Since the discovery of auxin, a debate has taken place as to whether the auxin distribution in elongating organs can account for the distinctive cell elongation pro®les that have been found. In an attempt to address this important issue, the elongation pro®les of cells have been compared in the hypocotyls of wild-type and auxin-hypersensitive axr3-1 Arabidopsis Columbia ecotype seedlings. Clear differences in cell elongation pro®les were found in the two types of seedling, whether they were

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... t-or dark-grown. However, it was not possible unambiguously to ascribe the cell elongation pro®le differences to the proposition that the axr3-1 mutation causes the hypocotyl to be hypersensitive to auxin. The possibility that the abnormal hypocotyl elongation pro®le of the mutant was a secondary effect, consequent on a more fundamental effect of the axr3-1 mutation, is considered. It is clear from this study that cell elongation and its control needs to be studied at the cell, and not the organ, level. To characterize a mutant as having a short, or long, hypocotyl is inadequate. To determine which factors control the timing and the magnitude of cell elongation requires the demonstration of correlations between the growth rate of cells and their content of regulating substances or their sensitivity to that substance. Studies of the cell elongation pro®les of the many hypocotyl length mutants could also be a very effective means of probing the co-ordination of root and shoot elongation.