AbstractAfter the onset of imbibition, the dormant seeds ofRumex obtusifoliusandR. crispusare stimulated to germinate by a change from an initial low temperature to a warmer temperature for a relatively brief period: the warmer that temperature the shorter is the optimum period spent at it, and this optimum value is unaffected by the initial temperature. The optimum period is more critical inR. crispusthan inR. obtusifolius(about 1 h and 2.5 to 4 h, respectively, for a warmer temperature of 35°C in the dark); in the light the length of the period at the warmer temperature is less critical in both species. The sensitivity of the seeds to the change to the warmer temperature increases with time from the start of imbibition at a rate which is positively related to the initial temperature. InR. obtusifoliusmaximum sensitivity was typically reached after 3 to 5 d when the initial temperature was 20°C and then remained constant, or declined only slightly, over the period investigated (10 d). At the same initial temperature, however,R. crispusshowed a cyclical pattern of sensitivity with peaks occurring at 3–4 d intervals from the start of imbibit