A cytological study of the oat coleoptile

Abstract

A study has been made of the cytoplasmic organization of epidermal and sub- epidermal parenchyma of oat coleoptiles both before and after the onset of extension growth. In the youngest cells studied (coleoptiles 0.5 mm long) the epidermal parenchyma was characterized by large stellate vacuoles and by the secretion of vesicles through the plasmalemma into the external periclinal cell wall. Vesicular secretion was not observed in the anticlinal walls or into the walls of subepidermal parenchyma. The subepidermal parenchyma was characterized by the presence of numerous thick-walled vacuolar structures and plastids rich in starch. In the extension phase (coleoptiles > 10 mm long) vesicular secretion was not observed, starch was absent from the plastids, and the cells contained large vacuoles with only a thin peripheral zone of cytoplasm. During the later stages of this phase the plasmalemma appeared to consist of two darkly stained membranes separated by a light zone, or of elaborations of this structure, e.g. dark, light, dark, light, dark. By contrast, in the pre-extension phase the plasmalemma appeared as a single membrane. The implications of these observations are discussed in relation to the process of wall formation.