4 - Origin and structure of the secondary plant body

Abstract

This chapter focuses on the origin and structure of the secondary plant body. All gymnosperms and most dicotyledons develop lateral meristems, which result in radial growth and are responsible for an increase in the width of the stems and the roots. Growth derived from the lateral meristems is called secondary growth and is composed of secondary tissues. Lateral meristems are termed “cambia” and function as biracial zones composed of initials and derivatives. The vascular cambium adds to the girth of the axis by the production of secondary xylem internally and secondary phloem externally. The outer protective layers of the axis are called periderm and are formed by the cork cambium, or phellogen. The vascular cambium develops a short distance behind the apical meristem. The cambium that forms within the vascular bundle is called fascicular cambium. The interfascicular cambium is initiated somewhat later from the renewed division of the parenchyma cells located between the original vascular bundles. A cambium in which the fusiform initials are overlapping and are of variable length is termed “unstratified.” A specialized stratified or stored cambium occurs when the fusiform initials are of nearly equal length, and are grouped in parallel and horizontal rows.