Asa Gray (1810–1888) was an American botanist who developed the study and classification of plants as a scientific discipline in the United States. Like many other life scientists, he trained first as a physician, but turned to botany and joined the Harvard faculty in 1842. His herbarium there remains the largest such collection in North America. His Manual of the Botany of the Northern United States, first published in 1848, and after many editions known as "Gray's Manual" remains an indispensable research and teaching guide to North American flora. A late edition cites Mendel's papers on peas, but the reference seems to have been gleaned from a secondary source, and there is no evidence that anyone read it. [Do not confuse Gray's Manual with Gray's Anatomy on the human body].

    Like Joseph Hooker, Gray was a friend and correspondent of Charles Darwin. He supplied Darwin with botanical information in support of his books, and Darwin's "Form of Flowers" is dedicated to Gray. Gray received one of the first copies of "Origin of Species" to arrive in the United States, read it between Christmas 1859 and New Year's Day, and was instantly convinced. He became the most prominent supporter and defender of Darwin's theory of Natural Selection in the United States, met Darwin on trips to Europe, and exchanged more than 300 letters with him. He firmly advocated that Darwin's theories could be reconciled with religious belief,

    Also like Hooker, he was an advocate of comparative plant biogeography, and drew attention to similarities between plants of eastern North America and eastern Asia, and posed questions about the history and movement of species.

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