Vascular bundle: The part of the transport system that runs through the plant and carries water, minerals, and nutrition to different parts of the plant.
Different types of vascular bundles:
The vascular bundles are classified into three types accordingly as per the relative location of xylem and phloem:-
(1) Radial vascular bundle
(2) Conjoint vascular bundles
(3) Concentric vascular bundles
(1) Radial vascular bundle:
- In these types of vascular bundles, xylem and phloem tissues occur at alternate radial locations in different groups.
- This can be seen at the roots. This is the most primitive type.
(2) Conjoint vascular bundles:
- In conjoint vascular bundles, the xylem and phloem tissues are present on the same radius and only opposed to each other.
- In dicot stems, this is a common phenomenon. Conjoint vascular bundles are of two forms, depending on the number and the location of the phloem group.
The two sub-types are:-
(i) Collateral (ii) Bi-collateral
(i) Collateral type –
- Except for the members of Cucurbitaceae and certain members of Convolvulaceae, vascular bundles are of a very specific nature and are found in dicotyledon stems.
- Cambium can be present or absent in between xylem and phloem patches that open or close the vascular bundle, accordingly.
(ii) Bi-collateral type –
- Vascular bundles comprise two patches of phloem in the same area on each side of the xylem.
- The outer phloem remains to the central cylinder periphery and the inner phloem remains toward the middle.
- Those vascular bundles consist of two patches of cambium.
- The external cambium distinguishes the outer phloem from the xylem, whereas the internal cambium distinguishes the xylem from the internal phloem.
- The outer cambium is concave in shape and much more active than the internal planoconcave cambium strips which are less active.
(3) Concentric vascular bundles:
- The either xylem surrounds the tissue of the phloem occasionally or vice versa.
- Such bundles of vascular substances are called concentered vascular bundles.
- When xylem surrounds the phloem tissue from all sides, the vascular bundle is called amphivasal vascular bundle (leptocentric type).
- Such bundles are seen after secondary growth in monocot plants like Dracaena.
- The vascular bundle is called amphicribal vascular bundle (hadrocentric type) when phloem covers the xylem tissue altogether.
- These vascular bundles occur in pteridophytes such as Selaginella etc.