From humble beginnings as a small desert laboratory in Tucson, Arizona, at the beginning of the twentieth century, the Carnegie Institution's Department of Plant Biology has evolved into a thriving international center of plant molecular biology that sits today on the campus of Stanford University. This fourth in a series of five histories of the Carnegie Institution touches on the tangled beginnings of ecology, the baroque complexities of photosynthesis, the great mid-century evolutionary synthesis and the adventurous start of the plant molecular revolution.