After Darwin, experimental biology sought to unravel organisms. By the early twentieth century, organisms were broadly conceived as the product of their heredity and their environment. Much historical work has explored the scientific attack on the genotype, particularly through the new science of genetics. This article explores the tandem efforts to assert experimental control over the environment in which plants grew and developed. The case described here concerns the creation of the first phytotron at Caltech by botanist and plant physiologist (...) Frits Went. Opening in 1949, the phytotron was a plant laboratory that, across a series of rooms and chambers, kept genes constant while regulating and maintaining defined ranges of known environments. This article details the context in which the phytotron emerged, how the phytotron gained its sobriquet, and how it served to cement the “environment” as a category of biological knowledge. Describing the institutional context of Caltech, its interdisciplinary culture, and its encouragement of adopting technology into biological science, I argue that the phytotron and the commensurate category of the “environment” were the product of the familiar movement to integrate the physical and biological sciences. In addition, however, the creation of the phytotron was also a broader story of plant physiologists establishing a definition of the “environment” in both physical and technological terms. (shrink)