Definitions / Explanation :

$(i)$ Endoarch vessels

$(ii)$ Exoarch vessels

$(i)$ The beginning of a development of a xylem tissue starts from centre, gradually forwards towards peripherry. Thus protoxylem is arranged towards centre and metaxylem towards metaxylem it is called endoarch developme

$(ii)$ The beginning of development of this type of xylem tissue starts with periphery and forwards towards centre. Thus protoxylem is arranged towards periphery and metaxylem is arranged towards centre. It is called exoarch vessels.