The mechanism of surface growth in parenchyma of Avena coleoptiles

Abstract

A study has been made of the organization of the cell wall in the parenchyma of Avena coleoptiles at successive stages of growth, using light and electron microscopic methods. It has been observed that extension of the parenchyma involves a progressive separation of the primary pit fields accompanied by an increasing dispersion of the cellulose microfibrils about their preferred direction of orientation. On the basis of this, and ancillary evidence from other cell types, it is suggested that extension growth involves stretching of the cell with the intercalation of new microfibrils into the expanding cell wall framework from the regions of the primary pit fields and penetration of the cell wall by plasmodesmata. It is considered that the evidence is consistent equally with the view either that the cell wall is stretched as water absorption accompanying enlargement takes place, or that cell enlargement is controlled by the synthesis of cell wall material at synthetic centres (pit fields and plasmodesmata) distributed over the cell surface. The concept of bipolar tip growth for coleoptile parenchyma is rejected.