[E- learning: XI & B.Sc. Part-II, Botany Hons.]

TYPES OF VASCULAR BUNDLES IN PLANTS

Dr. Arvind Kumar Department of Botany, T.N.B. College (T.M. Bhagalpur University), Bhagalpur-812007

Vascular bundles are a part of the transport system in vascular plants. The two elements of the vascular tissue are xylem and phloem. These are arranged in groups called as vascular bundles. Xylem is responsible for unidirectional flow of water. Phloem is responsible for the bidirectional flow of food. This, vascular bundles comprise the main transport system of the plant. The vascular bundles are arranged differently in plants depending on their internal structure. Plants classified as dicots and monocots have their vascular bundles arranged in a circle within the stem, with phloem on the outside and xylem on the inside. A layer of cambium in between each bundle is found in dicots and absent in monocots. It is xylem in these plants that can later become woody tissue, in many cases. According to relative position of xylem and phloem, the vascular bundles are classified variously. There are mainly three types of vascular bundles:

(i) Radial (ii) Conjoint (iii) Concentric (A) Radial vascular bundle

In these types of vascular bundle, xylem and phloem tissues occur in separate groups on alternate radial positions. Xylem is present in different radii and phloem in different alternating to each other and this type of arrangement is seen in roots.

(B) Conjoint vascular bundles

When the xylem and phloem tissues are present on the same radius and just opposed to each other then it is known as conjoint vascular bundles. In this case generally xylem is present towards the center and phloem is found outside to the xylem cells. It is a common occurrence in dicot stems. Few meristmatic cells called cambial cells are also found in between xylem and phloem cells. Whenever cambial cells are found in between the vascular bundles are called open type and when the cambial cells are not found the vascular bundles are known as closed type. Depending on the number and position of phloem group, conjoint vascular bundles are of two types: collateral type and bi-collateral type.

i. Collateral vascular bundles are very common type and seen in stems of dicotyledons except the members of Cucurbitaceae and some members of Convolvulaceae. In this condition xylem is present towards the center and phloem towards outward. Phloem surrounds the xylem within the bundle and cambial cells may be present or absent in between xylem and phloem patches. If the cambial cells are present in between the xylem and phloem then the vascular bundles are known as open type. But if the cambial cells are not present in between these two then it is known as closed type. ii. Bicollateral vascular bundles contain two patches of phloem on either sides of the xylem on the same radius. The outer phloem or external phloem remains towards the periphery of the central cylinder and inner or internal phloem remains towards the centre. However, there are two patches of cambium found in these vascular bundles. The outer cambium separates outer phloem and xylem; whereas the inner cambium separates the xylem and the inner phloem. The outer cambium is concave in shape and is more active than the inner cambium strip which is inactive or less active.

C) Concentric vascular bundles

Sometimes, either xylem surrounds the phloem tissue or vice versa. In this case xylem and phloem cells are not found in separate bundles instead they completely surround each other. Such vascular bundles are called concentric vascular bundles.

When xylem surrounds the phloem tissue from all sides the vascular bundle is called amphivasal vascular bundle or leptocentric type. Such bundles are seen in monocot plant like Dracaena after secondary growth. When phloem surrounds the xylem tissue entirely, the vascular bundle is called amphicribal vascular bundle or hadrocentric type. Such vascular bundles are seen in Pteridophytes like Lycopodium, Selaginella.

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