In this article we will discuss about the interxylary periderm in plants, explained with the help of suitable diagrams.
The periderm, which is formed in the wood and medullary tissues, is described as interxylary periderm. The formation of periderm at the periphery of stems and roots, and its protective function are generally recognized. But the periderm in the wood and medullary tissues has received little attention because it is less prevalent in plants in contrast to outer periderm.
Interxylary periderm is formed in the roots and rhizome of the following plants: Aconitum (Ranunculaceae), Sedum (Crassulaceae), Epilobium, Oenothera (Onagraceae), Mertensia (Boraginaceae), Salvia (Labiatae), Crepis, Artemisia (Compositae), Geranium (Geraniaceae), Polemonium (Polemoniaceae) etc.
The interxylary periderm is formed in addition to normal peripheral periderm. It develops in between the two concentric rings of secondary wood formed at different seasons. The interxylary periderm is formed by phellogen that is differentiated from the outer parenchymatous part of the wood of the previous year.
It may arise from the deeper portions also. The phellogen forms phellem and phelloid on the peripheral side. The suberized cork and non- suberized phelloid layers alternate each other. Normally two or three layers of cork and a similar number of phelloid layers are present in each interxylary periderm. There is little or no differentiation of phelloderm.
The phelloid cells present on the inner layers collapse soon. As growth continues, rings of secondary xylem are formed by vascular cambium ring.
Accordingly continuous and concentric rings of interxylary periderm are formed. So the mode of formation of interxylary periderm is centrifugal in contrast to peripheral periderm, which is centripetal in development. After the formation of interxylary periderm the tissues present on the inner side die.
On the peripheral side on inner periderm new vascular tissues are developed by the vascular cambium. These tissues are protected by the external periderm situated on the peripheral side and inner or interxylary periderm present on the inner side. In the old roots the number of rings of interxylary periderm commonly ranges from one to ten and the number may be as high as twenty (e.g. Epilobium angustifolium).
Usually interxylary periderm is formed in spring and it was observed that a single periderm is formed each year during the life of the organ except the first one. Therefore, by counting the number of rings of interxylary periderm the approximate age of the organ can be determined. Moss (1940) regards that the interxylary periderm protects the organ from desiccation and plant pathogen, and thus provides efficient functioning of perennating structures.