The ground tissue system is heterogeneous consisting of different type of cells performing different function. It includes cortex, pericycle, pith and medullary rays.
1. Cortex: The cortex is the extra stellar part which spread from the epidermis to the layer enveloping the stele. The cortex is divided into hypodermis, general cortex and endodermis.
Hypodermis: It is the layer just below the epidermis. In monocots the hypodermis is usually sclerenchymatous whereas in dicot it collenchymatous. The main function of the hypodermis is to provide protection to the internal tissue. It also gives mechanical support to peripheral regions.
General cortex: The layer of cells present between the hypodermis and the endodermis is known as the general cortex. It is usually characterized by the presence of parenchymatous cells with inter cellular space. The presence of the sclereids, resin ducts, tannin cells, oil cavities and laticiferous tissues are noticed in this region. The main function of the general cortex is to store food materials and also it takes part in the metabolic activities.
Endodermis: The innermost layer of the cortex is the endodermis. It is the layer that limits the stele. It is usually single layered formed of compactly arranged cells without inter cellular space. The cells are vertically elongated tubular or barrel shaped in the cross section. It is a protective layer which is also known as accessory epidermis.
2. Pericycle: Pericycle may be one to several layers which is found between endodermis and vascular bundle. It is formed of homogenous cells which is parenchymatous. The pericycle serves as region storage of food materials. During the secondary growth in the dicot stem pericycle forms the secondary cambium.
3. Pith: Pith is the innermost part of the stem which contains isodiametric and loosely arranged parenchymatous cells. In dicotyledons stem the pith is conspicuous with large and well developed, where as in the monocots the pith is not much distinct or completely absent. The pith acts as a region of storage of various substances like starch, fatty substance, mucilage, tannin etc.
4. Medullary rays: Medullary rays or pith rays are a long strip of parenchymatous tissue that passes between the vascular bundles. Since the medullary rays originate from the primary meristem it is known as primary medullary rays. Certain cells of the medullary rays become active during the secondary growth and produce the secondary medullary rays. The main function of medullary rays is conduction of food and water from the cortical region to the pith.