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Vascular Tissue System

The vascular system is composed of complex tissues namely xylem and phloem, which together are known as vascular bundles. There are two types of vascular bundles, namely open and closed

Open Vascular bundles:
In dicotyledonous stems, cambium is present between xylem and phloem. Due to the presence of cambium, secondary xylem and phloem tissues are formed; hence they are called as open vascular bundles.

Closed vascular bundles:
In monocotyledonous stems, the absence of cambium results in a lack of formation of secondary tissues. Hence they are referred to as closed vascular bundles.

Radial type of vascular bundles:
If the xylem and phloem tissues are arranged in an alternate manner on different radii within a vascular bundle, then this arrangement is called as radial and is usually seen in roots.

Figure 1 Various types of vascular bundles:

A.    Radial              B. Conjoint closed                  C. Conjoint open

Conjoint type of vascular bundles: In this type, the xylem and phloem are arranged in the same radius of vascular bundles. This type of arrangement is usually seen in stem and leaves. In conjoint type, the phloem is usually located on the outer side of xylem.

Anatomy of dicotyledonous root

In dicotyledonous roots, the outermost layer is covered thoroughly with epidermal cells. Some of these epidermal cells extend to unicellular root hairs. The cortex consists of layers of thin-walled parenchymatous cells with intracellular spaces. The innermost layer of cortex called endodermis is composed of a single layer of barrel-shaped cells lacking intercellular spaces.

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