The vascular bundle is a kind of bundle that consists of xylem and phloem tissues. The phloem transports sugar and any other photosynthetic products to the parts of that plant that need it. It is a strand of the particular vascular tissue of higher plants consisting mostly of xylem and phloem. These strands during the course of time distinguish into xylem and phloem and organize into vascular bundles.
- To carry water and dissolved food materials from root to stem and leaf. The prime function vascular bundle is the conduction of water and food materials in the primary growth stage of plants.
- To carry prepared food material from leaf to different parts. Phloem transport food materials form leaves (photosynthetic products) to all parts of the plants including roots.
- To give mechanical support to plant. They also provide mechanical support (a mechanical tissue) to the plant.
There are three types of the vascular bundle:
- Collateral Bundle,
- Bicollateral Bundle,
- Concentric Bundle,
- Radial Vascular Bundle.
Collateral Bundle: A vascular bundle in which a strand of phloem is present peripheral to the strand of xylem on the same radius side by side is known as a collateral bundle. Two types:
- Closed collateral bundle: In this type cambium is absent in between xylem and phloem. Ex. Monocotyledonous stem.
- Open collateral bundle: An open collateral vascular bundle has a cambium called fascicular cambium between xylem and phloem. Ex. Dicotyledonous stem.
Bicollateral Bundle: A vascular bundle with a phloem situated on the peripheral and inner side of xylem is known as bicollateral bundle. Ex. Cucurbita stem.
Concentric Bundle: The bundle in which one type of vascular tissue surrounds the other is known as a concentric bundle. Two types:
- Amphivasal bundle: xylem encircles the central strand of phloem is known as amphivasal bundle, also called leptocentric bundle. Ex. Dracaena, Yucca.
- Amphicribral bundle: phloem encircles the central strand of xylem is called an amphicribral bundle, also known as hadrocentric bundle. Ex. Selaginella.
Radial Vascular Bundle: The primary xylem and primary phloem strands are separated from each other by nonvascular tissues and they are situated on alternate radii of an axis, is known as radial vascular bundle or radial bundle.