Phi thickening occurs since epidermis till the endodermis' adjacent layer in roots of various angiosperm and gymnosperm species. Unlikely Casparian strips they are not suberized and they can be found in varied points of the cell wall in a unique cell. In some species the phi thickening development can be observed since the first centimeter from root apex. Phi thickening has received the role of mechanical support to tissues in radial expansion process and roots in longitudinal growing in substrates with high level of difficulty to root penetration, also as physical barrier against the fungi invasion to the cortex deep layers or to the roots central cylinder and also to ions selection which moves by apoplastic way. Instead phi thickenings continue being used as a root anatomic character to taxonomic studies of some species, it has been proven that its development is not necessarily constitutive, what can makes it improper for this use.
Casparian strips; ions uptake; mycorrhiza; phi layer