The region outside the stellar is called the outer stellar region. Its various parts are-
(i) Hypodermis: One or more cell layers under the skin is called hypodermis. Hypoderm is absent in plant roots. It is formed by skerenchyma cells in monocots and collenchyma cells in dicots.
Action: Strengthens and protects the stem.
(ii) Cortex: The part from below hypodermis to endodermis is called cortex. It is multilayered and composed of parenchyma cells. Cells are round and have intercellular spaces. It contains scarides, oil glands and resin ducts.
Functions: Cortex stores water and food, gives strength to the plant and participates in photosynthesis. It controls the root pressure. Shvetsar stores food.
(iii) Endodermis: The inner layer of cortex is called endodermis. It is composed of single-layered and barrel-shaped cells. The cells are densely packed and without gaps. Suberin and lignin accumulate in the inner walls of the cells to form a ribbon-like lining. This is called the Caspian strip. It is named after the scientist Caspery (1865). Endothelial cells that have thin walls are called passage cells or passage cells. The endodermis of the stem of dicots (sunflowers) contains many starchy grains called starch sheath. Stems of monocots do not have endosperms.
Functions of Endodermis: Vascular bundles and other cells act as a barrier to the endodermis by entrapping air and water to prevent obstruction. It protects the inner part. It stores food. It controls the root pressure.