to Harvard, he specialized in Hawaiian plants and prepared "Enumeratio of Hawaiian Plants" for his thesis, later published as Enumeration of Hawaiian Plants. Mann received his degree in 1867 and accompanied Professor Agassiz to Brazil, where he developed pulmonary tuberculosis.
Mann served as the curator of botany to the Boston Natural History Society, and was curator of the Harvard herbarium and, beginning in 1866, assistant to Professor Gray. When Gray left for Europe in September 1868, he placed Mann in charge of the botanical garden and botany department, apparently intending for Mann to succeed him when the latter retired. However, Mann’s health rapidly declined and he died of tuberculosis on November 11, 1868, at the age of 24, on the same day that he was elected to the American Academy of Arts and Sciences. During his lifetime, he published several articles and reviews, but his book, Flora of the Hawaiian Islands, was left unfinished at his death. His botanical collection of 7,500 species later formed the basis of Cornell University’s herbarium.