Root is the part of the plant axis that mostly remains underground. It is a simple organ and lacks any lateral appendages and nodes. The internal structure is very simple and shows little difference at different level. The primary structure of the root in a transverse section shows three distinct regions (a) Epidermis (b) Cortex (c) Stele.
Epidermis: It is the outer most layer of the root and is also known as epiblema or piliferous layer. Generally, it is one layered and bears tubular extension of the epidermal cells called, epidermal hairs (or root hairs). Root hairs are unicellular and continuation of the epidermal cell. They are short, thin so that they can enter into the soil capillaries and increase the surface area of absorption.
Cortex: It is a broad zone of parenchyma cells and there is no hypodermis. The cells are regularly arranged and with intercellular spaces. Endodermis is its inner layer and contains one layer of barrel shaped cells. Sometimes starch grains are found and in that case it is called the starch sheath. Passage cells are found in the endodermal layer
Stele: It is a very narrow zone bounded by pericycle. Generallypericycle is one or two layered and parenchymatous. Sometimes, patches of sclerenchyma may be seen. Stele contains vascular bundles. Vascular bundles are radial, closed and exarch in nature with centripetal development of xylem. The number of vascular bundles vary from dicotyledons to monocotyledons. Pith is very narrow or absent. When present it is parenchymatous in nature.
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