The "Elements of Botany" (1836), an introductory textbook, was the first of Gray's many works. In this book Gray espoused the idea that botany was useful not only to medicine, but also to farmers. Gray and Torrey published the "Flora of North America" together in 1838. By the mid-1850s the demands of teaching, research, gardening, collecting, and corresponding had become so great, and he had become so influential, that Gray wrote two high school-level texts in the late 1850s: "First Lessons in Botany and Vegetable Physiology" (1857) and "How Plants Grow: A Simple Introduction to Structural Botany" (1858). The publishers pressured Gray to make these two books non-technical enough so that high school students and non-scientists could understand them. As with most scientists in American academic institutions, Gray found it difficult to concentrate purely on research.