Difference between Collateral and Bicollateral Vascular Bundles

Vascular bundles are the transport system of higher plants. It consists of two transport elements, xylem and phloem. Additionally, the vascular bundles also contain surrounding and protective tissues, and a layer of tissue between the xylem and phloem called cambium.

According to the placement of xylem and phloem in the bundle, they are divided into four major types: i) Collateral Bundle, ii) Bicollateral Bundle, iii) Concentric Bundle and iv) Radial Vascular Bundle. Let us throw some light on the first two types, collateral and bicollateral bundles, and also differentiate between them.

Collateral Bundles

Collateral bundles are arranged in such a way that both xylem and phloem are present on a single radius, jointly. The phloem is present external to the xylem elements. A layer of cells called cambium is sometimes present between xylem and phloem, referred to as an open vascular bundle, e.g, dicot stem. In closed vascular bundles, there are no cambial cells present between the xylem and phloem, e.g, monocot stem.

Bicollateral Bundles

The arrangement of vascular bundles in a bicollateral bundle is such that phloem is present both on the periphery and inner side of the xylem. In simple words, xylem bundles are sandwiched between two phloem bundles. There are two cambial strips present in the arrangement, one between the peripheral phloem and xylem, and another between inner phloem and xylem. Since cambium is present, these bundles are always open. E.g, Cucurbita stem.

Collateral vs Bicollateral Bundles

Collateral Bundles	Bicollateral Bundles
Definition
The arrangement of xylem and phloem bundles jointly in the same radius is referred to as collateral bundles.	The arrangement of xylem between a peripheral and inner phloem is referred to as bicollateral bundles.
Number of Phloem
One strip of phloem is present.	Two strips of phloem are present.
Number of Cambium
A single cambium is found, if present.	Two cambiums are found.
Open/Closed
The bundle can be open or closed.	It is always open.
Example
Monocot and dicot stem	Cucurbita stem

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Frequently Asked Questions

Are collateral and conjoint vascular bundles the same?

Conjoint bundles are the ones that have xylem and phloem arranged on the same radius, jointly. Collateral and bicollateral bundles are two types of conjoint vascular bundles.

What is an Amphivasal vascular bundle?

Amphivasal vascular bundle is an arrangement where a phloem tissue is surrounded by xylem tissue on all the sides.

Which is an example of Amphivasal?

Amphivasal bundles are found in monocot stems, such as Yucca and Dracaena.