We used the plastochron index to study the relationship between plant age, leaf age and development, and net photosynthesis of black cherry (Prunus serotina Ehrh.) seedlings. Leaf area and net photosynthesis were measured on all leaves >/= 75 mm of plants ranging in age from 7 to 20 plastochrons. Effects of plant developmental stage on leaf area and net photosynthesis were evaluated for leaves of differing age (horizontal series), leaves on plants of constant age (vertical series), and leaves of constant age (oblique series). Regression techniques were used to estimate leaf area from leaf blade dimensions. The best equations for predicting leaf area had R(2) values of 0.991-0.992 and used linear or logarithmic functions of both leaf length and width. Suitable, but less precise, equations with R(2) values of 0.946-0.962 were developed from either leaf length or leaf width. Leaf area development in black cherry seedlings was similar to that in other indeterminate species. Leaves of young plants reached full expansion at a lower leaf plastochron age than leaves of older plants. Maximum net photosynthesis per unit leaf area occurred 2-3 plastochrons before full leaf expansion. There was strong ontogenetic drift in net photosynthesis with leaf age; net photosynthesis decreased as plant age increased in leaves of the same plastochron age. Plots of the oblique series were particularly useful in providing information about interaction effects.