In this article we will discuss about the classification of meristem based on origin.
1. Primary Meristem:
Primary meristem can be defined as a meristem that owes its origin from promeristem, differentiates into permanent tissues that compose the primary body of a plant and persists for the whole life of a plant.
The primary body of stem and root, as seen in cross-section, consists of epidermis, cortex, phloem, xylem and pith that compose the dermal, vascular and ground tissue system of a plant. All tissues of these tissue systems are the derivatives of primary meristem. Primary meristem composes the apical meristem of root and shoot.
Primary meristem also occurs at the leaf tip and forms the adaxial and abaxial epidermises of a leaf that enclose mesophyll tissues and vascular tissues. Primary meristems are also found to occur at the basal region of spine, at the basal region of internode, at the basal region of leaves of many monocotyledons and at the apex of axillary bud that develops into branches, inflorescences, thorns and tendrils etc.
It is to note that as soon as differentiation starts in the promeristem it passes to primary meristem stage, the cells of which are also capable of division. After complete differentiation certain cells of permanent tissue may regain the ability to divide and this phenomenon is referred to as dedifferentiation.
2. Secondary Meristem:
Secondary meristem can be defined as the meristem that owes its origin from primary permanent tissues by dedifferentiation and adds new tissues to dermal and vascular tissue systems for effective protection, conduction and mechanical strength.
Cork cambium also called phellogen, vascular cambium of dicotyledonous root and interfascicular cambium of dicotyledonous stem are examples of secondary meristem. Phellogen occurs at the extrastelar region of root and stem. It originates either adjacent to or within the epidermis.
It also originates in the peripheral or deeper region of the cortex. Phellogen donates cells on the peripheral and inner side. The peripheral derivatives are phellem and the inner derivatives are phelloderm. Phellem, phellogen and phelloderm together constitute periderm. Periderm forms protective tissue system in plant.
Vascular cambium of root occurs at the intrastelar region. The derivatives of vascular cambium supplement the vascular tissue —and mechanical tissue system of roots. Interfascicular cambium of stem originates from the primary permanent medullary ray cells present in between the two vascular bundles of stem by dedifferentiation.
The interfascicular cambium along with fascicular cambium (the derivative of primary meristem) forms a cambium ring. The inner and peripheral derivatives are respectfully secondary xylem and secondary phloem and they compose the secondary vascular tissue or conducting tissue system in plants.