The groups of complex connective tissues of the xylem and phloem are called vascular bundles. There are different types of vascular bundles in plants.
Vascular bundles have different patterns of arrangement in the primary structure of plant parts. However, they do not have any pronounced pattern in the secondary vascular tissues.
Types of Vascular Bundles
The particular arrangement of the xylem and phloem forms different types of vascular bundles. There are four types of vascular bundles based on the arrangement of the xylem and phloem.
- Collateral bundle
- Bicollateral bundle
- Concentric bundle
- Radial bundle
Collateral Bundle
The vascular bundle where the xylem and phloem are arranged on the same radius, one after the other is called a collateral bundle. Here, the xylem lies towards the center and the phloem towards the periphery. This is characteristic of the stems of dicots, monocots, and gymnosperms.
In the dicot stem there may be meristematic tissue between phloem and xylem. Such vascular bundles that have this meristematic tissue between the xylem and phloem are called open bundles. In monocot stem, this meristematic tissue or cambium is absent and such vascular bundles are said to be closed. Here, the external phloem is continuous with the internal xylem.
Bicollateral Bundles
A bicollateral bundle is a variation of a collateral bundle where there is an additional phloem patch on the inner side. This additional phloem patch is called an internal phloem. The presence of these additional phloem results in a vascular bundle having a xylem in the center and a phloem appearing on its upper and lower sides.
There are cambium or meristematic tissue strips present between the phloem and xylem on both sides, thus making the bicollateral type of vascular bundle an open bundle. So the sequence of arrangement is the outer phloem followed by the outer cambium, then comes the xylem and inner cambium with the inner phloem at the last.
This is one of the rare types of vascular bundles but is quite common in members of the family Cucurbitaceae.
Concentric Bundle
In a concentric bundle, the xylem and phloem are arranged in concentric rings one after the other. They lack a cambium between the conductive tissues and is therefore called a closed bundle. This type of vascular bundle is of two types.
- Amphivasal bundle: Here, the phloem occupies the center, and the xylem present surrounds it. It is commonly seen in Drasena stems.
- Amphicribral bundle: In this type, the phloem surrounds the xylem that occupies the center. It is also called a hydrocentric bundle and is common in ferns.
Radial Bundle
Radial bundles are characteristic of roots where the xylem and phloem appear in different radii and alternate with each other. The patches of groups of xylem and phloem are separated by non-conductive tissues.
Variations in Vascualr Bundles
- Asparagus stems have a ‘V’ shaped xylem and a phloem appears in between the arms.
- Plants of Solanaceae have bicollateral bundles in their stems but the internal phloem appears detached from the vascular bundle and is positioned towards the pith.
- In Amaranthaceae and Piperace, the vascular bundles are scattered in the pith.
- Vascular bundles in Cactaceae are seen in the cortex and are called cortical bundles.
References
- The Vascular Tissue Systems
- Sukumaran O R. Pre-Degree Botany. Murali Publications.
- Abraham P C. Anatomy, Embryology & Microtechnique. 1999. St. Mary’s Books & Publications.
