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. In the last hundred years it has witnessed immense changes in biological thinking, and been at the forefront of innovative research. 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.