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. A vascular bundle in which one type of vascular tissue surrounds the other is known as a concentric bundle. It is a plant vascular bundle in which (1) phloem surrounds xylem or (2) xylem surrounds phloem. Out of xylem and phloem, if one type of tissue is surrounded by the outer kind, it is said to be a concentric vascular bundle. One vascular element surrounds the other on all sides.
The concentric bundles, either amphivasal (Hadrocentric) or amphicribral (Leptocentric), are closed as there is no cambium in between xylem and phloem. It is of two types, such as; (a) Hadrocentric and (b) Leptocentric.
Hadrocentric: When xylem is surrounded by phloem, it is called Hadrocentric. It is a vascular bundle in which the phloem encircles the central strand of xylem. Here phloem is seen towards the outer side and xylem towards the inner side. Example: Vascular bundle of Pteris, Dryopteris, Ferns, etc.
Leptocentric: When Phloem tissue is surrounded by Xylem tissue the vascular bundle is called Leptocentric. It is a vascular bundle in which xylem encircles the central strand of phloem. This xylem is seen outside that surrounds the phloem. Example; Dracaena, Yucca, etc.