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Epidermis: It is the outermost layer of stem and is protective in function. Made up of compactly arranged, parenchymatous cells devoid of chloroplasts (except guard cells). The outer walls of epidermal cells are cuticularised. Stomata and multicellular hair are present in epidermis.
Cortex: Consists of thin-walled parenchymatous cells with inter- cellular spaces. Major function of cortex is food storage.
Endodermis: It is the innermost boundary of cortex made up of compactly arranged barrel-shaped cells without Casparian strips. Endodermal cells of stem store starch grain and are often referred to as starch sheath.
Medullary Ray: They are radial strips of parenchyma which are present between adjacent vascular bundles. They connect pith with pericycle and cortex. Ray cells are larger than cortical cells.
Hypodermis: It lies just below epidermis and consists of 3-5 layers of collenchymatous cells. The intercellular spaces are absent and corners of cells are thickened due to deposition of extra cellulose impregnated with pectic substances. These cells often possess chloroplasts.
Pericycle: Pericycle is heterogenous, i.e., made up of alternating bands of parenchymatous and sclerenchymatous cells. Sclerenchymatous cells are situated in between endodermis and phloem cells of vascular bundles whereas parenchymatous cells are present above the medullary rays. Vascular bundles: Arranged in a ring and are conjoint (with both phloem and xylem), collateral (phloem and xylem on same radius) and open (with a strip of cambium between phloem and xylem). Xylem is situated towards the inner side of each vascular bundle whereas phloem lies towards the pericycle on the outer side of vascular bundle. Xylem is endarch (protoxylem towards the centre).
Pith: It is extensively developed central portion of ground tissue, made up of large thin-walled polygonal parenchymatous cells with intercellular spaces.
Secondary growth in dicot stem: In a typical dicot stem the cambium is present in between the xylem and phloem. It is called fascicular or intrafascicular cambium. Along with this cambium, some medullary ray cells also become active forming interfascicular cambium. Interfascicular and intrafascicular cambia together form a ring of cambium. Cambial cells give rise to secondary phloem on the outer side and secondary xylem on the inner side. Phellogen or cork cambium arises from permanent living cells of hypodermis or outer cortex. It divides to give rise to phellem (cork) on the outerside and phelloderm (secondary cortex) on the inner side.
