Britain is known for its avid gardeners, but what first stimulated this popularity came largely from elsewhere. In The Brother Gardeners (Knopf, $35) Andrea Wulf traces the chain of adventures, discoveries, and friendships that brought about a revolution in botany in the 18th century. When the period began, gardens were formal, geometric constructions enjoyed by the aristocracy and based on French models. By 1760, gardens were everywhere, and even amateurs cultivated their own plots. Wulf focuses on the nearly 40-year trade in plants between the Pennsylvania farmer, John Bartram, and Peter Collinson, the London merchant eager to have specimens of every tree, shrub, flower, and weed he could get from the colonies. Soon English nurseries were supplying European buyers with North American species, while the archetypal English landscaper, Capability Brown, designed gardens full of exotic magnolias and tulip poplars. British gardens continued to reflect the spread of the British Empire, incorporating plants brought back from voyages to Tahiti, the Antipodes, and China. Meanwhile, the introduction of West Indian cotton seeds to Georgia in 1732 set the course for future events.