Abstract.In white light(W) stem elongation of green plants is stimulated when some far‐red light (F) is added to W (Holmes&Smith, 1975, 1977b). The effect of adding F to a background W was quantified usingVigna sinensisL. plants and relatively high fluence rate W. F was given selectively to the 1st internode and the internode elongation rate (E.R) was recorded using linear voltage displacement transducers. Results confirm that adding F to background W stimulates internode E. R. The stimulation is very quickly measurable after switching F on and falls off rapidly after switching F off. The stimulation is not dependent on F fluence rate perper sebut is strictly dependent on the quantum flux ratio F/W. These data are strongly in favour of phytochrome as the receptor, whose activity is controlled by the PFRcontent or the [PFR]/[P]ratio. A common mode of action for phytochrome in W and in darkness is suggested. The results indicate independence and additivity of the E. R.modulation by W light quantity and of the modulation by light qualit