referring to cells differentiating towards the inside of the stem, contrasting with exarch.

a type of xylem maturation in which protoxylem is internal to metaxylem and development prodeeds centrifugally (from the inside out); for comparison see exarch and mesarch

Where the maturation and development of the xylem strand is centrifugal or 'from the inside outwards'. In this case the small xylem cells that are the first to differentiate and mature (the protoxylem) are surrounded by the later developed, longer metaxylem cells.