The mature seed of celery (Apium graveolens, L.) contains a small axile linear embryo surrounded by endosperm which occpies the bulk of the seed. The endosperm is living and consists of mostly large angular thick-walled cells containing aleurone grains (often with globoids) and lipid droplets. - Using de-embryonated seeds, it has been shown that the endosperm was induced to break down by gibberellin. The aleurone grains became swollen and lost their contents and the bulk of each cell wall was hydrolyzed. However, a thin resistant layer of wall remained around each protoplast. The wall hydrolysis caused the endosperm to break down into individual cells which could be plasmolyzed and therefore appeared to be still living. All cells of the endosperm responded to gibberellin in a similar way although the cells near the radicle appeared to degrade more rapidly than those elsewhere. There was no change in the absence of the hormone. The response was apparently specific to gibberellin and did not occur in the presence of ethylene, kinetin, abscisic acid and indole acetic acid. The results were the same in light and in darkness. - It has been thought that endosperm breakdown during germination of seed such as celery involved release of hydrolases from the expanding embryo. The results of this study indicate that endosperm breakdown might be caused by hydrolases arising in the endosperm itself in response to gibberellin released from the embryo.