Gray traveled to the American West on two separate occasions, the first in 1872 by train, and then again with Joseph Dalton Hooker, son of William Hooker, in 1877. His wife accompanied him on both trips. Both times his goal was botanical research, and he avidly collected plant specimens to bring back with him to Harvard. On his second trip through the American West, he and Hooker reportedly collected over 1,000 specimens. Gray's and Hooker's research was reported in their joint 1880 publication, "The Vegetation of the Rocky Mountain Region and a Comparison with that of Other Parts of the World," which appeared in volume six of Hayden's "Bulletin of the United States Geological and Geophysical Survey of the Territories".