Abstract: The stomatal densities of herbarium specimens of Trema angustifolia BL. and T. cannabina Lour were investigated ,and the intrinsic water use efficiency of both species was computed from δ13C values in their leaves. From 1920 to 1990,the stomatal densities of T. angustifolia and T. cannabina reduced by 28 % and 40% respectively, while their intrinsic water use efficiency increased by 20% and 42% respectively. Changes in these parameters with time (and therefore with increasing of atmospheric CO2 concentration)were significant in both species :with stomatal densities declining and water use efficiency increasing with time.