Difference Between Collateral and Bicollateral Vascular Bundles

The key difference between collateral and bicollateral vascular bundles is that collateral vascular bundle has a strand of phloem and xylem situated on the same radius while bicollateral vascular bundle has two phloem strands situated on the peripheral and inner side of xylem.

Vascular plants have transporting tissues known as vascular bundles. Vascular bundles contain two main types of conducting tissues: xylem and phloem. Xylem is responsible for the transportation of water and minerals from the soil to the plant while phloem is responsible for the transportation of carbohydrates from the photosynthetic parts to other parts of the plant. Therefore, vascular bundles can be seen in the cross-sections of stems and roots. There are four main types of vascular bundles in vascular plants. They are collateral bundle, bicollateral bundle, concentric bundle and radial vascular bundle.

CONTENTS

1. Overview and Key Difference
2. What are Collateral Vascular Bundles
3. What are Bicollateral Vascular Bundles
4. Similarities Between Collateral and Bicollateral Vascular Bundles
5. Side by Side Comparison – Collateral vs Bicollateral Vascular Bundles in Tabular Form
6. Summary

What are Collateral Vascular Bundles?

Collateral vascular bundles have a strand of phloem present external to the strand of xylem on the same radius side by side. Collateral vascular bundles may or may not have a cambium between the phloem and xylem. Cambium is absent in closed collateral vascular bundles while there is a cambium between the phloem and xylem on open collateral vascular bundles. Since there is no cambium in closed collateral vascular bundles, those stems cannot increase in diameter by secondary growth.

Almost all monocotyledon plants have closed collateral vascular bundles. However, stems which have open collateral vascular bundles show a secondary growth. Hence, they can increase in diameter. Open collateral vascular bundles are characteristic of dicotyledons.

What are Bicollateral Vascular Bundles?

Bicollateral vascular bundle is a conjoint vascular bundle in which the xylem is situated in between two phloem strands. Therefore, there are two phloem strands (outer phloem and inner phloem) in a bicollateral vascular bundle.

Moreover, there are two strands of cambium in a bicollateral vascular bundle. One cambium strand is present between the peripheral phloem and xylem. The other one is present in between xylem and inner phloem. Therefore, bicollateral vascular bundles are always open. Secondary thickening happens due to outer cambium. Cucurbita and cephalandra have bicollateral vascular bundles.

What are the Similarities Between Collateral and Bicollateral Vascular Bundles?

• Collateral and bicollateral vascular bundles are two types of conjoint vascular bundles.
• Plant stems show both types of vascular bundles.
• Only one strand of xylem is present in both types of vascular bundles.

What is the Difference Between Collateral and Bicollateral Vascular Bundles?

Collateral vascular bundle is a type of conjoint vascular bundle which has a phloem and a xylem situated on the same radius. In contrast, bicollateral vascular bundle is a type of conjoint vascular bundle which has two phloems located peripheral and inner side of the xylem. So, this is the key difference between collateral and bicollatera vascular bundles. Collaterall vascular bundles may be closed or open, but, bicollateral vascular bundles are always open.

The below infographic summarizes the difference between collateral and bicollateral vascular bundles.

Summary – Collateral vs Bicollateral Vascular Bundles

A collateral vascular bundle is a type of conjoint vascular bundle in which the phloem and xylem lie on the same radius. In contrast, a bicollateral vascular bundle is a type of conjoint vascular bundle in which two phloem strands are situated on the peripheral and inner side of xylem. Moreover, collateral vascular bundles are either open or closed while bicollateral vascular bundles are always open. Furthermore, open collateral vascular bundle has only one cambium strand while bicollateral vascular bundle has two cambium strands. Thus, this is the summary of the difference between collateral and bicollateral vascular bundles.

Reference:

1. “4 Main Types Of Vascular Bundle | Plants”. Biology Discussion, 2020, Available here.

Image Courtesy:

1. “Monocot Stem: Closed Collateral Vascular Bundle in Sparganium” By Berkshire Community College Bioscience Image Library (Public Domain) via Flickr
2. “Herbaceous Dicot Stem Bicollateral Vascular Bundle in Cucurbita (37066303760)” By Berkshire Community College Bioscience Image Library – Herbaceous Dicot Stem: Bicollateral Vascular Bundle in Cucurbita (CC0) via Commons Wikimedia