Rapid mobilisation of storage products, including xyloglucan, in cotyledons of germinating nasturtium (Tropaeolurn majus L.) normally starts about 7-8 d after imbibition and growth of the seedling at 20 25 ~ C. Levels of activity of endo-l,4-fl-glucanase (EC 3.2.1.4) in cotyledons, as assayed viscometrically with xyloglucan as substrate, varied in parallel with the rate of breakdown of xyloglucan. When cotyledons were excised from the seedling axis and incubated on moist filter paper at any point before 7 d, the catabolic reactions which normally occurred in the intact seedling were suspended. If, however, cotyledons excised at 8 d were incubated in 10 -6 M 2,4-dichlorophenoxyacetic acid, a rise in endo-l,4-flglucanase (xyloglucanase) activity was observed and a sharp decrease in fresh and dry weight as well as xyloglucan levels ensued at rates comparable to those observed in cotyledons attached to the seedling. Neither gibberellin nor kinetin treatments promoted xyloglucan breakdown or enhanced xyloglucanase activity. Addition of auxin to excised cotyledons before 7 d did not evoke premature breakdown, indicating that the tissue became receptive to auxin only at this time. The triggering process took place in darkness and was unaffected by various light-dark cycles. It is concluded that the sudden degradation of xyloglucan which occurs in nasturtium seeds about a week after germination begins is the result of enhanced activity of a depolymerizing xyloglucanase, this activity being evoked by auxin originating in the emerging seedling axis.