The effect of environmental changes on the rate of leaf emergence in wheat Triticum aesiveum L. must be understood to accurately simulate the development of the crop canopy. We determined the phyllochron for 'stephens' winter wheat plants in growth chambers at two constant temperatures (10 and 18℃) at a daylength of 14h and at two constant daylengths (8 and 18h) at a temperature of 20℃, and for plants transferred between the two temperatures and between the two daylengths at the first-, second--, third-, and fourth. leaf stages. The phyllochron of all plants transferred from low to high temperature increased to that of plants kept continually at the high timperature. The phyllochron of plants transterred from high to low temperature at the one-leaf stage was identical with the phyllochron of plants kept continually at the low temperature, but was intermedite between that of the high and low temperatures for gransfers at the second-, third, or fourth-leaf stage. The phyllochron of plants transferred from short to lone days decreased to values identical to those plants transferred from long to short daylengths increased to values equal to or greater than that of plants kept continually at the short daylengths. Thus. plants adapted to either tmperature or day length environments that favor rapid leaf environment for leaf growth than plants that were kept continually in the less favorable environment. The results help explain why the phyllochron is often constant for a particular field planting where plants are subject to seasonal changes in tmperature and daylength.