On Nature of Intercalary Growth and Cellular Differentiation in Internodes of Avena sativa

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Abstract
Growth and cellular differentiation follow a basipetal course in the next to the last intemode (p-1 intemode) in Avena sativa L. During internodal extension, inter-calary-meristem activity becomes localized to the base and peripheral portion of the intemode with predominant cytokinetic activity occurring in the lower portions of the epidermal and outer ground-parenchyma systems. The intercalary meristem in the p-1 internode of Avena consists of parallel rows of more or less isodiametric initials which give rise to more differentiated elements both above and below the intercalary-meristem zone. Cytokinetic activity in the intercalary meristem is continuous; that is, no evidence of a diurnal rhythm in frequency of mitoses was found. Below the intercalary meristem, only epidermal cells differentiate from the meristem. In these cells the nuclei become remarkably elongate and corkscrew-shaped; these are considered to be senescent nuclei. Cellular differentiation above the intercalary meristem is continuously basipetal and results in formation of a remarkably ordered series of long epidermal cells which alternate with pairs of cork and silica cells, trichomes, or stomates. The earliest stages of cytodifferentiation of these epidermal idioblasts and the long epidermal cells can be seen in the intercalary meristem and just above it. Changes and rates of change in cell number and cell length have been analyzed for different stages in the development of the p-1 intermode. Cell division continues until p-1 is 3-4 cm in length; then the cell-division component of intercalary-meristem activity ceases to be of any significance. Cell division and cell elongation are clearly overlapping processes during the earliest stages of internodal development. Most of the linear extension of p-1 internode in Avena, however, is explained on the basis of growth by cell elongation.