Concentric vascular bundle: Concentric bundle is a plant vascular bundle in which- phloem surrounds xylem or xylem surrounds phloem called also amphicribral bundle and amphivasal bundle. Out of xylem and phloem, if one type of tissue is surrounded by the outer kind, it is said to be concentric vascular bundle.
It is of two types, such as; (a) Hadrocentric and (b) Leptocentric
Hadrocentric: When xylem is surrounded by phloem, it is called Hadrocentric. Example: Vascular bundle of Pteris, Dryopteris, Ferns etc.
Leptocentric: When Phloem tissue is surrounded by Xylem tissue the vascular bundle is called Leptocentric. Example; Dracaena, Yucca etc.