The purpose of this paper is to establish principles for determining the quantitative relationships between climate and plant growth. It is known that the amount of dry matter produced by any plant and the water it transpires are proportional, and that this relation is constant for each plant species or variety under a given set of growing conditions. Using the experimental data of earlier workers, dry-matter production is plotted against water use—separately for each level of soil fertility—and equations are formulated to show the relationships under different conditions. This new analysis of the published data shows that a correction based on atmospheric humidity accounts for about 90 per cent of the variation in the ratio of dry-matter production to transpiration when soil fertility is constant, and that a correction for soil fertility accounts for about 75 per cent of the variation when climate is constant.
