Attempts to relate plant metabolic activity with developmental stage are often hindered by lack of an appropriate developmental index. Existing indices of morphological development are unsuitable for use with plants having a semideterminate, recurrently flushing pattern of growth as displayed by Quercus seedlings. We propose the following morphological index (QMI) to define the stages of Quercus seedling ontogeny: (1) radicle emergence; (2) epicotyl emergence from the soil; and (for each flush) (3) termination of elongation of the second internode, which corresponds with the period of most rapid stem elongation; (4) completion of elongation by all internodes, which corresponds with the period of most rapid leaf elongation; and (5) completion of elongation of the last leaf but one, which usually precedes closely the pause between one growth flush and another. The relationship between QMI and net photosynthesis by individual leaves of Quercus rubra L. seedlings was determined. Net photosynthesis increased with QMI during a flush, but at a particular QMI stage, generally decreased from one flush to the next.