Plant cell extension: structural implications for the origin of the plasma membrane

Abstract. The proposal that rapidly elongating plant cells cannot maintain plasma membrane synthesis by means of the normal endomembrane system has been examined in elongating segments of Avena sativa coleoptiles. Segments were sampled and fixed for electron microscopy, before and after elongation on auxin solutions. Mean cell extensions, cytoplasmic volumes, dictyosomc numbers, and vesicle sizes and numbers were determined. It was shown that there are sufficient dictyosomes present to sustain the vesicle production necessary for the observed plasma membrane extension.