Abstract Elongation of successive leaves was measured following defoliation of tall fescue plants in controlled environments. Measurements were made under constant temperatures of 24 °C and 14 °C, and after temperature changes from 24 to 14 °C andvice versa. A morphological analysis of the growing leaf was made from the time it was 1 mm long until it was fully elongated. The time elapsed from initiation until the leaf was 1 mm long was estimated. Young leaves less than 1.5 mm long elongated slowly at a constant leaf elongation rate (LER). By extrapolating this LER back to leaf initiation from the apex it was calculated that elongation lasted 42.5 d at 24 °C and 51 d at 14 °C. Lengths of the division zone (DZ) and the extension-only zone (E-OZ) increased to a maximum and then decreased during leaf development. Temperature change had an immediate effect on LER but the response varied depending on the direction of the temperature change. To describe these different features, an empirical model of DZ and E-OZ was designed. Its five parameters were optimized at constant temperature. The model was then used to simulate the LER of plants subjected to temperature changes. Instant and lasting effects of the initial temperature on mean LER in plants transferred from 14 to 24 °C andvice versawere well simulated. It was concluded that the major reason for differences was due to the growth stage (DZ and E-OZ lengths) at which the changes occurred at both temperatures.