Climate is one–and perhaps the most important–of the fundamental determinants of what plants can be grown in a given area. Climate, in this broad sense, includes temperature, total annual rainfall and seasonal distribution, as well as atmospheric humidity, air movement and light and other factors.

F. W. Went, Director of the Earhart Laboratory, California Institute of Technology, Pasadena, in Experimental Control of Plant Growth, 1957, reported the research on the effect of four-dimensional relationship of day and night temperatures on plant growth which was basic information for the study reported in the foregoing article. He also devised the system of relating United States Weather Bureau data to phytotron temperatures, called effective day and effective night temperatures in this article.

G. C. Hanna, Olericulturist, University of California, Davis, reported the effect on tomatoes of the high night temperatures that occurred in 1958 in the Sacramento-San Joaquin growing areas.