The following points highlight the four main types of vascular bundle. The types are: 1. Collateral Bundle 2. Bicollateral Bundle 3. Concentric Bundle 4. Radial Vascular Bundle.
1. Collateral Bundle
A vascular bundle in which a strand c f phloem is present external to the strand of xylem on the same radius side by side is known as collateral bundle (Figure 1). Between xylem and phloem, cambium may be present or missing, resulting in the following two forms of collateral bundle: (a) Closed collateral bundle and (b) Open collateral bundle.
(a) Closed collateral bundle: Between the xylem and the phloem in this kind, there is no cambium. As a result, stems with this kind of bundle do not exhibit typical secondary growth. Ex. the stem of a monocotyledon. Usually these bundles are enclosed within bundle sheath made up of sclerenchyma and those that lack the sheath are considered as anomalous (e.g. Asparagus stem).
(b) Open collateral bundle: Between the xylem and phloem of an open collateral vascular bundle is cambium known as fascicular cambium. With the aid of fascicular cambium, the bundles may become larger by regular secondary development. Ex. Dicotyledonous stem.
Figure 1: Diagram showing types of vascular bundle
2. Bicollateral Bundle
Bicollateral bundle refers to a vascular bundle with phloem located on the inner and peripheral side of xylem. Between the peripheral phloem and xylem, there is a cambium strip known as the outer cambium; a second cambium strip known as the inner cambium is also present between the inner phloem and xylem.
While the inner or internal phloem is known as inner phloem, the peripheral or external phloem is referred to as outer phloem. In the bicollateral bundles, the vascular tissues are arranged in the following order: outer phloem, outer cambium, xylem, inner cambium, and inner phloem.
These bundles are open type as strips of cambia are present but the secondary thickening occurs only by the outer cambium, i.e. cambium present between the outer phloem and xylem. Ex. Cucurbita stem.
3. Concentric Bundle
A concentric bundle is a vascular bundle in which one type of vascular tissue surrounds the other. The amphivasal or amphicribral concentric bundles are closed because there is no cambium between the xylem and phloem. In this bundle, xylem either encircles or is encircled by phloem, and thus two types are recognised:
(a) Amphivasal bundle: Amphivasal bundle, also called leptocentric bundle, is a group of blood vessels in which the xylem wraps around the phloem in the middle. Ex. Dracaena, Yucca.
(b) Amphicribral bundle: A bundle of blood vessels in which the phloem wraps around the xylem in the middle is called an amphicribral bundle or a hadrocentric bundle. Ex. Selaginella.
4. Radial Vascular Bundle
Radial vascular bundle or radial bundle is a type of vascular bundle in which the primary xylem and primary phloem strands are separated by nonvascular tissues and are on opposite radii of an axis (Figure 2B).
These bundles are the defining feature of roots. This bundle contains no primary cambium, and the secondary thickening is caused by the secondary cambium that arises at the time of secondary root growth in only dicotyledonous roots.
Figure 2A: Diagram illustrating the different arrangements of the number of protoxylem groups in roots in cross-sectional view. Figure 2B: Chart summarizing the different types of vascular bundle.
Dicot roots typically have between four and six protoxylem poles, whereas monocot roots typically have more than six xylem poles. The number of protoxylem poles in a root can range from 1 to 6. Consequently, they are referred to as monarch, diarch, triarch, tetrarch, etc (Figure 2A). The term polyarch is used when there are more than six protoxylem poles.