(i) Periderm: The cork, cork cambium and secondary cortex collectively form a protective tissue called periderm. In order to increase in girth and prevent rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells. Phellogen cells divide on both the outer side as well as the inner side to form secondary tissue. The secondary tissue produced on the inner side of the phellogen is parenchymatous. It is called secondary cortex or phelloderm. Phellogen produce cork or phellem on the outer side. The phelloderm, phellogen and phellem together constitute the periderm.
(ii) The vascular bundles are arranged in the form of ring around the central pit. The xylem bundles are exarch, i.e., protoxylem lies towards the outer side while the metaxylem faces inwards. Phloem bundles alternate with xylem bundles.
