Coleoptiles of oats (Avena sativaL.) are often peeled in order to observe hormone-enhanced acidification of the external medium. It is shown by means of the scanning electron microscope that peeling largely removes a single layer of cells, the epidermis with its cuticle. Strips of intact and damaged epidermal cells remain, but most of the newly exposed surface is composed of cortical cells. The cortical face is relatively intact, except that some cells appear punctured and some are broken when a vascular bundle is pulled out with the epidermis. The surface of the cortex is covered by a thin “film” which is partially digested by 2 pectinase. The pectinase pretreatment also inhibits indoleacetic-acid- and fusicoccin-enhanced acidification. Thus, although peeling could be involved in proton extrusion, physical damage to the coleoptile cells per se does not seem to be the major stimulus leading to hormone-enhanced acidificat
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