Vascular Bundles: Definition, Types and Function

Vascular Bundles in Plants: Have you ever noticed the veins at the bottom side of your arms? Some of these veins carry blood from your hands to your heart and vice versa. These veins are an example of Vascular Tissue. These vascular tissue transport blood which carries life-sustaining substances essential for an active human body.

Like humans, plants also have vascular tissue, but these tissues transport water and nutrients that circulates throughout the plant body. Like our veins are spread around our internal body parts, the arrangement of vascular tissue in the plant body occurs in Vascular Bundles; these vascular bundles are spread like thin tubes in plants. Generally, vascular bundles are described as a part of the transport system in vascular plants which involved 2 tissues namely, the Xylem and Phloem. In this article, we will provide all the detailed information about Vascular Bundles. Scroll down to read more!

Define Vascular Bundle

The Vascular bundle is a strand of special vascular tissue in plants. It consists of two complex tissues, xylem and phloem. Vascular bundles are one of the major components of the vascular tissue system in plants.

What are Vascular Bundles?

The vascular bundle is a part of the transport system in vascular plants; these are specialized tissues that run through the plant and carry water and nutrition to different parts of the plants. In higher plants, vascular bundles are present inside the stele. Members of the vascular bundles are derived from the procambium of the apical meristem.

Vascular Bundles Components

The main components of Vascular bundles are (a) the Xylem and (b) Phloem. Although Cambium is not the utmost important part of a vascular bundle, it is present in dicotyledonous plants.

1. Xylem

The xylem is the primary water-conducting tissue. It consists of tracheids, trachea, xylem fibres and xylem parenchyma. The primary function of the xylem is to transport water from roots to stem and leaves.

The primary xylem consists of Protoxylem or first-formed xylem, and Metaxylem or late-formed xylem. There are three basic arrangements of these two types of xylem in plants:
(i) Endarch xylem is found in the stems of dicots and monocots. The protoxylem occurs towards the centre and the metaxylem towards the periphery.
(ii) Exarch xylem is found in the roots of dicots and monocots. The protoxylem occurs towards the periphery and the metaxylem towards the centre.
(iii) Mesarch xylem is found in leaves of dicots and monocots and in some ferns. The metaxylem element is surrounded by the protoxylem.

2. Phloem

The phloem is the leading food conducting tissue. It consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma. The primary function of phloem is to transport food and nutrients from leaves to different parts of the plant.
Primary phloem is also divided into Protophloem or first formed phloem and Metaphloem or late-formed phloem, depending on their origin.

Cambium: The cambium is a lateral meristem from which secondary xylem and secondary phloem tissue arise. Cambium mainly helps in secondary growth in plants. Cambium is composed of two types of cells: 

1. Fusiform initials – These cells are elongated with tapering ends, and their size varies greatly.
2. Ray initials – These cells are more or less isodiametric, and their size is more or less uniform.

Fig.  Components of Vascular Bundles in Monocots and Dicots

Vascular Bundles Diagrams

Fig. Diagram of Vascular Bundles

Vascular Bundle Types

Vascular Bundles are classified into:

1. Based on Presence or Absence of Cambium
2. According to Mode of Occurrence of the Elements

1. Based on Presence or Absence of Cambium

Open Vascular Bundles: The Cambium is present between the xylem and phloem. They help in secondary growth in plants. These are found in Dicots. Fascicular cambium is the cambium present in the xylem and phloem.

Closed Vascular Bundles: The Cambium is absent in between the xylem and phloem. The secondary growth is absent in closed vascular bundles. These are mainly found in Monocots. Fascicular cambium is absent. Example: Asparagus stem.

2. According to Mode of Occurrence of the Elements

Radial

In the roots of monocots and dicots, xylem and phloem are arranged in an alternate manner on different radii. It is called radial arrangement. The vascular tissues are separated by non-vascular strands of tissue. There is no primary cambium in these bundles. Thus, the secondary growth or secondary thickening occurs by the secondary cambium, which originates later.
The dicot roots usually have four to six numbers protoxylem poles (termed as diarch to tetrarch). Conversely, in the monocot roots, there are many poles of the xylem (more than six) are present, and are called polyarch. 

