A seasonal cycle in δ13C, varying between −22‰ in early spring and −28‰ in late fall, has been observed in maple leaves growing under natural conditions. A similar δ13C cycle, varying between −25‰ and −30‰ is indicated for grass growing in the same location as the maple leaves. The seasonal cycle appears to be analogous to the diurnal δ13C cycle.14C/12C ratios of various plants from different parts of Canada show that the 14C produced by thermonuclear bombs has approximately a half-life of eleven years in the atmosphere over Canada. When due allowance is made for the isotope fractionation effect produced by plants, no significant variations in the 14C content in plants from different localities across Canada can be detected. The reported 14C peak observed in atmospheric CO2 in late summer also shows up in maple leaves but with reduced intensity. 14C levels show clearly that the first leaves in spring are composed primarily of carbon assimilated in the previous fall.