We propose a new method, the leaf measuring-interval index (LMI), to estimate leaf age in morphological and physiological studies of leaves. When the plastochron, the interval between the initiation of successive leaves, is constant, the well-known leaf plastochron index (LPI) provides a robust measure of leaf age. When the duration of the plastochron is not uniform, however, we show that the LPI can (in simulations) and does (with actual data) turn variation in duration of the plastochron into variance about the regression estimates of leaf growth curves. The method we present in this paper, the LMI, is plastochron independent. This new method is particularly suited, therefore, for studies of plants growing in natural environments rather than in controlled growth facilities where the assumptions of the LPI method can be met.