Question

A concentric amphivasal vascular bundle is that in which
A. Centrally located xylem is surrounded by phloem
B. Centrally located phloem is surrounded by xylem
C. Phloem is flanked by xylem on interior side only
D. Xylem is flanked by phloem on exterior side only

Answer 1

Concentric vascular bundles are those within which one vascular component surrounds another on all sides. It is divided into two types which are :- amphivasal and amphicribral. This kind of concentric amphivasal vascular bundle is found in Yucca and Dracaena.

Complete answer:
Vascular bundles are distinguished according to the spatial relation of xylem and phloem. The presence of xylem and phloem in the same bundle surrounding one other is referred to as a concentric bundle. These vascular bundles are classified as amphicribral and amphivasal vascular bundles. The presence of xylem in the centre enclosed by phloem is described as amphicribral (or hadrocentric),like in Ferns.
When xylem tissue surrounds the phloem tissue from all different sides then the vascular bundle formed is called as 'amphivasal vascular bundle''. Such types of bundles are seen in monocot plants, like Dracaena.

Hence, the correct answer is option (B).

Additional information:
The other type of amphivasal vascular bundle is Collateral Vascular bundle. A vascular bundle in which the strand of xylem is surrounded externally by the strand of phloem on the same radius side by side is known as a collateral bundle. Cambium may or may not be present in between xylem and phloem, and so there are the following two types of a collateral bundle:
(a) Closed collateral bundle: In this type of bundle cambium is not present in between xylem and phloem. Therefore stems having this sort of bundle does not have normal secondary growth. Ex. Monocotyledonous stem. Usually, these bundles are surrounded around a bundle sheath which is made up of sclerenchyma cells and those that lack the sheath are considered as anomalous (e.g. Asparagus stem).
(b) Open collateral bundle: In this open collateral vascular bundle cambium is present known as fascicular cambium between xylem and phloem. The bundles will increase in size by increase in diameter by normal secondary growth with the assistance of fascicular cambium. Ex. Dicotyledonous stem.

Note: In a collateral vascular bundle xylem and phloem are present on the similar radius and the phloem is located on the outer side of the xylem. Whereas in concentric vascular bundles one variety of vascular tissue (xylem or phloem) is surrounded by the other.