A phi layer in roots of Ceratonia siliqua L.

The central cylinder of the primary root of the carob tree (Ceratonio siliqua) is encircled by a layer of cells with wall thickenings, known as a phi (Phi) cell layer. The development of the Phi layer and the chemical composition of the cell wall thickenings have been studied in roots of C. siliqua. The results reveal the presence of condensed tannins in the mature phi thickenings and that the development of the Phi layer is asynchronous: at 0-1 cm from the root tip Phi thickenings appear before endodermis differentiation at the sites opposite phloem, at 1-4 cm new Phi thickenings are developed at the sites opposite xylem, at 4-7 cm the Phi layer consists of two layers of cells and it completely encloses the central cylinder.