In plants which show secondary growth the outer appearance of the stem differ in different species f plants. This difference results from the manner of growth of the periderm, the structure of the phellum and the nature and amount of tissue that are separated by the periderm from the stem.
The periderm consists of three parts: the phellogen which is the cork cambium, the phellum which is the cork produced centrifugally by the phellogen and phelloderm which is parenchymatous tissues produced centripetally by the phellogen.
In trees which produce successive periderms by the formation of successive phellogens up to the depth of phloem, there will be many cork layers.
All the cork layers together with cortical and phloem tissues external to the innermost phellogen are termed rhytidome. In such trees the colour of the stem will be dark brown and never green.
In plants like citrus, eucalyptus, acer and acacia the development of periderm commences only after the production of the secondary vascular tissue has reached considerable dimensions. In such cases the circumference of the epidermis increases together with secondary and other tissues on the outer side of the cambium.
In viscum cork tissue is never formed and the epidermis increases in circumference and persists on the stem throughout the life of the plant. In all these plants stem surface looks green even after secondary growth.
In plants like solanum, guava, pyrus and nerium the first phellogen is formed in the epidermis itself and iln plant like populous, jugulans and ulmus the first periderm is formed in the outer most cortical layer next to the epidermis.
In such cases the subsequent periderms are not formed to the full circumference of the stem similar to the formed one. But they develop in the form of scales. So in these plants absence of well marked rhytidome give the stem a green appearance even after the secondary growth.