Conjoint

In stems and leaves of angiosperms and gymnosperms, xylem and phloem are arranged at the same radius and form a vascular bundle together. Xylem is present towards the centre and phloem towards the periphery.

Such vascular bundles are of two types:
(a) Collateral: A vascular bundle in which the phloem is present towards the outer side and xylem towards the inner side is called collateral. A collateral bundle may be open or closed on the presence or absence of cambium between the xylem and phloem. These are found in the Sunflower, Maize. 
(b) Bicollateral: If phloem occurs on both the outer and inner sides of the xylem, the bundle is called bicollateral. In these two patches of phloem present, one outside the xylem and the other inside the xylem. The sequence of vascular bundles is outer phloem, outer cambium, xylem, inner cambium, and inner phloem. The bundles are always open. These are mainly found in Cucurbitaceae. 
These bundles are essentially open type as strips of cambia are present. However, secondary thickening occurs only by the outer cambium, i.e., the cambium present between the outer phloem and xylem.

Concentric

The concentric vascular bundle is one in which either phloem surrounds the xylem, or the xylem covers the phloem. Such vascular bundles are of two types:
(a) Amphivasal (or Leptocentric): In amphivasal concentric vascular bundles, the xylem completely surrounds the phloem. These are found in Dracaena, Yucca.
(b) Amphicribral (or Hadrocentric): In amphicribral concentric vascular bundles, the phloem completely surrounds the xylem. These are found in Selaginella. This is considered a primitive feature.

Fig. Types of Vascular Bundles

Vascular Bundle Occurrence

Vascular bundles are found in all plant organs. It is mainly present in the stem, leaves and roots of the plant. 
1. In the stem, the vascular bundles are scattered or found around the stem axis.
2. In leaves, the vascular bundles are present in the mesophyll tissue.
3. In roots, they are present at the centre.
4. Sometimes vascular bundles are also situated in the cortex, pith, and medullary bundle.

Functions of Vascular Bundles

The vascular bundle is made up of several tissues and these tissues have specific functions:

Xylem: It plays a vital role in the transportation of water components to different parts of the plant.

Phloem: This tissue is responsible for transporting food and nutrients to the plant.

Cambium: It is involved in the secondary growth in plants by creating new xylem and phloem.

Summary

We can say that vascular bundles act as a mode of transport in plants. It consists of specific tissues which help in carrying water and nutrition to different parts of the plants. This article will allow students to go through the essential aspects of vascular bundles and their major components. It also gives a clear concept about the types of the vascular bundle and their functions. So, the vascular bundles contribute a vital role to provide essential and life-sustaining substances to the plant.

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FAQs on Vascular Bundles

Q.1. What does a vascular bundle consist of?
Ans: The Vascular bundles consist of (a) Xylem (present in both monocots and dicots) and (b) Phloem (present in both monocots and dicots). (c) Cambium (present in the case of dicots) is not an utmost important component of vascular bundles.

Q.2. Where in the leaves are vascular bundles located?
Ans: Vascular bundles are located within the mesophyll tissue in leaves.

Q.3. Where are vascular bundles located?
Ans: The vascular bundles are located near the edge of the stem, with the phloem on the outside and the xylem on the inside.

Q.4. Which type of vascular bundle is found in Dracaena?
Ans: Amphivasal or leptocentric vascular bundle is found in Dracaena.

Q.5. What is a closed vascular bundle?
Ans: If the cambium is absent in between the xylem and phloem of the vascular bundles, it is called a closed vascular bundle, as it does not show secondary growth. This is found in monocot stems.

Q.6. What are the functions of vascular bundles?
Ans: The functions of vascular bundles are as follows:
1. Xylem: It plays a vital role in the transportation of water components to different parts of the plant.
2. Phloem: This tissue is responsible for transporting food and nutrients to the plant.
3. Cambium: It is involved in the secondary growth in plants by creating new xylem and phloem.

Q.7. What are vascular bundles and their types?
Ans: The vascular bundle is a part of the transport system in vascular plants; these are specialized tissues that run through the plant and carry water and nutrition to different parts of the plants.
The different types of vascular bundles are:
(A) Based on presence or absence of cambium:
(1) Open (with cambium) and (2) Closed (without cambium).
(B) According to the mode of occurrence of the elements:
(1) Radial, (2) Conjoint and (3) Concentric

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