Young's modulus and tensile strength have been measured on mango leaves at weekly intervals up to the age of 35 days. Both increase with age, but there is a fall in the rate of increase of Young's modulus at 21 days. The relative plastic/elastic extension increased to a maximum at day 21. Prior to day 21, turgidity played an important role in the elasticity of the leaf, but not after that date. These changes are discussed in relation to the composition of the cell wall. Removing the cuticle and cell wall constituents sequentially showed that pectin was responsible for all the mechanical properties of the leaf up to day 14, when leaf expansion ceased. At day 35 the cuticle and the lignin together contributed about 80% to the tensile strength, and the cuticle alone about 70% to the Young's modulus.