Walpole and Morgan1 have suggested that differences in absolute growth rates between grains at different spikelet nodes in barley result from differences in quantity of assimilate supplied by each subtending awn, and that this quantity varies in proportion to awn size. They assume implicitly that awns affect grain growth rates by functioning solely as additional assimilate sources. If this is so, then differences in awn size between different nodal positions could determine these growth rate differences only if the assimilate requirements for grain growth were in excess of the total potential pool available for grain growth. Results from other work2,3 suggest that in barley (as in wheat4,5) the converse situation may occur. From studies of temperature effects on the relationship between grain growth and leaf area duration6,7 and on CO2 exchange rates8 in barley, it is a reasonable supposition that under the ‘cool greenhouse’ conditions used by Walpole and Morgan (with presumably a high light intensity and adequate water supply) the potential assimilate supply may have exceeded the grain growth demand in their plants. Support for this hypothesis is provided by their data as these show no evidence of any competition for assimilate between grains such as might be expected if demand had exceeded supply. If grain growth rates were not limited by assimilate supply, then either awns did not affect them or they did so primarily by affecting potential growth rates independently of assimilate supply, that is, by influencing the ‘sink demand’ (or ‘sink capacity’) of each grain.