THE relative importance of various photosynthetic organs as suppliers of assimilates to developing grain in barley and as determinants of the yield of grain per ear has attracted much attention. It has been shown that the major contributions are made by the flag leaves and photosynthetic tissues of the ears themselves1,2; little or no information is available regarding the relative contributions of the various organs to the individual grain within the ear; furthermore there is little understanding of the mechanisms by which differences in growth rates between grains develop.