It forms a multi-layered zone between the endodermis and the central vascular cylinder. Pericycle is present in both stem and root but is not found in many hydrophytes and monocot stems.
In dicot stems, it occurs as a cylinder encircling the vascular bundles and the pith. Pericycle originates from apical meristem. Pericycle may be parenchymatous as in roots or sclerenchymatous as in dicot stems.
In dicot stems the sclerenchymatous pericycle may be continuous or discontinuous. The scleremchymal form isolated strands which associate with the phloem or bast of the vascular bundle into the form of a cap known as the hard bast.
In roots the pericycle consists of a single Layer of small very thin-walled, more or less barrel-shaped cells.
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Function: In roots the pericycle is the seat of origin of lateral roots. In dicot roots 1 pericycle give rise to lateral meristem. In stems pericycle is the seat of origin of adventitious roots. Parenchymatous pericycle serves as storage layer.
3. Pith: The central core part of the ground tissue is known as pith or medulla. It is usually made up of thin-walled, large-celled parenchyma with or without intercellular spaces.
In dicot stems, the pith is often large and well developed. Here, it extends outwards to the pericycle between the vascular bundles.
Each such extension of pith is a strip of parenchyma known as pith ray or modularly ray. In dicot root, the pith is either small or absent whereas in monocot root a distinct large pith is present. In dicot root often bigger vessels of xylem meet at the centre.
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Function: It chiefly serves as the store house of several excretory substances such as tannins, phenols, calcium oxalate etc. It also provides mechanical strength. The modularly rays, at the time of secondary growth, become partly meristematic and produce secondary meristem.