Depending upon the time of origin and internal structure, the xylem is of two types: (1) Protoxylem: It is the first or easily formed xylem. It is made up of small tracheids and vessels, with annular or spiral thickenings. (2) Metaxylem : It is the later formed xylem. It consist of larger tracheids and vessels and have reticulate, scalariform and pitted thickenings.